Moved the classes to individual files

12 files changed, 703 insertions, 482 deletions

diff --git a/common/CMakeLists.txt b/common/CMakeLists.txt
index 79b627a..3c6a083 100644
--- a/common/CMakeLists.txt
+++ b/common/CMakeLists.txt
@@ -68,6 +68,10 @@ set(command_SRCS
     query.cpp
     changereplay.cpp
     adaptorfactoryregistry.cpp
+    synchronizer.cpp
+    entitystore.cpp
+    remoteidmap.cpp
+    sourcewriteback.cpp
     ${storage_SRCS})
 
 add_library(${PROJECT_NAME} SHARED ${command_SRCS})
diff --git a/common/entitystore.cpp b/common/entitystore.cpp
new file mode 100644
index 0000000..2c15abf
--- /dev/null
+++ b/common/entitystore.cpp
@@ -0,0 +1,48 @@
+/*
+ * Copyright (C) 2016 Christian Mollekopf <mollekopf@kolabsys.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) version 3, or any
+ * later version accepted by the membership of KDE e.V. (or its
+ * successor approved by the membership of KDE e.V.), which shall
+ * act as a proxy defined in Section 6 of version 3 of the license.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include "entitystore.h"
+
+using namespace Sink;
+
+EntityStore::EntityStore(const QByteArray &resourceType, const QByteArray &resourceInstanceIdentifier, Sink::Storage::Transaction &transaction)
+    : mResourceType(resourceType), mResourceInstanceIdentifier(resourceInstanceIdentifier),
+    mTransaction(transaction)
+{
+
+}
+
+QSharedPointer<Sink::ApplicationDomain::BufferAdaptor> EntityStore::getLatest(const Sink::Storage::NamedDatabase &db, const QByteArray &uid, DomainTypeAdaptorFactoryInterface &adaptorFactory)
+{
+    QSharedPointer<Sink::ApplicationDomain::BufferAdaptor> current;
+    db.findLatest(uid,
+        [&current, &adaptorFactory](const QByteArray &key, const QByteArray &data) -> bool {
+            Sink::EntityBuffer buffer(const_cast<const char *>(data.data()), data.size());
+            if (!buffer.isValid()) {
+                Warning() << "Read invalid buffer from disk";
+            } else {
+                Trace() << "Found value " << key;
+                current = adaptorFactory.createAdaptor(buffer.entity());
+            }
+            return false;
+        },
+        [](const Sink::Storage::Error &error) { Warning() << "Failed to read current value from storage: " << error.message; });
+    return current;
+}
+
diff --git a/common/entitystore.h b/common/entitystore.h
new file mode 100644
index 0000000..17156ec
--- /dev/null
+++ b/common/entitystore.h
@@ -0,0 +1,53 @@
+/*
+ * Copyright (C) 2016 Christian Mollekopf <mollekopf@kolabsys.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) version 3, or any
+ * later version accepted by the membership of KDE e.V. (or its
+ * successor approved by the membership of KDE e.V.), which shall
+ * act as a proxy defined in Section 6 of version 3 of the license.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include "sink_export.h"
+#include <domainadaptor.h>
+
+#include "storage.h"
+#include "adaptorfactoryregistry.h"
+
+namespace Sink {
+
+class SINK_EXPORT EntityStore
+{
+public:
+    EntityStore(const QByteArray &resourceType, const QByteArray &mResourceInstanceIdentifier, Sink::Storage::Transaction &transaction);
+
+    template<typename T>
+    T read(const QByteArray &identifier) const
+    {
+        auto typeName = ApplicationDomain::getTypeName<T>();
+        auto mainDatabase = Storage::mainDatabase(mTransaction, typeName);
+        auto bufferAdaptor = getLatest(mainDatabase, identifier, *Sink::AdaptorFactoryRegistry::instance().getFactory<T>(mResourceType));
+        Q_ASSERT(bufferAdaptor);
+        return T(mResourceInstanceIdentifier, identifier, 0, bufferAdaptor);
