diff --git a/src/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri b/src/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri
index b77118bb983..558c86baebf 100644
--- a/src/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri
+++ b/src/tools/clangrefactoringbackend/source/clangrefactoringbackend-source.pri
@@ -27,7 +27,10 @@ HEADERS += \
     $$PWD/symbolindexertaskqueue.h \
     $$PWD/symbolindexertaskscheduler.h \
     $$PWD/symbolscollectormanagerinterface.h \
-    $$PWD/symbolindexertaskqueueinterface.h
+    $$PWD/symbolindexertaskqueueinterface.h \
+    $$PWD/symbolindexertaskschedulerinterface.h \
+    $$PWD/symbolscollectormanager.h \
+    $$PWD/symbolindexertask.h
 
 !isEmpty(LIBTOOLING_LIBS) {
 SOURCES += \
@@ -75,4 +78,5 @@ SOURCES += \
     $$PWD/filestatuscache.cpp \
     $$PWD/projectpartqueue.cpp \
     $$PWD/symbolindexertaskqueue.cpp \
-    $$PWD/symbolindexertaskscheduler.cpp
+    $$PWD/symbolindexertaskscheduler.cpp \
+    $$PWD/symbolscollectormanager.cpp
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexer.cpp b/src/tools/clangrefactoringbackend/source/symbolindexer.cpp
index 58450cdc4d8..cb47f87406a 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexer.cpp
+++ b/src/tools/clangrefactoringbackend/source/symbolindexer.cpp
@@ -25,15 +25,18 @@
 
 #include "symbolindexer.h"
 
+#include <symbolscollector.h>
+#include <symbolindexertaskqueue.h>
+
 namespace ClangBackEnd {
 
-SymbolIndexer::SymbolIndexer(SymbolsCollectorInterface &symbolsCollector,
+SymbolIndexer::SymbolIndexer(SymbolIndexerTaskQueueInterface &symbolIndexerTaskQueue,
                              SymbolStorageInterface &symbolStorage,
                              ClangPathWatcherInterface &pathWatcher,
                              FilePathCachingInterface &filePathCache,
                              FileStatusCache &fileStatusCache,
                              Sqlite::TransactionInterface &transactionInterface)
-    : m_symbolsCollector(symbolsCollector),
+    : m_symbolIndexerTaskQueue(symbolIndexerTaskQueue),
       m_symbolStorage(symbolStorage),
       m_pathWatcher(pathWatcher),
       m_filePathCache(filePathCache),
@@ -52,41 +55,60 @@ void SymbolIndexer::updateProjectParts(V2::ProjectPartContainers &&projectParts,
 void SymbolIndexer::updateProjectPart(V2::ProjectPartContainer &&projectPart,
                                       const V2::FileContainers &generatedFiles)
 {
-    m_symbolsCollector.clear();
+    Sqlite::ImmediateTransaction transaction{m_transactionInterface};
+    const auto optionalArtefact = m_symbolStorage.fetchProjectPartArtefact(projectPart.projectPartId);
+    int projectPartId = m_symbolStorage.insertOrUpdateProjectPart(projectPart.projectPartId,
+                                                                  projectPart.arguments,
+                                                                  projectPart.compilerMacros,
+                                                                  projectPart.includeSearchPaths);
+    transaction.commit();
 
-    const Utils::optional<ProjectPartArtefact> optionalArtefact = m_symbolStorage.fetchProjectPartArtefact(
-                projectPart.projectPartId);
+    if (optionalArtefact)
+        projectPartId = optionalArtefact->projectPartId;
 
     FilePathIds sourcePathIds = updatableFilePathIds(projectPart, optionalArtefact);
 
-    if (!sourcePathIds.empty()) {
-        m_symbolsCollector.addFiles(projectPart.sourcePathIds,
-                                    compilerArguments(projectPart, optionalArtefact));
+    if (sourcePathIds.empty())
+        return;
 
-        m_symbolsCollector.addUnsavedFiles(generatedFiles);
+    Utils::SmallStringVector arguments = compilerArguments(projectPart, optionalArtefact);
 
-        m_symbolsCollector.collectSymbols();
+    std::vector<SymbolIndexerTask> symbolIndexerTask;
+    symbolIndexerTask.reserve(projectPart.sourcePathIds.size());
 
-        Sqlite::ImmediateTransaction transaction{m_transactionInterface};
+    for (FilePathId sourcePathId : projectPart.sourcePathIds) {
+        auto indexing = [projectPart, arguments, generatedFiles, sourcePathId]
+                (SymbolsCollectorInterface &symbolsCollector,
+                SymbolStorageInterface &symbolStorage,
+                Sqlite::TransactionInterface &transactionInterface) {
+            symbolsCollector.addFile(sourcePathId, arguments);
 
-        m_symbolStorage.addSymbolsAndSourceLocations(m_symbolsCollector.symbols(),
-                                                     m_symbolsCollector.sourceLocations());
+            symbolsCollector.addUnsavedFiles(generatedFiles);
 
-        m_symbolStorage.insertOrUpdateProjectPart(projectPart.projectPartId,
-                                                  projectPart.arguments,
-                                                  projectPart.compilerMacros,
-                                                  projectPart.includeSearchPaths);
-        m_symbolStorage.updateProjectPartSources(projectPart.projectPartId,
-                                                 m_symbolsCollector.sourceFiles());
+            symbolsCollector.collectSymbols();
 
-        m_symbolStorage.insertOrUpdateUsedMacros(m_symbolsCollector.usedMacros());
+            Sqlite::ImmediateTransaction transaction{transactionInterface};
 
-        m_symbolStorage.insertFileStatuses(m_symbolsCollector.fileStatuses());
+            symbolStorage.addSymbolsAndSourceLocations(symbolsCollector.symbols(),
+                                                       symbolsCollector.sourceLocations());
 
-        m_symbolStorage.insertOrUpdateSourceDependencies(m_symbolsCollector.sourceDependencies());
+            symbolStorage.updateProjectPartSources(projectPart.projectPartId,
+                                                   symbolsCollector.sourceFiles());
 
-        transaction.commit();
+            symbolStorage.insertOrUpdateUsedMacros(symbolsCollector.usedMacros());
+
+            symbolStorage.insertFileStatuses(symbolsCollector.fileStatuses());
+
+            symbolStorage.insertOrUpdateSourceDependencies(symbolsCollector.sourceDependencies());
+
+            transaction.commit();
+        };
+
+        symbolIndexerTask.emplace_back(sourcePathId, projectPartId, std::move(indexing));
     }
+
+    m_symbolIndexerTaskQueue.addOrUpdateTasks(std::move(symbolIndexerTask));
+    m_symbolIndexerTaskQueue.processTasks();
 }
 
 void SymbolIndexer::pathsWithIdsChanged(const Utils::SmallStringVector &)
@@ -95,40 +117,57 @@ void SymbolIndexer::pathsWithIdsChanged(const Utils::SmallStringVector &)
 
 void SymbolIndexer::pathsChanged(const FilePathIds &filePathIds)
 {
+    std::vector<SymbolIndexerTask> symbolIndexerTask;
+    symbolIndexerTask.reserve(filePathIds.size());
+
     for (FilePathId filePathId : filePathIds)
-        updateChangedPath(filePathId);
+        updateChangedPath(filePathId, symbolIndexerTask);
+
+    m_symbolIndexerTaskQueue.addOrUpdateTasks(std::move(symbolIndexerTask));
+    m_symbolIndexerTaskQueue.processTasks();
 }
 
-void SymbolIndexer::updateChangedPath(FilePathId filePathId)
+void SymbolIndexer::updateChangedPath(FilePathId filePathId,
+                                      std::vector<SymbolIndexerTask> &symbolIndexerTask)
 {
-    m_symbolsCollector.clear();
     m_fileStatusCache.update(filePathId);
 
+    Sqlite::DeferredTransaction transaction{m_transactionInterface};
     const Utils::optional<ProjectPartArtefact> optionalArtefact = m_symbolStorage.fetchProjectPartArtefact(filePathId);
+    transaction.commit();
 
     if (optionalArtefact && !optionalArtefact.value().compilerArguments.empty()) {
+
         const ProjectPartArtefact &artefact = optionalArtefact.value();
 
-        m_symbolsCollector.addFiles({filePathId},
-                                    compilerArguments(artefact.compilerArguments, artefact.projectPartId));
+        Utils::SmallStringVector arguments = compilerArguments(artefact.compilerArguments,
+                                                               artefact.projectPartId);
 
-        m_symbolsCollector.collectSymbols();
+        auto indexing = [projectPartId=artefact.projectPartId, arguments, filePathId]
+                (SymbolsCollectorInterface &symbolsCollector,
+                SymbolStorageInterface &symbolStorage,
+                Sqlite::TransactionInterface &transactionInterface) {
+            symbolsCollector.addFile(filePathId, arguments);
 
-        Sqlite::ImmediateTransaction transaction{m_transactionInterface};
+            symbolsCollector.collectSymbols();
 
-        m_symbolStorage.addSymbolsAndSourceLocations(m_symbolsCollector.symbols(),
-                                                     m_symbolsCollector.sourceLocations());
+            Sqlite::ImmediateTransaction transaction{transactionInterface};
 
-        m_symbolStorage.updateProjectPartSources(artefact.projectPartId,
-                                                 m_symbolsCollector.sourceFiles());
+            symbolStorage.addSymbolsAndSourceLocations(symbolsCollector.symbols(),
+                                                       symbolsCollector.sourceLocations());
 
-        m_symbolStorage.insertOrUpdateUsedMacros(m_symbolsCollector.usedMacros());
+            symbolStorage.updateProjectPartSources(projectPartId, symbolsCollector.sourceFiles());
 
-        m_symbolStorage.insertFileStatuses(m_symbolsCollector.fileStatuses());
+            symbolStorage.insertOrUpdateUsedMacros(symbolsCollector.usedMacros());
 
-        m_symbolStorage.insertOrUpdateSourceDependencies(m_symbolsCollector.sourceDependencies());
+            symbolStorage.insertFileStatuses(symbolsCollector.fileStatuses());
 