+    }
+
+
+    static QSharedPointer<Sink::ApplicationDomain::BufferAdaptor> getLatest(const Sink::Storage::NamedDatabase &db, const QByteArray &uid, DomainTypeAdaptorFactoryInterface &adaptorFactory);
+private:
+    QByteArray mResourceType;
+    QByteArray mResourceInstanceIdentifier;
+    Sink::Storage::Transaction &mTransaction;
+};
+
+}
diff --git a/common/genericresource.cpp b/common/genericresource.cpp
index cb2ef21..568e066 100644
--- a/common/genericresource.cpp
+++ b/common/genericresource.cpp
@@ -34,6 +34,7 @@
 #include "definitions.h"
 #include "bufferutils.h"
 #include "adaptorfactoryregistry.h"
+#include "synchronizer.h"
 
 #include <QUuid>
 #include <QDataStream>
@@ -461,347 +462,6 @@ void GenericResource::setLowerBoundRevision(qint64 revision)
     updateLowerBoundRevision();
 }
 
-
-
-
-EntityStore::EntityStore(const QByteArray &resourceType, const QByteArray &resourceInstanceIdentifier, Sink::Storage::Transaction &transaction)
-    : mResourceType(resourceType), mResourceInstanceIdentifier(resourceInstanceIdentifier),
-    mTransaction(transaction)
-{
-
-}
-
-template<typename T>
-T EntityStore::read(const QByteArray &identifier) const
-{
-    auto typeName = ApplicationDomain::getTypeName<T>();
-    auto mainDatabase = Storage::mainDatabase(mTransaction, typeName);
-    auto bufferAdaptor = getLatest(mainDatabase, identifier, *Sink::AdaptorFactoryRegistry::instance().getFactory<T>(mResourceType));
-    Q_ASSERT(bufferAdaptor);
-    return T(mResourceInstanceIdentifier, identifier, 0, bufferAdaptor);
-}
-
-QSharedPointer<Sink::ApplicationDomain::BufferAdaptor> EntityStore::getLatest(const Sink::Storage::NamedDatabase &db, const QByteArray &uid, DomainTypeAdaptorFactoryInterface &adaptorFactory)
-{
-    QSharedPointer<Sink::ApplicationDomain::BufferAdaptor> current;
-    db.findLatest(uid,
-        [&current, &adaptorFactory](const QByteArray &key, const QByteArray &data) -> bool {
-            Sink::EntityBuffer buffer(const_cast<const char *>(data.data()), data.size());
-            if (!buffer.isValid()) {
-                Warning() << "Read invalid buffer from disk";
-            } else {
-                Trace() << "Found value " << key;
-                current = adaptorFactory.createAdaptor(buffer.entity());
-            }
-            return false;
-        },
-        [](const Sink::Storage::Error &error) { Warning() << "Failed to read current value from storage: " << error.message; });
-    return current;
-}
-
-
-
-SyncStore::SyncStore(Sink::Storage::Transaction &transaction)
-    : mTransaction(transaction)
-{
-
-}
-
-void SyncStore::recordRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId)
-{
-    Index("rid.mapping." + bufferType, mTransaction).add(remoteId, localId);
-    Index("localid.mapping." + bufferType, mTransaction).add(localId, remoteId);
-}
-
-void SyncStore::removeRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId)
-{
-    Index("rid.mapping." + bufferType, mTransaction).remove(remoteId, localId);
-    Index("localid.mapping." + bufferType, mTransaction).remove(localId, remoteId);
-}
-
-void SyncStore::updateRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId)
-{
-    const auto oldRemoteId = Index("localid.mapping." + bufferType, mTransaction).lookup(localId);
-    removeRemoteId(bufferType, localId, oldRemoteId);
-    recordRemoteId(bufferType, localId, remoteId);
-}
-
-QByteArray SyncStore::resolveRemoteId(const QByteArray &bufferType, const QByteArray &remoteId)
-{
-    // Lookup local id for remote id, or insert a new pair otherwise
-    Index index("rid.mapping." + bufferType, mTransaction);
-    QByteArray sinkId = index.lookup(remoteId);
-    if (sinkId.isEmpty()) {
-        sinkId = QUuid::createUuid().toString().toUtf8();
-        index.add(remoteId, sinkId);
-        Index("localid.mapping." + bufferType, mTransaction).add(sinkId, remoteId);
-    }
-    return sinkId;
-}
-
-QByteArray SyncStore::resolveLocalId(const QByteArray &bufferType, const QByteArray &localId)
-{
-    QByteArray remoteId = Index("localid.mapping." + bufferType, mTransaction).lookup(localId);
-    if (remoteId.isEmpty()) {
-        Warning() << "Couldn't find the remote id for " << localId;
-        return QByteArray();
-    }
-    return remoteId;
-}
-
-
-
-
-
-
-
-
-Synchronizer::Synchronizer(const QByteArray &resourceType, const QByteArray &resourceInstanceIdentifier)
-    : mStorage(Sink::storageLocation(), resourceInstanceIdentifier, Sink::Storage::ReadOnly),
-    mSyncStorage(Sink::storageLocation(), resourceInstanceIdentifier + ".synchronization", Sink::Storage::ReadWrite),
-    mResourceType(resourceType),
-    mResourceInstanceIdentifier(resourceInstanceIdentifier)
-{
-    Trace() << "Starting synchronizer: " << resourceType << resourceInstanceIdentifier;
-
-}
-
-void Synchronizer::setup(const std::function<void(int commandId, const QByteArray &data)> &enqueueCommandCallback)
-{
-    mEnqueue = enqueueCommandCallback;
-}
-
-void Synchronizer::enqueueCommand(int commandId, const QByteArray &data)
-{
-    Q_ASSERT(mEnqueue);
-    mEnqueue(commandId, data);
-}
-
-EntityStore &Synchronizer::store()
-{
-    if (!mEntityStore) {
-        mEntityStore = QSharedPointer<EntityStore>::create(mResourceType, mResourceInstanceIdentifier, mTransaction);
-    }
-    return *mEntityStore;
-}
-
-SyncStore &Synchronizer::syncStore()
-{
-    if (!mSyncStore) {
-        mSyncStore = QSharedPointer<SyncStore>::create(mSyncTransaction);
-    }
-    return *mSyncStore;
-}
-
-void Synchronizer::createEntity(const QByteArray &sinkId, const QByteArray &bufferType, const Sink::ApplicationDomain::ApplicationDomainType &domainObject,
-    DomainTypeAdaptorFactoryInterface &adaptorFactory, std::function<void(const QByteArray &)> callback)
-{
-    // These changes are coming from the source
-    const auto replayToSource = false;
-    flatbuffers::FlatBufferBuilder entityFbb;
-    adaptorFactory.createBuffer(domainObject, entityFbb);
-    flatbuffers::FlatBufferBuilder fbb;
-    // This is the resource type and not the domain type
-    auto entityId = fbb.CreateString(sinkId.toStdString());
-    auto type = fbb.CreateString(bufferType.toStdString());
-    auto delta = Sink::EntityBuffer::appendAsVector(fbb, entityFbb.GetBufferPointer(), entityFbb.GetSize());
-    auto location = Sink::Commands::CreateCreateEntity(fbb, entityId, type, delta, replayToSource);
-    Sink::Commands::FinishCreateEntityBuffer(fbb, location);
-    callback(BufferUtils::extractBuffer(fbb));
-}
-
-void Synchronizer::modifyEntity(const QByteArray &sinkId, qint64 revision, const QByteArray &bufferType, const Sink::ApplicationDomain::ApplicationDomainType &domainObject,
-    DomainTypeAdaptorFactoryInterface &adaptorFactory, std::function<void(const QByteArray &)> callback)
-{
-    // These changes are coming from the source
-    const auto replayToSource = false;
-    flatbuffers::FlatBufferBuilder entityFbb;
-    adaptorFactory.createBuffer(domainObject, entityFbb);
-    flatbuffers::FlatBufferBuilder fbb;
-    auto entityId = fbb.CreateString(sinkId.toStdString());
-    // This is the resource type and not the domain type
-    auto type = fbb.CreateString(bufferType.toStdString());