-        transaction.commit();
+            symbolStorage.insertOrUpdateSourceDependencies(symbolsCollector.sourceDependencies());
+
+            transaction.commit();
+        };
+
+        symbolIndexerTask.emplace_back(filePathId, optionalArtefact->projectPartId, std::move(indexing));
     }
 }
 
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexer.h b/src/tools/clangrefactoringbackend/source/symbolindexer.h
index 4fc66fee0ff..ffa2269e9b3 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexer.h
+++ b/src/tools/clangrefactoringbackend/source/symbolindexer.h
@@ -26,7 +26,7 @@
 #pragma once
 
 #include "filestatuscache.h"
-#include "symbolscollectorinterface.h"
+#include "symbolindexertaskqueueinterface.h"
 #include "symbolstorageinterface.h"
 #include "clangpathwatcher.h"
 
@@ -35,10 +35,12 @@
 
 namespace ClangBackEnd {
 
+class SymbolsCollectorInterface;
+
 class SymbolIndexer : public ClangPathWatcherNotifier
 {
 public:
-    SymbolIndexer(SymbolsCollectorInterface &symbolsCollector,
+    SymbolIndexer(SymbolIndexerTaskQueueInterface &symbolIndexerTaskQueue,
                   SymbolStorageInterface &symbolStorage,
                   ClangPathWatcherInterface &pathWatcher,
                   FilePathCachingInterface &filePathCache,
@@ -52,7 +54,8 @@ public:
 
     void pathsWithIdsChanged(const Utils::SmallStringVector &ids) override;
     void pathsChanged(const FilePathIds &filePathIds) override;
-    void updateChangedPath(FilePathId filePath);
+    void updateChangedPath(FilePathId filePath,
+                           std::vector<SymbolIndexerTask> &symbolIndexerTask);
 
     bool compilerMacrosOrIncludeSearchPathsAreDifferent(
             const V2::ProjectPartContainer &projectPart,
@@ -70,7 +73,7 @@ public:
                                                int projectPartId) const;
 
 private:
-    SymbolsCollectorInterface &m_symbolsCollector;
+    SymbolIndexerTaskQueueInterface &m_symbolIndexerTaskQueue;
     SymbolStorageInterface &m_symbolStorage;
     ClangPathWatcherInterface &m_pathWatcher;
     FilePathCachingInterface &m_filePathCache;
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexertask.h b/src/tools/clangrefactoringbackend/source/symbolindexertask.h
new file mode 100644
index 00000000000..3b18681e072
--- /dev/null
+++ b/src/tools/clangrefactoringbackend/source/symbolindexertask.h
@@ -0,0 +1,87 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include <filepathid.h>
+
+#include <functional>
+
+namespace Sqlite {
+class TransactionInterface;
+}
+
+namespace ClangBackEnd {
+
+class SymbolIndexerTaskSchedulerInterface;
+class SymbolsCollectorInterface;
+class SymbolStorageInterface;
+
+class SymbolIndexerTask
+{
+public:
+    using Callable = std::function<void(SymbolsCollectorInterface &symbolsCollector,
+                                        SymbolStorageInterface &symbolStorage,
+                                        Sqlite::TransactionInterface &transaction)>;
+
+    SymbolIndexerTask(FilePathId filePathId,
+                      int projectPartId,
+                      Callable &&callable)
+        : callable(std::move(callable)),
+          filePathId(filePathId),
+          projectPartId(projectPartId)
+    {
+    }
+
+    SymbolIndexerTask clone() const
+    {
+        return *this;
+    }
+
+    friend
+    bool operator==(const SymbolIndexerTask &first, const SymbolIndexerTask &second)
+    {
+        return first.filePathId == second.filePathId && first.projectPartId == second.projectPartId;
+    }
+
+    friend
+    bool operator<(const SymbolIndexerTask &first, const SymbolIndexerTask &second)
+    {
+        return std::tie(first.filePathId, first.projectPartId)
+             < std::tie(second.filePathId, second.projectPartId);
+    }
+
+    operator Callable&&()
+    {
+        return std::move(callable);
+    }
+
+public:
+    Callable callable;
+    FilePathId filePathId;
+    int projectPartId;
+};
+
+} // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.cpp b/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.cpp
index aa4fd382157..4bfed1fe31f 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.cpp
+++ b/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.cpp
@@ -25,6 +25,8 @@
 
 #include "symbolindexertaskqueue.h"
 
+#include <symbolindexertaskschedulerinterface.h>
+
 namespace ClangBackEnd {
 
 namespace {
@@ -60,11 +62,6 @@ OutputIt set_union_merge(InputIt1 first1,
 
 }
 
-SymbolIndexerTaskQueue::SymbolIndexerTaskQueue()
-{
-
-}
-
 void SymbolIndexerTaskQueue::addOrUpdateTasks(std::vector<SymbolIndexerTask> &&tasks)
 {
     std::vector<SymbolIndexerTask> mergedTasks;
@@ -133,7 +130,12 @@ std::vector<std::size_t> SymbolIndexerTaskQueue::projectPartNumberIds(const Util
 
 void SymbolIndexerTaskQueue::processTasks()
 {
+    int taskCount = m_symbolIndexerScheduler.freeSlots();
 
+    auto newEnd = std::prev(m_tasks.end(), std::min<int>(taskCount, int(m_tasks.size())));
+    m_symbolIndexerScheduler.addTasks({std::make_move_iterator(newEnd),
+                                       std::make_move_iterator(m_tasks.end())});
+    m_tasks.erase(newEnd, m_tasks.end());
 }
 
 } // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.h b/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.h
index a4fb44de0ac..979129eb998 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.h
+++ b/src/tools/clangrefactoringbackend/source/symbolindexertaskqueue.h
@@ -26,6 +26,7 @@
 #pragma once
 
 #include "symbolindexertaskqueueinterface.h"
+#include "symbolindexertask.h"
 
 #include <filepathid.h>
 
@@ -34,59 +35,25 @@
 #include <functional>
 #include <vector>
 
+namespace Sqlite {
+class TransactionInterface;
+}
+
 namespace ClangBackEnd {
 
+class SymbolIndexerTaskSchedulerInterface;
 class SymbolsCollectorInterface;
 class SymbolStorageInterface;
 
-class SymbolIndexerTask
+class SymbolIndexerTaskQueue final : public SymbolIndexerTaskQueueInterface
 {
 public:
-    using Callable = std::function<void(SymbolsCollectorInterface &symbolsCollector,
-                                        SymbolStorageInterface &symbolStorage)>;
+    SymbolIndexerTaskQueue(SymbolIndexerTaskSchedulerInterface &symbolIndexerTaskScheduler)
+        : m_symbolIndexerScheduler(symbolIndexerTaskScheduler)
+    {}
 
-    SymbolIndexerTask(FilePathId filePathId,
-                      std::size_t projectPartId,
-                      Callable &&callable)
-        : callable(std::move(callable)),
-          filePathId(filePathId),
-          projectPartId(projectPartId)
-    {
-    }
-
-    SymbolIndexerTask clone() const
-    {
-        return *this;
-    }
-
-    friend
-    bool operator==(const SymbolIndexerTask &first, const SymbolIndexerTask &second)
-    {
-        return first.filePathId == second.filePathId && first.projectPartId == second.projectPartId;
-    }
-
-    friend
-    bool operator<(const SymbolIndexerTask &first, const SymbolIndexerTask &second)
-    {
-        return std::tie(first.filePathId, first.projectPartId)
-             < std::tie(second.filePathId, second.projectPartId);
-    }
-
-public:
-    Callable callable;
-    FilePathId filePathId;
-    std::size_t projectPartId;
-};
-
-class SymbolIndexerTaskQueue
-{
-public:
-    SymbolIndexerTaskQueue();
-
-    void addOrUpdateTasks(std::vector<SymbolIndexerTask> &&tasks)
-    /* [[expects: std::is_sorted(tasks)]] */;
-    void removeTasks(const Utils::SmallStringVector &projectPartIds)
-    /* [[expects: std::is_sorted(projectPartIds)]] */;
+    void addOrUpdateTasks(std::vector<SymbolIndexerTask> &&tasks);
+    void removeTasks(const Utils::SmallStringVector &projectPartIds);
 
     const std::vector<SymbolIndexerTask> &tasks() const;
 
@@ -94,11 +61,13 @@ public:
     std::vector<std::size_t> projectPartNumberIds(const Utils::SmallStringVector &projectPartIds)
     /* [[ensures result: std::is_sorted(result)]] */;
 
-    void processTasks() ;
+    void processTasks();
+    void syncTasks();
 
 private:
     std::vector<Utils::SmallString> m_projectPartIds;
     std::vector<SymbolIndexerTask> m_tasks;
+    SymbolIndexerTaskSchedulerInterface &m_symbolIndexerScheduler;
 };
 
 } // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexertaskqueueinterface.h b/src/tools/clangrefactoringbackend/source/symbolindexertaskqueueinterface.h
index 09b760d0a32..d4d79ec6cfb 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexertaskqueueinterface.h
+++ b/src/tools/clangrefactoringbackend/source/symbolindexertaskqueueinterface.h
@@ -25,11 +25,23 @@
 
 #pragma once
 
+#include <utils/smallstringvector.h>
+
 namespace ClangBackEnd {
 
+class SymbolIndexerTask;
+
 class SymbolIndexerTaskQueueInterface
 {
 public:
+    SymbolIndexerTaskQueueInterface() = default;
+    SymbolIndexerTaskQueueInterface(const SymbolIndexerTaskQueueInterface &) = delete;
+    SymbolIndexerTaskQueueInterface &operator=(const SymbolIndexerTaskQueueInterface &) = delete;
+
+    virtual void addOrUpdateTasks(std::vector<SymbolIndexerTask> &&tasks) = 0
+    /* [[expects: std::is_sorted(tasks)]] */;
+    virtual void removeTasks(const Utils::SmallStringVector &projectPartIds) = 0
+    /* [[expects: std::is_sorted(projectPartIds)]] */;
     virtual void processTasks() = 0;
 
 protected:
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexertaskscheduler.cpp b/src/tools/clangrefactoringbackend/source/symbolindexertaskscheduler.cpp
index bd065311055..e9f5cbe285b 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexertaskscheduler.cpp
+++ b/src/tools/clangrefactoringbackend/source/symbolindexertaskscheduler.cpp
@@ -87,7 +87,7 @@ void SymbolIndexerTaskScheduler::addTasks(std::vector<Task> &&tasks)
         auto callWrapper = [&, task=std::move(task)] (
                 std::reference_wrapper<SymbolsCollectorInterface> symbolsCollector)
                 -> SymbolsCollectorInterface& {
-            task(symbolsCollector.get(), m_symbolStorage);
+            task(symbolsCollector.get(), m_symbolStorage, m_transactionInterface);
             executeInLoop([&] {
                 m_symbolIndexerTaskQueue.processTasks();
             });
@@ -105,11 +105,14 @@ const std::vector<SymbolIndexerTaskScheduler::Future> &SymbolIndexerTaskSchedule
     return m_futures;
 }
 