-    auto delta = Sink::EntityBuffer::appendAsVector(fbb, entityFbb.GetBufferPointer(), entityFbb.GetSize());
-    // TODO removals
-    auto location = Sink::Commands::CreateModifyEntity(fbb, revision, entityId, 0, type, delta, replayToSource);
-    Sink::Commands::FinishModifyEntityBuffer(fbb, location);
-    callback(BufferUtils::extractBuffer(fbb));
-}
-
-void Synchronizer::deleteEntity(const QByteArray &sinkId, qint64 revision, const QByteArray &bufferType, std::function<void(const QByteArray &)> callback)
-{
-    // These changes are coming from the source
-    const auto replayToSource = false;
-    flatbuffers::FlatBufferBuilder fbb;
-    auto entityId = fbb.CreateString(sinkId.toStdString());
-    // This is the resource type and not the domain type
-    auto type = fbb.CreateString(bufferType.toStdString());
-    auto location = Sink::Commands::CreateDeleteEntity(fbb, revision, entityId, type, replayToSource);
-    Sink::Commands::FinishDeleteEntityBuffer(fbb, location);
-    callback(BufferUtils::extractBuffer(fbb));
-}
-
-void Synchronizer::scanForRemovals(const QByteArray &bufferType, const std::function<void(const std::function<void(const QByteArray &key)> &callback)> &entryGenerator, std::function<bool(const QByteArray &remoteId)> exists)
-{
-    entryGenerator([this, bufferType, &exists](const QByteArray &key) {
-        auto sinkId = Sink::Storage::uidFromKey(key);
-        Trace() << "Checking for removal " << key;
-        const auto remoteId = syncStore().resolveLocalId(bufferType, sinkId);
-        // If we have no remoteId, the entity hasn't been replayed to the source yet
-        if (!remoteId.isEmpty()) {
-            if (!exists(remoteId)) {
-                Trace() << "Found a removed entity: " << sinkId;
-                deleteEntity(sinkId, Sink::Storage::maxRevision(mTransaction), bufferType,
-                    [this](const QByteArray &buffer) { enqueueCommand(Sink::Commands::DeleteEntityCommand, buffer); });
-            }
-        }
-    });
-}
-
-void Synchronizer::createOrModify(const QByteArray &bufferType, const QByteArray &remoteId, const Sink::ApplicationDomain::ApplicationDomainType &entity)
-{
-    Trace() << "Create or modify" << bufferType << remoteId;
-    auto mainDatabase = Storage::mainDatabase(mTransaction, bufferType);
-    const auto sinkId = syncStore().resolveRemoteId(bufferType, remoteId);
-    const auto found = mainDatabase.contains(sinkId);
-    auto adaptorFactory = Sink::AdaptorFactoryRegistry::instance().getFactory(mResourceType, bufferType);
-    if (!found) {
-        Trace() << "Found a new entity: " << remoteId;
-        createEntity(
-            sinkId, bufferType, entity, *adaptorFactory, [this](const QByteArray &buffer) { enqueueCommand(Sink::Commands::CreateEntityCommand, buffer); });
-    } else { // modification
-        if (auto current = store().getLatest(mainDatabase, sinkId, *adaptorFactory)) {
-            bool changed = false;
-            for (const auto &property : entity.changedProperties()) {
-                if (entity.getProperty(property) != current->getProperty(property)) {
-                    Trace() << "Property changed " << sinkId << property;
-                    changed = true;
-                }
-            }
-            if (changed) {
-                Trace() << "Found a modified entity: " << remoteId;
-                modifyEntity(sinkId, Sink::Storage::maxRevision(mTransaction), bufferType, entity, *adaptorFactory,
-                    [this](const QByteArray &buffer) { enqueueCommand(Sink::Commands::ModifyEntityCommand, buffer); });
-            }
-        } else {