-int SymbolIndexerTaskScheduler::freeSlots()
+uint SymbolIndexerTaskScheduler::freeSlots()
 {
     removeFinishedFutures();
 
-    return std::max(m_hardware_concurrency - int(m_futures.size()), 0);
+    if (m_isDisabled)
+        return 0;
+
+    return uint(std::max(int(m_hardware_concurrency) - int(m_futures.size()), 0));
 }
 
 void SymbolIndexerTaskScheduler::syncTasks()
@@ -118,6 +121,11 @@ void SymbolIndexerTaskScheduler::syncTasks()
         future.wait();
 }
 
+void SymbolIndexerTaskScheduler::disable()
+{
+    m_isDisabled = true;
+}
+
 void SymbolIndexerTaskScheduler::removeFinishedFutures()
 {
     auto notReady = [] (Future &future) {
@@ -127,7 +135,9 @@ void SymbolIndexerTaskScheduler::removeFinishedFutures()
     auto split = std::partition(m_futures.begin(), m_futures.end(), notReady);
 
     std::for_each(split, m_futures.end(), [] (Future &future) {
-        future.get().setIsUsed(false);
+        SymbolsCollectorInterface &symbolCollector = future.get();
+        symbolCollector.setIsUsed(false);
+        symbolCollector.clear();
     });
 
     m_futures.erase(split, m_futures.end());
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexertaskscheduler.h b/src/tools/clangrefactoringbackend/source/symbolindexertaskscheduler.h
index 919b8c1fd7b..6182376dab0 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexertaskscheduler.h
+++ b/src/tools/clangrefactoringbackend/source/symbolindexertaskscheduler.h
@@ -25,10 +25,17 @@
 
 #pragma once
 
+#include "symbolindexertaskschedulerinterface.h"
+#include "symbolindexertask.h"
+
 #include <functional>
 #include <future>
 #include <vector>
 
+namespace Sqlite {
+class TransactionInterface;
+};
+
 namespace ClangBackEnd {
 
 class FilePathCachingInterface;
@@ -37,20 +44,21 @@ class SymbolsCollectorManagerInterface;
 class SymbolIndexerTaskQueueInterface;
 class SymbolStorageInterface;
 
-class SymbolIndexerTaskScheduler
+class SymbolIndexerTaskScheduler final : public SymbolIndexerTaskSchedulerInterface
 {
 public:
-    using Task = std::function<void(SymbolsCollectorInterface &symbolsCollector,
-                                    SymbolStorageInterface &symbolStorage)>;
+    using Task = SymbolIndexerTask::Callable;
     using Future = std::future<SymbolsCollectorInterface&>;
 
     SymbolIndexerTaskScheduler(SymbolsCollectorManagerInterface &symbolsCollectorManager,
                                SymbolStorageInterface &symbolStorage,
+                               Sqlite::TransactionInterface &transactionInterface,
                                SymbolIndexerTaskQueueInterface &symbolIndexerTaskQueue,
-                               int hardware_concurrency,
+                               uint hardware_concurrency,
                                std::launch launchPolicy = std::launch::async)
         : m_symbolsCollectorManager(symbolsCollectorManager),
           m_symbolStorage(symbolStorage),
+          m_transactionInterface(transactionInterface),
           m_symbolIndexerTaskQueue(symbolIndexerTaskQueue),
           m_hardware_concurrency(hardware_concurrency),
           m_launchPolicy(launchPolicy)
@@ -60,10 +68,12 @@ public:
 
     const std::vector<Future> &futures() const;
 
-    int freeSlots();
+    uint freeSlots();
 
     void syncTasks();
 
+    void disable();
+
 private:
     void removeFinishedFutures();
 
@@ -71,9 +81,11 @@ private:
     std::vector<Future> m_futures;
     SymbolsCollectorManagerInterface &m_symbolsCollectorManager;
     SymbolStorageInterface &m_symbolStorage;
+    Sqlite::TransactionInterface &m_transactionInterface;
     SymbolIndexerTaskQueueInterface &m_symbolIndexerTaskQueue;
-    int m_hardware_concurrency;
+    uint m_hardware_concurrency;
     std::launch m_launchPolicy;
+    bool m_isDisabled = false;
 };
 
 } // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexertaskschedulerinterface.h b/src/tools/clangrefactoringbackend/source/symbolindexertaskschedulerinterface.h
new file mode 100644
index 00000000000..c71454df3d8
--- /dev/null
+++ b/src/tools/clangrefactoringbackend/source/symbolindexertaskschedulerinterface.h
@@ -0,0 +1,60 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include <functional>
+#include <vector>
+
+namespace Sqlite {
+class TransactionInterface;
+}
+
+namespace ClangBackEnd {
+
+class SymbolsCollectorInterface;
+class SymbolStorageInterface;
+
+using uint = unsigned int;
+
+class SymbolIndexerTaskSchedulerInterface
+{
+public:
+    using Task = std::function<void(SymbolsCollectorInterface &symbolsCollector,
+                                    SymbolStorageInterface &symbolStorage,
+                                    Sqlite::TransactionInterface &transaction)>;
+
+    SymbolIndexerTaskSchedulerInterface() = default;
+    SymbolIndexerTaskSchedulerInterface(const SymbolIndexerTaskSchedulerInterface &) = delete;
+    SymbolIndexerTaskSchedulerInterface &operator=(const SymbolIndexerTaskSchedulerInterface &) = delete;
+
+    virtual void addTasks(std::vector<Task> &&tasks) = 0;
+    virtual uint freeSlots() = 0;
+
+protected:
+    ~SymbolIndexerTaskSchedulerInterface() = default;
+};
+
+} // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/symbolindexing.h b/src/tools/clangrefactoringbackend/source/symbolindexing.h
index 96f6ecb1e5a..07188690c7f 100644
--- a/src/tools/clangrefactoringbackend/source/symbolindexing.h
+++ b/src/tools/clangrefactoringbackend/source/symbolindexing.h
@@ -30,6 +30,9 @@
 #include "storagesqlitestatementfactory.h"
 #include "symbolindexer.h"
 #include "symbolscollector.h"
+#include "symbolscollectormanager.h"
+#include "symbolindexertaskqueue.h"
+#include "symbolindexertaskscheduler.h"
 #include "symbolstorage.h"
 
 #include <refactoringdatabaseinitializer.h>
@@ -41,6 +44,8 @@
 
 #include <QFileSystemWatcher>
 
+#include <thread>
+
 namespace ClangBackEnd {
 
 class SymbolIndexing final : public SymbolIndexingInterface
@@ -52,26 +57,44 @@ public:
     SymbolIndexing(Sqlite::Database &database,
                    FilePathCachingInterface &filePathCache)
         : m_filePathCache(filePathCache),
-          m_statementFactory(database)
+          m_statementFactory(database),
+          m_collectorManger(database),
+          m_indexerScheduler(m_collectorManger, m_symbolStorage, database, m_indexerQueue, std::thread::hardware_concurrency())
     {
     }
 
+    ~SymbolIndexing()
+    {
+        syncTasks();
+    }
+
     SymbolIndexer &indexer()
     {
         return m_indexer;
     }
 
+    void syncTasks()
+    {
+        m_indexerScheduler.disable();
+        while (!m_indexerScheduler.futures().empty()) {
+            m_indexerScheduler.syncTasks();
+            m_indexerScheduler.freeSlots();
+        }
+    }
+
     void updateProjectParts(V2::ProjectPartContainers &&projectParts,
                             const V2::FileContainers &generatedFiles) override;
 
 private:
     FilePathCachingInterface &m_filePathCache;
-    SymbolsCollector m_collector{m_filePathCache};
     StatementFactory m_statementFactory;
     Storage m_symbolStorage{m_statementFactory};
     ClangPathWatcher<QFileSystemWatcher, QTimer> m_sourceWatcher{m_filePathCache};
     FileStatusCache m_fileStatusCache{m_filePathCache};
-    SymbolIndexer m_indexer{m_collector,
+    SymbolsCollectorManager<SymbolsCollector> m_collectorManger;
+    SymbolIndexerTaskScheduler m_indexerScheduler;
+    SymbolIndexerTaskQueue m_indexerQueue{m_indexerScheduler};
+    SymbolIndexer m_indexer{m_indexerQueue,
                             m_symbolStorage,
                             m_sourceWatcher,
                             m_filePathCache,
diff --git a/src/tools/clangrefactoringbackend/source/symbolscollector.cpp b/src/tools/clangrefactoringbackend/source/symbolscollector.cpp
index 490c0745fa2..57815da0acb 100644
--- a/src/tools/clangrefactoringbackend/source/symbolscollector.cpp
+++ b/src/tools/clangrefactoringbackend/source/symbolscollector.cpp
@@ -29,11 +29,11 @@
 
 namespace ClangBackEnd {
 
-SymbolsCollector::SymbolsCollector(FilePathCachingInterface &filePathCache)
-    : m_indexDataConsumer(std::make_shared<IndexDataConsumer>(m_symbolEntries, m_sourceLocationEntries, filePathCache, m_sourcesManager)),
+SymbolsCollector::SymbolsCollector(Sqlite::Database &database)
+    : m_filePathCache(database),
+      m_indexDataConsumer(std::make_shared<IndexDataConsumer>(m_symbolEntries, m_sourceLocationEntries, m_filePathCache, m_sourcesManager)),
       m_collectSymbolsAction(m_indexDataConsumer),
-      m_collectMacrosSourceFileCallbacks(m_symbolEntries, m_sourceLocationEntries, filePathCache, m_sourcesManager),
-      m_filePathCache(filePathCache)
+      m_collectMacrosSourceFileCallbacks(m_symbolEntries, m_sourceLocationEntries, m_filePathCache, m_sourcesManager)
 {
 }
 
@@ -44,6 +44,11 @@ void SymbolsCollector::addFiles(const FilePathIds &filePathIds,
     m_collectMacrosSourceFileCallbacks.addSourceFiles(filePathIds);
 }
 
+void SymbolsCollector::addFile(FilePathId filePathId, const Utils::SmallStringVector &arguments)
+{
+    addFiles({filePathId}, arguments);
+}
+
 void SymbolsCollector::addUnsavedFiles(const V2::FileContainers &unsavedFiles)
 {
     m_clangTool.addUnsavedFiles(unsavedFiles);
diff --git a/src/tools/clangrefactoringbackend/source/symbolscollector.h b/src/tools/clangrefactoringbackend/source/symbolscollector.h
index d14fef53fe4..1b6c4948959 100644
--- a/src/tools/clangrefactoringbackend/source/symbolscollector.h
+++ b/src/tools/clangrefactoringbackend/source/symbolscollector.h
@@ -31,17 +31,22 @@
 #include "sourcesmanager.h"
 #include "symbolscollectorinterface.h"
 
-#include <filepathcachingfwd.h>
+#include <filepathcaching.h>
+
+namespace Sqlite {
+class Database;
+}
 
 namespace ClangBackEnd {
 
 class SymbolsCollector final : public SymbolsCollectorInterface
 {
 public:
-    SymbolsCollector(FilePathCachingInterface &filePathCache);
+    SymbolsCollector(Sqlite::Database &database);
 
     void addFiles(const FilePathIds &filePathIds,
-                  const Utils::SmallStringVector &arguments) override;
+                  const Utils::SmallStringVector &arguments);
+    void addFile(FilePathId filePathId, const Utils::SmallStringVector &arguments) override;
 
     void addUnsavedFiles(const V2::FileContainers &unsavedFiles) override;
 
@@ -60,6 +65,7 @@ public:
     void setIsUsed(bool isUsed) override;
 
 private:
+    FilePathCaching m_filePathCache;
     ClangTool m_clangTool;
     SymbolEntries m_symbolEntries;
     SourceLocationEntries m_sourceLocationEntries;
@@ -67,7 +73,6 @@ private:
     CollectSymbolsAction m_collectSymbolsAction;
     CollectMacrosSourceFileCallbacks m_collectMacrosSourceFileCallbacks;
     SourcesManager m_sourcesManager;
-    FilePathCachingInterface &m_filePathCache;
     bool m_isUsed = false;
 };
 
diff --git a/src/tools/clangrefactoringbackend/source/symbolscollectorinterface.h b/src/tools/clangrefactoringbackend/source/symbolscollectorinterface.h
index 715e26be1cc..2bf344d4b11 100644
--- a/src/tools/clangrefactoringbackend/source/symbolscollectorinterface.h
+++ b/src/tools/clangrefactoringbackend/source/symbolscollectorinterface.h
@@ -44,11 +44,11 @@ class SymbolsCollectorInterface
 {
 public:
     SymbolsCollectorInterface() = default;
+    virtual ~SymbolsCollectorInterface() = default;
     SymbolsCollectorInterface(const SymbolsCollectorInterface &) = delete;
     SymbolsCollectorInterface &operator=(const SymbolsCollectorInterface &) = delete;
 
-    virtual void addFiles(const FilePathIds &filePathIds,
-                          const Utils::SmallStringVector &arguments) = 0;
+    virtual void addFile(FilePathId filePathId, const Utils::SmallStringVector &arguments) = 0;
 
     virtual void addUnsavedFiles(const V2::FileContainers &unsavedFiles) = 0;
 
@@ -65,9 +65,6 @@ public:
 
     virtual bool isUsed() const = 0;
     virtual void setIsUsed(bool isUsed) = 0;
-
-protected:
-    ~SymbolsCollectorInterface() = default;
 };
 
 } // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/symbolscollectormanager.cpp b/src/tools/clangrefactoringbackend/source/symbolscollectormanager.cpp
new file mode 100644
index 00000000000..c6cc5e283eb
--- /dev/null
+++ b/src/tools/clangrefactoringbackend/source/symbolscollectormanager.cpp
@@ -0,0 +1,35 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "symbolscollectormanager.h"
+
+#include "symbolscollector.h"
+
+namespace ClangBackEnd {
+
+
+
+
+} // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/symbolscollectormanager.h b/src/tools/clangrefactoringbackend/source/symbolscollectormanager.h
new file mode 100644
index 00000000000..5443911640d
--- /dev/null
+++ b/src/tools/clangrefactoringbackend/source/symbolscollectormanager.h
@@ -0,0 +1,83 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include "symbolscollectormanagerinterface.h"
+#include "symbolscollectorinterface.h"
+
+#include <memory>
+
+namespace Sqlite {
+class Database;
+}
+
+namespace ClangBackEnd {
+
+class SymbolsCollector;
+template<typename SymbolsCollector>
+class SymbolsCollectorManager final : public SymbolsCollectorManagerInterface
+{
+public:
+    SymbolsCollectorManager(Sqlite::Database &database)
+        : m_database(database)
+    {}
+
+    SymbolsCollector &unusedSymbolsCollector() override
+    {
+        auto split = std::partition(m_collectors.begin(),
+                                    m_collectors.end(),
+                                    [] (const auto &collector) {
+            return collector->isUsed();
+        });
+
+        auto freeCollectors = std::distance(split, m_collectors.end());
+
+        if (freeCollectors > 0)
+            return usedCollector(*split->get());
+
+        m_collectors.emplace_back(std::make_unique<SymbolsCollector>(m_database));
+
+        return  usedCollector(*m_collectors.back().get());
+    }
+
+    const std::vector<std::unique_ptr<SymbolsCollector>> &collectors() const
+    {
+        return m_collectors;
+    }
+
+private:
+    SymbolsCollector &usedCollector(SymbolsCollector &collector)
+    {
+        collector.setIsUsed(true);
+        return collector;
+    }
+
+private:
+    std::vector<std::unique_ptr<SymbolsCollector>> m_collectors;
+    Sqlite::Database &m_database;
+};
+
+} // namespace ClangBackEnd
diff --git a/src/tools/clangrefactoringbackend/source/symbolscollectormanagerinterface.h b/src/tools/clangrefactoringbackend/source/symbolscollectormanagerinterface.h
index 82a169ff0c3..12a96990701 100644
--- a/src/tools/clangrefactoringbackend/source/symbolscollectormanagerinterface.h
+++ b/src/tools/clangrefactoringbackend/source/symbolscollectormanagerinterface.h
@@ -34,6 +34,10 @@ class SymbolsCollectorInterface;
 class SymbolsCollectorManagerInterface
 {
 public:
+    SymbolsCollectorManagerInterface() = default;
+    SymbolsCollectorManagerInterface(const SymbolsCollectorManagerInterface &) = delete;
+    SymbolsCollectorManagerInterface &operator=(const SymbolsCollectorManagerInterface &) = delete;
+
     virtual SymbolsCollectorInterface &unusedSymbolsCollector() = 0;
 
 protected:
diff --git a/src/tools/clangrefactoringbackend/source/symbolstorage.h b/src/tools/clangrefactoringbackend/source/symbolstorage.h
index 9a474d4e85a..abea7854ae5 100644
--- a/src/tools/clangrefactoringbackend/source/symbolstorage.h
+++ b/src/tools/clangrefactoringbackend/source/symbolstorage.h
@@ -64,7 +64,7 @@ public:
         deleteNewLocationsTable();
     }
 
-    void insertOrUpdateProjectPart(Utils::SmallStringView projectPartName,
+    int insertOrUpdateProjectPart(Utils::SmallStringView projectPartName,
                                    const Utils::SmallStringVector &commandLineArguments,
                                    const CompilerMacros &compilerMacros,
                                    const Utils::SmallStringVector &includeSearchPaths) override
@@ -88,6 +88,8 @@ public:
                                   includeSearchPathsAsJason,
                                   projectPartName);
         }
+
+        return int(m_statementFactory.database.lastInsertedRowId());
     }
 
     Utils::optional<ProjectPartArtefact> fetchProjectPartArtefact(FilePathId sourceId) const override
diff --git a/src/tools/clangrefactoringbackend/source/symbolstorageinterface.h b/src/tools/clangrefactoringbackend/source/symbolstorageinterface.h
index 70f5901447d..ffd85103c20 100644
--- a/src/tools/clangrefactoringbackend/source/symbolstorageinterface.h
+++ b/src/tools/clangrefactoringbackend/source/symbolstorageinterface.h
@@ -49,10 +49,10 @@ public:
 
     virtual void addSymbolsAndSourceLocations(const SymbolEntries &symbolEntries,
                                               const SourceLocationEntries &sourceLocations) = 0;
-    virtual void insertOrUpdateProjectPart(Utils::SmallStringView projectPartName,
-                                           const Utils::SmallStringVector &commandLineArguments,
-                                           const CompilerMacros &compilerMacros,
-                                           const Utils::SmallStringVector &includeSearchPaths) = 0;
+    virtual int insertOrUpdateProjectPart(Utils::SmallStringView projectPartName,
+                                          const Utils::SmallStringVector &commandLineArguments,
+                                          const CompilerMacros &compilerMacros,
+                                          const Utils::SmallStringVector &includeSearchPaths) = 0;
     virtual void updateProjectPartSources(Utils::SmallStringView projectPartName,
                                           const FilePathIds &sourceFilePathIds) = 0;
     virtual void updateProjectPartSources(int projectPartId,
diff --git a/tests/unit/unittest/google-using-declarations.h b/tests/unit/unittest/google-using-declarations.h
index 321cd1b540b..e7fe88065b6 100644
--- a/tests/unit/unittest/google-using-declarations.h
+++ b/tests/unit/unittest/google-using-declarations.h
@@ -39,6 +39,7 @@ using testing::ElementsAre;
 using testing::Field;
 using testing::HasSubstr;
 using testing::InSequence;
+using testing::Invoke;
 using testing::IsEmpty;
 using testing::Matcher;
 using testing::Mock;
diff --git a/tests/unit/unittest/mocksymbolindexertaskqueue.h b/tests/unit/unittest/mocksymbolindexertaskqueue.h
index 1eca733ade4..276dcf2c480 100644
--- a/tests/unit/unittest/mocksymbolindexertaskqueue.h
+++ b/tests/unit/unittest/mocksymbolindexertaskqueue.h
@@ -32,5 +32,9 @@
 class MockSymbolIndexerTaskQueue : public ClangBackEnd::SymbolIndexerTaskQueueInterface
 {
 public:
+    MOCK_METHOD1(addOrUpdateTasks,
+                 void (std::vector< ClangBackEnd::SymbolIndexerTask> &&tasks));
+    MOCK_METHOD1(removeTasks,
+                 void (const Utils::SmallStringVector &projectPartIds));
     MOCK_METHOD0(processTasks, void());
 };
diff --git a/tests/unit/unittest/mocksymbolindexertaskscheduler.h b/tests/unit/unittest/mocksymbolindexertaskscheduler.h
new file mode 100644
index 00000000000..c3b70ca93c9
--- /dev/null
+++ b/tests/unit/unittest/mocksymbolindexertaskscheduler.h
@@ -0,0 +1,44 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#pragma once
+
+#include "googletest.h"
+
+#include <symbolindexertaskschedulerinterface.h>
+
+class MockSymbolIndexerTaskScheduler : public ClangBackEnd::SymbolIndexerTaskSchedulerInterface
+{
+public:
+    MOCK_METHOD1(addTasks,
+                 void (const std::vector<ClangBackEnd::SymbolIndexerTaskSchedulerInterface::Task> &));
+    MOCK_METHOD0(freeSlots,
+                 uint ());
+
+    void addTasks(std::vector<ClangBackEnd::SymbolIndexerTaskSchedulerInterface::Task> &&tasks) override
+    {
+        addTasks(tasks);
+    }
+};
diff --git a/tests/unit/unittest/mocksymbolscollector.h b/tests/unit/unittest/mocksymbolscollector.h
index 1eb5eb03cb6..bf82f538ae1 100644
--- a/tests/unit/unittest/mocksymbolscollector.h
+++ b/tests/unit/unittest/mocksymbolscollector.h
@@ -29,14 +29,28 @@
 
 #include <symbolscollectorinterface.h>
 
+namespace Sqlite {
+class Database;
+}
+
 class MockSymbolsCollector : public ClangBackEnd::SymbolsCollectorInterface
 {
 public:
+    MockSymbolsCollector()
+    {
+    }
+
+    MockSymbolsCollector(const Sqlite::Database &)
+    {
+        ON_CALL(*this, setIsUsed(_)).WillByDefault(Invoke(this, &MockSymbolsCollector::setIsUsed2));
+        ON_CALL(*this, isUsed()).WillByDefault(Invoke(this, &MockSymbolsCollector::isUsed2));
+    }
+
     MOCK_METHOD0(collectSymbols,
                  void());
 
-    MOCK_METHOD2(addFiles,
-                 void(const ClangBackEnd::FilePathIds &filePathIds,
+    MOCK_METHOD2(addFile,
+                 void(ClangBackEnd::FilePathId filePathId,
                       const Utils::SmallStringVector &arguments));
 
     MOCK_METHOD1(addUnsavedFiles,
@@ -60,12 +74,25 @@ public:
     MOCK_CONST_METHOD0(fileStatuses,
                        const ClangBackEnd::FileStatuses &());
 
-   MOCK_CONST_METHOD0(sourceDependencies,
-                      const ClangBackEnd::SourceDependencies &());
+    MOCK_CONST_METHOD0(sourceDependencies,
+                       const ClangBackEnd::SourceDependencies &());
 
-   MOCK_CONST_METHOD0(isUsed,
-                      bool());
+    MOCK_CONST_METHOD0(isUsed,
+                       bool());
 
-   MOCK_METHOD1(setIsUsed,
-                void(bool));
+    MOCK_METHOD1(setIsUsed,
+                 void(bool));
+
+    void setIsUsed2(bool isUsed)
+    {
+        used = isUsed;
+    }
+
+    bool isUsed2() const
+    {
+        return used;
+    }
+
+public:
+    bool used = false;
 };
diff --git a/tests/unit/unittest/mocksymbolstorage.h b/tests/unit/unittest/mocksymbolstorage.h
index 90142fe9ee1..ca5f06a1501 100644
--- a/tests/unit/unittest/mocksymbolstorage.h
+++ b/tests/unit/unittest/mocksymbolstorage.h
@@ -38,10 +38,10 @@ public:
                 void(const ClangBackEnd::SymbolEntries &symbolEentries,
                      const ClangBackEnd::SourceLocationEntries &sourceLocations));
    MOCK_METHOD4(insertOrUpdateProjectPart,
-                void(Utils::SmallStringView projectPartName,
-                     const Utils::SmallStringVector &commandLineArgument,
-                     const ClangBackEnd::CompilerMacros &compilerMacros,
-                     const Utils::SmallStringVector &includeSearchPaths));
+                int(Utils::SmallStringView projectPartName,
+                    const Utils::SmallStringVector &commandLineArgument,
+                    const ClangBackEnd::CompilerMacros &compilerMacros,
+                    const Utils::SmallStringVector &includeSearchPaths));
    MOCK_METHOD2(updateProjectPartSources,
                 void(Utils::SmallStringView projectPartName,
                      const ClangBackEnd::FilePathIds &sourceFilePathIds));
diff --git a/tests/unit/unittest/symbolindexer-test.cpp b/tests/unit/unittest/symbolindexer-test.cpp
index 08293a05877..8ce8e40c7a4 100644
--- a/tests/unit/unittest/symbolindexer-test.cpp
+++ b/tests/unit/unittest/symbolindexer-test.cpp
@@ -34,9 +34,13 @@
 #include <filestatuscache.h>
 #include <projectpartcontainerv2.h>
 #include <refactoringdatabaseinitializer.h>
+#include <symbolscollectormanager.h>
 #include <symbolindexer.h>
+#include <symbolindexertaskqueue.h>
+#include <symbolindexertaskscheduler.h>
 #include <updateprojectpartsmessage.h>
 
+#include <QCoreApplication>
 #include <QDateTime>
 
 #include <fstream>
@@ -54,6 +58,10 @@ using ClangBackEnd::V2::ProjectPartContainers;
 using ClangBackEnd::V2::FileContainers;
 using ClangBackEnd::SymbolEntries;
 using ClangBackEnd::SymbolEntry;
+using ClangBackEnd::SymbolIndexerTask;
+using ClangBackEnd::SymbolIndexerTaskQueue;
+using ClangBackEnd::SymbolIndexerTaskScheduler;
+using ClangBackEnd::SymbolsCollectorManager;
 using ClangBackEnd::SourceDependencies;
 using ClangBackEnd::SourceLocationEntries;
 using ClangBackEnd::SourceLocationEntry;
@@ -89,6 +97,21 @@ protected:
         ON_CALL(mockCollector, sourceDependencies()).WillByDefault(ReturnRef(sourceDependencies));
         ON_CALL(mockStorage, fetchProjectPartArtefact(A<FilePathId>())).WillByDefault(Return(artefact));
         ON_CALL(mockStorage, fetchLowestLastModifiedTime(A<FilePathId>())).WillByDefault(Return(QDateTime::currentSecsSinceEpoch()));
+
+        mockCollector.setIsUsed(false);
+    }
+
+    void TearDown()
+    {
+        syncTasks();
+    }
+
+    void syncTasks()
+    {
+        while (!indexerQueue.tasks().empty() || !indexerScheduler.futures().empty()) {
+            indexerScheduler.syncTasks();
+            QCoreApplication::processEvents();
+        }
     }
 
     static void SetUpTestCase()
@@ -153,23 +176,26 @@ protected:
     ClangBackEnd::ProjectPartArtefact emptyArtefact{"", "", "", 74};
     ClangBackEnd::ProjectPartPch projectPartPch{"/path/to/pch", 4};
     NiceMock<MockSqliteTransactionBackend> mockSqliteTransactionBackend;
-    NiceMock<MockSymbolsCollector> mockCollector;
     NiceMock<MockSymbolStorage> mockStorage;
     NiceMock<MockClangPathWatcher> mockPathWatcher;
     ClangBackEnd::FileStatusCache fileStatusCache{filePathCache};
-    ClangBackEnd::SymbolIndexer indexer{mockCollector,
+    SymbolsCollectorManager<NiceMock<MockSymbolsCollector>> collectorManger{data->database};
+    SymbolIndexerTaskScheduler indexerScheduler{collectorManger, mockStorage, mockSqliteTransactionBackend, indexerQueue, 1};
+    SymbolIndexerTaskQueue indexerQueue{indexerScheduler};
+    ClangBackEnd::SymbolIndexer indexer{indexerQueue,
                                         mockStorage,
                                         mockPathWatcher,
                                         filePathCache,
                                         fileStatusCache,
                                         mockSqliteTransactionBackend};
+    NiceMock<MockSymbolsCollector> &mockCollector{collectorManger.unusedSymbolsCollector()};
 };
 
 std::unique_ptr<Data> SymbolIndexer::data;
 
 TEST_F(SymbolIndexer, UpdateProjectPartsCallsAddFilesInCollector)
 {
-    EXPECT_CALL(mockCollector, addFiles(projectPart1.sourcePathIds, projectPart1.arguments));
+    EXPECT_CALL(mockCollector, addFile(main1PathId, projectPart1.arguments));
 
     indexer.updateProjectParts({projectPart1}, Utils::clone(unsaved));
 }
@@ -179,14 +205,14 @@ TEST_F(SymbolIndexer, UpdateProjectPartsCallsAddFilesWithPrecompiledHeaderInColl
     ON_CALL(mockStorage, fetchProjectPartArtefact(TypedEq<Utils::SmallStringView>(projectPart1.projectPartId))).WillByDefault(Return(emptyArtefact));
     ON_CALL(mockStorage, fetchPrecompiledHeader(Eq(artefact.projectPartId))).WillByDefault(Return(projectPartPch));
 
-    EXPECT_CALL(mockCollector, addFiles(projectPart1.sourcePathIds,
-                                        ElementsAre(Eq("-I"),
-                                                    Eq(TESTDATA_DIR),
-                                                    Eq("-Wno-pragma-once-outside-header"),
-                                                    Eq("-Xclang"),
-                                                    Eq("-include-pch"),
-                                                    Eq("-Xclang"),
-                                                    Eq("/path/to/pch"))));
+    EXPECT_CALL(mockCollector, addFile(main1PathId,
+                                       ElementsAre(Eq("-I"),
+                                                   Eq(TESTDATA_DIR),
+                                                   Eq("-Wno-pragma-once-outside-header"),
+                                                   Eq("-Xclang"),
+                                                   Eq("-include-pch"),
+                                                   Eq("-Xclang"),
+                                                   Eq("/path/to/pch"))));
 
     indexer.updateProjectParts({projectPart1}, Utils::clone(unsaved));
 }
@@ -195,10 +221,10 @@ TEST_F(SymbolIndexer, UpdateProjectPartsCallsAddFilesWithoutPrecompiledHeaderInC
 {
     ON_CALL(mockStorage, fetchProjectPartArtefact(TypedEq<Utils::SmallStringView>(projectPart1.projectPartId))).WillByDefault(Return(emptyArtefact));
 
-    EXPECT_CALL(mockCollector, addFiles(projectPart1.sourcePathIds,
-                                        ElementsAre(Eq("-I"),
-                                                    Eq(TESTDATA_DIR),
-                                                    Eq("-Wno-pragma-once-outside-header"))));
+    EXPECT_CALL(mockCollector, addFile(main1PathId,
+                                       ElementsAre(Eq("-I"),
+                                                   Eq(TESTDATA_DIR),
+                                                   Eq("-Wno-pragma-once-outside-header"))));
 
     indexer.updateProjectParts({projectPart1}, Utils::clone(unsaved));
 }
@@ -212,14 +238,14 @@ TEST_F(SymbolIndexer, UpdateProjectPartsCallsClearInCollector)
 
 TEST_F(SymbolIndexer, UpdateProjectPartsCallsAddFilesInCollectorForEveryProjectPart)
 {
-    EXPECT_CALL(mockCollector, addFiles(_, _)).Times(2);
+    EXPECT_CALL(mockCollector, addFile(_, _)).Times(2);
 
     indexer.updateProjectParts({projectPart1, projectPart2}, Utils::clone(unsaved));
 }
 
 TEST_F(SymbolIndexer, UpdateProjectPartsDoesNotCallAddFilesInCollectorForEmptyEveryProjectParts)
 {
-    EXPECT_CALL(mockCollector, addFiles(_, _))
+    EXPECT_CALL(mockCollector, addFile(_, _))
             .Times(0);
 
     indexer.updateProjectParts({}, {});
@@ -255,9 +281,7 @@ TEST_F(SymbolIndexer, UpdateProjectPartsCallsAddUnsavedFilesInCollector)
 
 TEST_F(SymbolIndexer, UpdateProjectPartsCallsAddSymbolsAndSourceLocationsInStorage)
 {
-    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin()).Times(2);
     EXPECT_CALL(mockStorage, addSymbolsAndSourceLocations(symbolEntries, sourceLocations)).Times(2);
-    EXPECT_CALL(mockSqliteTransactionBackend, commit()).Times(2);
 
     indexer.updateProjectParts({projectPart1, projectPart2}, Utils::clone(unsaved));
 }
@@ -320,14 +344,15 @@ TEST_F(SymbolIndexer, UpdateProjectPartsCallsInOrder)
 {
     InSequence s;
 
-    EXPECT_CALL(mockCollector, clear());
+    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
     EXPECT_CALL(mockStorage, fetchProjectPartArtefact(TypedEq<Utils::SmallStringView>(projectPart1.projectPartId)));
-    EXPECT_CALL(mockCollector, addFiles(_, _));
+    EXPECT_CALL(mockStorage, insertOrUpdateProjectPart(Eq(projectPart1.projectPartId), Eq(projectPart1.arguments), Eq(projectPart1.compilerMacros), Eq(projectPart1.includeSearchPaths)));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockCollector, addFile(main1PathId, projectPart1.arguments));
     EXPECT_CALL(mockCollector, addUnsavedFiles(unsaved));
     EXPECT_CALL(mockCollector, collectSymbols());
     EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
     EXPECT_CALL(mockStorage, addSymbolsAndSourceLocations(symbolEntries, sourceLocations));
-    EXPECT_CALL(mockStorage, insertOrUpdateProjectPart(Eq(projectPart1.projectPartId), Eq(projectPart1.arguments), Eq(projectPart1.compilerMacros), Eq(projectPart1.includeSearchPaths)));
     EXPECT_CALL(mockStorage, updateProjectPartSources(TypedEq<Utils::SmallStringView>(projectPart1.projectPartId), Eq(sourceFileIds)));
     EXPECT_CALL(mockStorage, insertOrUpdateUsedMacros(Eq(usedMacros)));
     EXPECT_CALL(mockStorage, insertFileStatuses(Eq(fileStatus)));
@@ -341,7 +366,7 @@ TEST_F(SymbolIndexer, CallSetNotifier)
 {
     EXPECT_CALL(mockPathWatcher, setNotifier(_));
 
-    ClangBackEnd::SymbolIndexer indexer{mockCollector, mockStorage, mockPathWatcher, filePathCache, fileStatusCache, mockSqliteTransactionBackend};
+    ClangBackEnd::SymbolIndexer indexer{indexerQueue, mockStorage, mockPathWatcher, filePathCache, fileStatusCache, mockSqliteTransactionBackend};
 }
 
 TEST_F(SymbolIndexer, PathChangedCallsFetchProjectPartArtefactInStorage)
@@ -356,9 +381,10 @@ TEST_F(SymbolIndexer, UpdateChangedPathCallsInOrder)
 {
     InSequence s;
 
-    EXPECT_CALL(mockCollector, clear());
-    EXPECT_CALL(mockStorage, fetchProjectPartArtefact(sourceFileIds[0]));
-    EXPECT_CALL(mockCollector, addFiles(ElementsAre(sourceFileIds[0]), Eq(artefact.compilerArguments)));
+    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin()).Times(0);
+    EXPECT_CALL(mockStorage, fetchProjectPartArtefact(sourceFileIds[0])).WillOnce(Return(artefact));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockCollector, addFile(Eq(sourceFileIds[0]), Eq(artefact.compilerArguments)));
     EXPECT_CALL(mockCollector, collectSymbols());
     EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
     EXPECT_CALL(mockStorage, addSymbolsAndSourceLocations(symbolEntries, sourceLocations));
@@ -368,17 +394,17 @@ TEST_F(SymbolIndexer, UpdateChangedPathCallsInOrder)
     EXPECT_CALL(mockStorage, insertOrUpdateSourceDependencies(Eq(sourceDependencies)));
     EXPECT_CALL(mockSqliteTransactionBackend, commit());
 
-    indexer.updateChangedPath(sourceFileIds[0]);
+    indexer.pathsChanged({sourceFileIds[0]});
 }
 
 TEST_F(SymbolIndexer, HandleEmptyOptionalArtifactInUpdateChangedPath)
 {
-    ON_CALL(mockStorage, fetchProjectPartArtefact(TypedEq<FilePathId>(sourceFileIds[0])))
-        .WillByDefault(Return(emptyArtefact));
+    InSequence s;
 
-    EXPECT_CALL(mockCollector, clear());
-    EXPECT_CALL(mockStorage, fetchProjectPartArtefact(sourceFileIds[0]));
-    EXPECT_CALL(mockCollector, addFiles(_, _)).Times(0);
+    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin()).Times(0);
+    EXPECT_CALL(mockStorage, fetchProjectPartArtefact(sourceFileIds[0])).WillOnce(Return(emptyArtefact));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockCollector, addFile(_, _)).Times(0);
     EXPECT_CALL(mockCollector, collectSymbols()).Times(0);
     EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin()).Times(0);
     EXPECT_CALL(mockStorage, addSymbolsAndSourceLocations(_, _)).Times(0);
@@ -388,7 +414,7 @@ TEST_F(SymbolIndexer, HandleEmptyOptionalArtifactInUpdateChangedPath)
     EXPECT_CALL(mockStorage, insertOrUpdateSourceDependencies(_)).Times(0);
     EXPECT_CALL(mockSqliteTransactionBackend, commit()).Times(0);
 
-    indexer.updateChangedPath(sourceFileIds[0]);
+    indexer.pathsChanged({sourceFileIds[0]});
 }
 
 TEST_F(SymbolIndexer, UpdateChangedPathIsUsingPrecompiledHeader)
@@ -397,26 +423,28 @@ TEST_F(SymbolIndexer, UpdateChangedPathIsUsingPrecompiledHeader)
             .WillByDefault(Return(artefact));
     ON_CALL(mockStorage, fetchPrecompiledHeader(Eq(artefact.projectPartId)))
             .WillByDefault(Return(projectPartPch));
+    std::vector<SymbolIndexerTask> symbolIndexerTask;
 
-    EXPECT_CALL(mockCollector, addFiles(projectPart1.sourcePathIds,
-                                        ElementsAre(Eq("-DFOO"),
-                                                    Eq("-Xclang"),
-                                                    Eq("-include-pch"),
-                                                    Eq("-Xclang"),
-                                                    Eq("/path/to/pch"))));
+    EXPECT_CALL(mockCollector, addFile(Eq(sourceFileIds[0]),
+                                       ElementsAre(Eq("-DFOO"),
+                                                   Eq("-Xclang"),
+                                                   Eq("-include-pch"),
+                                                   Eq("-Xclang"),
+                                                   Eq("/path/to/pch"))));
 
-    indexer.updateChangedPath(sourceFileIds[0]);
+    indexer.pathsChanged({sourceFileIds[0]});
 }
 
 TEST_F(SymbolIndexer, UpdateChangedPathIsNotUsingPrecompiledHeaderIfItNotExists)
 {
     ON_CALL(mockStorage, fetchProjectPartArtefact(TypedEq<FilePathId>(sourceFileIds[0])))
             .WillByDefault(Return(artefact));
+    std::vector<SymbolIndexerTask> symbolIndexerTask;
 
-    EXPECT_CALL(mockCollector, addFiles(projectPart1.sourcePathIds,
-                                        ElementsAre(Eq("-DFOO"))));
+    EXPECT_CALL(mockCollector, addFile(Eq(sourceFileIds[0]),
+                                       ElementsAre(Eq("-DFOO"))));
 
-    indexer.updateChangedPath(sourceFileIds[0]);
+    indexer.pathsChanged({sourceFileIds[0]});
 }
 
 
@@ -458,11 +486,13 @@ TEST_F(SymbolIndexer, IncludeSearchPathsAreDifferent)
 
 TEST_F(SymbolIndexer, DontReparseInUpdateProjectPartsIfDefinesAreTheSame)
 {
-    ON_CALL(mockStorage, fetchProjectPartArtefact(An<Utils::SmallStringView>())).WillByDefault(Return(artefact));
+    InSequence s;
 
-    EXPECT_CALL(mockCollector, clear());
-    EXPECT_CALL(mockStorage, fetchProjectPartArtefact(TypedEq<Utils::SmallStringView>(projectPart1.projectPartId)));
-    EXPECT_CALL(mockCollector, addFiles(_, _)).Times(0);
+    EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin());
+    EXPECT_CALL(mockStorage, fetchProjectPartArtefact(TypedEq<Utils::SmallStringView>(projectPart1.projectPartId))).WillRepeatedly(Return(artefact));
+    EXPECT_CALL(mockStorage, insertOrUpdateProjectPart(Eq(projectPart1.projectPartId), Eq(projectPart1.arguments), Eq(projectPart1.compilerMacros), Eq(projectPart1.includeSearchPaths)));
+    EXPECT_CALL(mockSqliteTransactionBackend, commit());
+    EXPECT_CALL(mockCollector, addFile(_, _)).Times(0);
     EXPECT_CALL(mockCollector, collectSymbols()).Times(0);
     EXPECT_CALL(mockSqliteTransactionBackend, immediateBegin()).Times(0);
     EXPECT_CALL(mockStorage, addSymbolsAndSourceLocations(_, _)).Times(0);
@@ -540,11 +570,12 @@ TEST_F(SymbolIndexer, UpToDateFilesDontPassFilteredUpdatableFilePathIds)
 TEST_F(SymbolIndexer, OutdatedFilesAreParsedInUpdateProjectParts)
 {
     indexer.pathsChanged({main1PathId});
+    indexerScheduler.syncTasks();
     ON_CALL(mockStorage, fetchProjectPartArtefact(An<Utils::SmallStringView>())).WillByDefault(Return(artefact));
     ON_CALL(mockStorage, fetchLowestLastModifiedTime(A<FilePathId>()))
             .WillByDefault(Return(0));
 
-    EXPECT_CALL(mockCollector, addFiles(ElementsAre(main1PathId), _));
+    EXPECT_CALL(mockCollector, addFile(Eq(main1PathId), _));
 
     indexer.updateProjectParts({projectPart1}, {});
 }
@@ -552,11 +583,12 @@ TEST_F(SymbolIndexer, OutdatedFilesAreParsedInUpdateProjectParts)
 TEST_F(SymbolIndexer, UpToDateFilesAreNotParsedInUpdateProjectParts)
 {
     indexer.pathsChanged({main1PathId});
+    indexerScheduler.syncTasks();
     ON_CALL(mockStorage, fetchProjectPartArtefact(An<Utils::SmallStringView>())).WillByDefault(Return(artefact));
     ON_CALL(mockStorage, fetchLowestLastModifiedTime(A<FilePathId>()))
             .WillByDefault(Return(QDateTime::currentSecsSinceEpoch()));
 
-    EXPECT_CALL(mockCollector, addFiles(_, _)).Times(0);
+    EXPECT_CALL(mockCollector, addFile(_, _)).Times(0);
 
     indexer.updateProjectParts({projectPart1}, {});
 }
diff --git a/tests/unit/unittest/symbolindexertaskqueue-test.cpp b/tests/unit/unittest/symbolindexertaskqueue-test.cpp
index 17b9f370c4b..5838fed0f92 100644
--- a/tests/unit/unittest/symbolindexertaskqueue-test.cpp
+++ b/tests/unit/unittest/symbolindexertaskqueue-test.cpp
@@ -25,6 +25,8 @@
 
 #include "googletest.h"
 
+#include "mocksymbolindexertaskscheduler.h"
+
 #include <symbolindexertaskqueue.h>
 
 namespace {
@@ -48,12 +50,13 @@ MATCHER_P2(IsTask, filePathId, projectPartId,
 class SymbolIndexerTaskQueue : public testing::Test
 {
 protected:
-    std::size_t projectPartId(const Utils::SmallString &projectPartId)
+    int projectPartId(const Utils::SmallString &projectPartId)
     {
-        return queue.projectPartNumberId(projectPartId);
+        return int(queue.projectPartNumberId(projectPartId));
     }
 protected:
-    ClangBackEnd::SymbolIndexerTaskQueue queue;
+    NiceMock<MockSymbolIndexerTaskScheduler> mockSymbolIndexerTaskScheduler;
+    ClangBackEnd::SymbolIndexerTaskQueue queue{mockSymbolIndexerTaskScheduler};
 };
 
 TEST_F(SymbolIndexerTaskQueue, AddTasks)
@@ -162,4 +165,52 @@ TEST_F(SymbolIndexerTaskQueue, GetProjectPartIds)
 
     ASSERT_THAT(ids , ElementsAre(0, 2, 3));
 }
+
+TEST_F(SymbolIndexerTaskQueue, ProcessTasksCallsFreeSlotsAndAddTasksInScheduler)
+{
+    InSequence s;
+    queue.addOrUpdateTasks({{{1, 1}, projectPartId("yi"), Callable{}},
+                            {{1, 3}, projectPartId("yi"), Callable{}},
+                            {{1, 5}, projectPartId("yi"), Callable{}}});
+
+    EXPECT_CALL(mockSymbolIndexerTaskScheduler, freeSlots()).WillRepeatedly(Return(2));
+    EXPECT_CALL(mockSymbolIndexerTaskScheduler, addTasks(SizeIs(2)));
+
+    queue.processTasks();
+}
+
+TEST_F(SymbolIndexerTaskQueue, ProcessTasksCallsFreeSlotsAndAddTasksWithNoTaskInSchedulerIfTaskAreEmpty)
+{
+    InSequence s;
+
+    EXPECT_CALL(mockSymbolIndexerTaskScheduler, freeSlots()).WillRepeatedly(Return(2));
+    EXPECT_CALL(mockSymbolIndexerTaskScheduler, addTasks(IsEmpty()));
+
+    queue.processTasks();
+}
+
+TEST_F(SymbolIndexerTaskQueue, ProcessTasksCallsFreeSlotsAndMoveAllTasksInSchedulerIfMoreSlotsAreFree)
+{
+    InSequence s;
+    queue.addOrUpdateTasks({{{1, 1}, projectPartId("yi"), Callable{}},
+                            {{1, 3}, projectPartId("yi"), Callable{}},
+                            {{1, 5}, projectPartId("yi"), Callable{}}});
+
+    EXPECT_CALL(mockSymbolIndexerTaskScheduler, freeSlots()).WillRepeatedly(Return(4));
+    EXPECT_CALL(mockSymbolIndexerTaskScheduler, addTasks(SizeIs(3)));
+
+    queue.processTasks();
+}
+
+TEST_F(SymbolIndexerTaskQueue, ProcessTasksRemovesProcessedTasks)
+{
+    queue.addOrUpdateTasks({{{1, 1}, projectPartId("yi"), Callable{}},
+                            {{1, 3}, projectPartId("yi"), Callable{}},
+                            {{1, 5}, projectPartId("yi"), Callable{}}});
+    ON_CALL(mockSymbolIndexerTaskScheduler, freeSlots()).WillByDefault(Return(2));
+
+    queue.processTasks();
+
+    ASSERT_THAT(queue.tasks(), SizeIs(1));
+}
 }
diff --git a/tests/unit/unittest/symbolindexertaskscheduler-test.cpp b/tests/unit/unittest/symbolindexertaskscheduler-test.cpp
index d4dff54a388..585102706a6 100644
--- a/tests/unit/unittest/symbolindexertaskscheduler-test.cpp
+++ b/tests/unit/unittest/symbolindexertaskscheduler-test.cpp
@@ -29,6 +29,7 @@
 #include "mocksymbolscollectormanager.h"
 #include "mocksymbolscollector.h"
 #include "mocksymbolstorage.h"
+#include "mocksqlitetransactionbackend.h"
 
 #include <symbolindexertaskscheduler.h>
 
@@ -55,18 +56,27 @@ protected:
 protected:
     MockFunction<void()> mock;
     ClangBackEnd::SymbolIndexerTaskScheduler::Task call{
-        [&] (SymbolsCollectorInterface &symbolsCollector,
-                SymbolStorageInterface &symbolStorage) {
+        [&] (SymbolsCollectorInterface &, SymbolStorageInterface &, Sqlite::TransactionInterface &) {
             mock.Call(); }};
     ClangBackEnd::SymbolIndexerTaskScheduler::Task nocall{
-        [&] (SymbolsCollectorInterface &symbolsCollector,
-                SymbolStorageInterface &symbolStorage) {}};
+        [&] (SymbolsCollectorInterface &, SymbolStorageInterface &, Sqlite::TransactionInterface &) {
+        }};
     NiceMock<MockSymbolsCollectorManager> mockSymbolsCollectorManager;
     NiceMock<MockSymbolsCollector> mockSymbolsCollector;
     MockSymbolStorage mockSymbolStorage;
     NiceMock<MockSymbolIndexerTaskQueue> mockSymbolIndexerTaskQueue;
-    ClangBackEnd::SymbolIndexerTaskScheduler scheduler{mockSymbolsCollectorManager, mockSymbolStorage, mockSymbolIndexerTaskQueue, 4};
-    ClangBackEnd::SymbolIndexerTaskScheduler deferedScheduler{mockSymbolsCollectorManager, mockSymbolStorage, mockSymbolIndexerTaskQueue, 4, std::launch::deferred};
+    MockSqliteTransactionBackend mockSqliteTransactionBackend;
+    ClangBackEnd::SymbolIndexerTaskScheduler scheduler{mockSymbolsCollectorManager,
+                mockSymbolStorage,
+                mockSqliteTransactionBackend,
+                mockSymbolIndexerTaskQueue,
+                4};
+    ClangBackEnd::SymbolIndexerTaskScheduler deferedScheduler{mockSymbolsCollectorManager,
+                mockSymbolStorage,
+                mockSqliteTransactionBackend,
+                mockSymbolIndexerTaskQueue,
+                4,
+                std::launch::deferred};
 };
 
 TEST_F(SymbolIndexerTaskScheduler, AddTasks)
@@ -123,6 +133,7 @@ TEST_F(SymbolIndexerTaskScheduler, NoFuturesAfterFreeSlots)
 
 TEST_F(SymbolIndexerTaskScheduler, FreeSlotsCallSymbolsCollectorSetIsUnused)
 {
+    InSequence s;
     scheduler.addTasks({nocall, nocall});
     scheduler.syncTasks();
 
@@ -131,6 +142,17 @@ TEST_F(SymbolIndexerTaskScheduler, FreeSlotsCallSymbolsCollectorSetIsUnused)
     scheduler.freeSlots();
 }
 
+TEST_F(SymbolIndexerTaskScheduler, FreeSlotsCallClearsSymbolsCollector)
+{
+    InSequence s;
+    scheduler.addTasks({nocall, nocall});
+    scheduler.syncTasks();
+
+    EXPECT_CALL(mockSymbolsCollector, clear()).Times(2);
+
+    scheduler.freeSlots();
+}
+
 TEST_F(SymbolIndexerTaskScheduler, AddTaskCallSymbolsCollectorManagerUnusedSymbolsCollector)
 {
     EXPECT_CALL(mockSymbolsCollectorManager, unusedSymbolsCollector()).Times(2);
@@ -140,8 +162,12 @@ TEST_F(SymbolIndexerTaskScheduler, AddTaskCallSymbolsCollectorManagerUnusedSymbo
 
 TEST_F(SymbolIndexerTaskScheduler, CallProcessTasksInQueueAfterFinishedTasks)
 {
-    EXPECT_CALL(mockSymbolIndexerTaskQueue, processTasks()).Times(2);
+    InSequence s;
 
-    scheduler.addTasks({nocall, nocall});
+    EXPECT_CALL(mock, Call());
+    EXPECT_CALL(mockSymbolIndexerTaskQueue, processTasks());
+
+    scheduler.addTasks({call});
+    scheduler.syncTasks();
 }
 }
diff --git a/tests/unit/unittest/symbolindexing-test.cpp b/tests/unit/unittest/symbolindexing-test.cpp
index 1971379fd6f..916867e4b3b 100644
--- a/tests/unit/unittest/symbolindexing-test.cpp
+++ b/tests/unit/unittest/symbolindexing-test.cpp
@@ -96,6 +96,7 @@ protected:
 TEST_F(SymbolIndexing, Locations)
 {
     indexing.indexer().updateProjectParts({projectPart1}, {});
+    indexing.syncTasks();
 
     auto locations = query.locationsAt(filePathId(TESTDATA_DIR "/symbolindexing_main1.cpp"), 1, 6);
     ASSERT_THAT(locations,
@@ -108,6 +109,7 @@ TEST_F(SymbolIndexing, Locations)
 TEST_F(SymbolIndexing, DISABLED_TemplateFunction)
 {
     indexing.indexer().updateProjectParts({projectPart1}, {});
+    indexing.syncTasks();
 
     auto locations = query.locationsAt(filePathId(TESTDATA_DIR "/symbolindexing_main1.cpp"), 21, 24);
     ASSERT_THAT(locations,
@@ -122,6 +124,8 @@ TEST_F(SymbolIndexing, PathsAreUpdated)
 
     indexing.indexer().pathsChanged({filePathId(main1Path)});
     indexing.indexer().pathsChanged({filePathId(main1Path)});
+    indexing.syncTasks();
+
     auto locations = query.locationsAt(filePathId(TESTDATA_DIR "/symbolindexing_main1.cpp"), 1, 6);
     ASSERT_THAT(locations,
                 ElementsAre(
diff --git a/tests/unit/unittest/symbolscollector-test.cpp b/tests/unit/unittest/symbolscollector-test.cpp
index 7b8bafe8f08..f20e26b7f51 100644
--- a/tests/unit/unittest/symbolscollector-test.cpp
+++ b/tests/unit/unittest/symbolscollector-test.cpp
@@ -173,7 +173,7 @@ protected:
     Sqlite::Database database{":memory:", Sqlite::JournalMode::Memory};
     ClangBackEnd::RefactoringDatabaseInitializer<Sqlite::Database> initializer{database};
     FilePathCaching filePathCache{database};
-    ClangBackEnd::SymbolsCollector collector{filePathCache};
+    ClangBackEnd::SymbolsCollector collector{database};
 };
 
 TEST_F(SymbolsCollector, CollectSymbolName)
diff --git a/tests/unit/unittest/symbolscollectormanager-test.cpp b/tests/unit/unittest/symbolscollectormanager-test.cpp
new file mode 100644
index 00000000000..18b210b80a2
--- /dev/null
+++ b/tests/unit/unittest/symbolscollectormanager-test.cpp
@@ -0,0 +1,80 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of Qt Creator.
+**
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+****************************************************************************/
+
+#include "googletest.h"
+
+#include "mocksymbolscollector.h"
+
+#include <sqlitedatabase.h>
+#include <refactoringdatabaseinitializer.h>
+#include <symbolscollectormanager.h>
+
+namespace {
+
+class SymbolsCollectorManager : public testing::Test
+{
+protected:
+    Sqlite::Database database{":memory:", Sqlite::JournalMode::Memory};
+    ClangBackEnd::RefactoringDatabaseInitializer<Sqlite::Database> initializer{database};
+    ClangBackEnd::SymbolsCollectorManager<NiceMock<MockSymbolsCollector>> manager{database};
+};
+
+TEST_F(SymbolsCollectorManager, CreateUnsedSystemCollector)
+{
+    manager.unusedSymbolsCollector();
+
+    manager.unusedSymbolsCollector();
+
+    ASSERT_THAT(manager.collectors(), SizeIs(2));
+}
+
+TEST_F(SymbolsCollectorManager, ReuseUnsedSystemCollector)
+{
+    auto &collector = manager.unusedSymbolsCollector();
+    collector.setIsUsed(false);
+
+    manager.unusedSymbolsCollector();
+
+    ASSERT_THAT(manager.collectors(), SizeIs(1));
+}
+
+TEST_F(SymbolsCollectorManager, AsGetNewUnusedSymbolsCollectorItIsSetUsed)
+{
+    auto &collector = manager.unusedSymbolsCollector();
+
+    ASSERT_TRUE(collector.isUsed());
+}
+
+TEST_F(SymbolsCollectorManager, AsGetReusedUnusedSymbolsCollectorItIsSetUsed)
+{
+    auto &collector = manager.unusedSymbolsCollector();
+    collector.setIsUsed(false);
+
+    auto &collector2 = manager.unusedSymbolsCollector();
+
+    ASSERT_TRUE(collector2.isUsed());
+}
+
+}
diff --git a/tests/unit/unittest/symbolstorage-test.cpp b/tests/unit/unittest/symbolstorage-test.cpp
index a0faff8dddb..c35d8c7144a 100644
--- a/tests/unit/unittest/symbolstorage-test.cpp
+++ b/tests/unit/unittest/symbolstorage-test.cpp
@@ -194,7 +194,7 @@ TEST_F(SymbolStorage, InsertProjectPart)
                       TypedEq<Utils::SmallStringView>("[\"foo\"]"),
                       TypedEq<Utils::SmallStringView>("{\"FOO\":\"1\"}"),
                       TypedEq<Utils::SmallStringView>("[\"/includes\"]")));
-    EXPECT_CALL(mockDatabase, lastInsertedRowId());
+    EXPECT_CALL(mockDatabase, lastInsertedRowId()).Times(2);
 
     storage.insertOrUpdateProjectPart("project",  {"foo"}, {{"FOO", "1"}}, {"/includes"});
 }
@@ -216,6 +216,7 @@ TEST_F(SymbolStorage, UpdateProjectPart)
                       TypedEq<Utils::SmallStringView>("{\"FOO\":\"1\"}"),
                       TypedEq<Utils::SmallStringView>("[\"/includes\"]"),
                       TypedEq<Utils::SmallStringView>("project")));
+    EXPECT_CALL(mockDatabase, lastInsertedRowId());
 
     storage.insertOrUpdateProjectPart("project",  {"foo"}, {{"FOO", "1"}}, {"/includes"});
 }
diff --git a/tests/unit/unittest/unittest.pro b/tests/unit/unittest/unittest.pro
index 126113d9a45..d329900d1ea 100644
--- a/tests/unit/unittest/unittest.pro
+++ b/tests/unit/unittest/unittest.pro
@@ -97,7 +97,8 @@ SOURCES += \
     generatedfiles-test.cpp \
     sourcesmanager-test.cpp \
     symbolindexertaskqueue-test.cpp \
-    symbolindexertaskscheduler-test.cpp
+    symbolindexertaskscheduler-test.cpp \
+    symbolscollectormanager-test.cpp
 
 !isEmpty(LIBCLANG_LIBS) {
 SOURCES += \
@@ -174,7 +175,7 @@ SOURCES += \
     symbolindexing-test.cpp \
     symbolscollector-test.cpp \
     symbolfinder-test.cpp \
-    testclangtool.cpp \
+    testclangtool.cpp
 }
 
 exists($$GOOGLEBENCHMARK_DIR) {
@@ -235,7 +236,9 @@ HEADERS += \
     mockprecompiledheaderstorage.h \
     mockeditormanager.h \
     mocksymbolscollectormanager.h \
-    mocksymbolindexertaskqueue.h
+    mocksymbolindexertaskqueue.h \
+    mocksymbolindexertaskscheduler.h
+
 !isEmpty(LIBCLANG_LIBS) {
 HEADERS += \
     chunksreportedmonitor.h \