-            Warning() << "Failed to get current entity";
-        }
-    }
-}
-
-KAsync::Job<void> Synchronizer::synchronize()
-{
-    mTransaction = mStorage.createTransaction(Sink::Storage::ReadOnly);
-    mSyncTransaction = mSyncStorage.createTransaction(Sink::Storage::ReadWrite);
-    return synchronizeWithSource().then<void>([this]() {
-        mTransaction.abort();
-        mSyncTransaction.commit();
-        mSyncStore.clear();
-        mEntityStore.clear();
-    });
-}
-
-
-
-SourceWriteBack::SourceWriteBack(const QByteArray &resourceType, const QByteArray &resourceInstanceIdentifier)
-    : ChangeReplay(resourceInstanceIdentifier),
-    mSyncStorage(Sink::storageLocation(), resourceInstanceIdentifier + ".synchronization", Sink::Storage::ReadWrite),
-    mResourceType(resourceType),
-    mResourceInstanceIdentifier(resourceInstanceIdentifier)
-{
-
-}
-
-EntityStore &SourceWriteBack::store()
-{
-    if (!mEntityStore) {
-        mEntityStore = QSharedPointer<EntityStore>::create(mResourceType, mResourceInstanceIdentifier, mTransaction);
-    }
-    return *mEntityStore;
-}
-
-SyncStore &SourceWriteBack::syncStore()
-{
-    if (!mSyncStore) {
-        mSyncStore = QSharedPointer<SyncStore>::create(mSyncTransaction);
-    }
-    return *mSyncStore;
-}
-
-KAsync::Job<void> SourceWriteBack::replay(const QByteArray &type, const QByteArray &key, const QByteArray &value)
-{
-    mTransaction = mStorage.createTransaction(Sink::Storage::ReadOnly);
-    mSyncTransaction = mSyncStorage.createTransaction(Sink::Storage::ReadWrite);
-
-    Sink::EntityBuffer buffer(value);
-    const Sink::Entity &entity = buffer.entity();
-    const auto metadataBuffer = Sink::EntityBuffer::readBuffer<Sink::Metadata>(entity.metadata());
-    Q_ASSERT(metadataBuffer);
-    if (!metadataBuffer->replayToSource()) {
-        Trace() << "Change is coming from the source";
-        return KAsync::null<void>();
-    }
-    const qint64 revision = metadataBuffer ? metadataBuffer->revision() : -1;
-    const auto operation = metadataBuffer ? metadataBuffer->operation() : Sink::Operation_Creation;
-    const auto uid = Sink::Storage::uidFromKey(key);
-    QByteArray oldRemoteId;
-
-    if (operation != Sink::Operation_Creation) {
-        oldRemoteId = syncStore().resolveLocalId(type, uid);
-    }
-    Trace() << "Replaying " << key << type;
-
-    KAsync::Job<QByteArray> job = KAsync::null<QByteArray>();
-    if (type == ENTITY_TYPE_FOLDER) {
-        auto folder = store().read<ApplicationDomain::Folder>(uid);
-        // const Sink::ApplicationDomain::Folder folder(mResourceInstanceIdentifier, uid, revision, mAdaptorFactories.value(type)->createAdaptor(entity));
-        job = replay(folder, operation, oldRemoteId);
-    } else if (type == ENTITY_TYPE_MAIL) {
-        auto mail = store().read<ApplicationDomain::Mail>(uid);
-        // const Sink::ApplicationDomain::Mail mail(mResourceInstanceIdentifier, uid, revision, mAdaptorFactories.value(type)->createAdaptor(entity));
-        job = replay(mail, operation, oldRemoteId);
-    }
-
-    return job.then<void, QByteArray>([this, operation, type, uid](const QByteArray &remoteId) {
-        Trace() << "Replayed change with remote id: " << remoteId;
-        if (operation == Sink::Operation_Creation) {
-            if (remoteId.isEmpty()) {
-                Warning() << "Returned an empty remoteId from the creation";
-            } else {
-                syncStore().recordRemoteId(type, uid, remoteId);
-            }
-        } else if (operation == Sink::Operation_Modification) {
-            if (remoteId.isEmpty()) {
-                Warning() << "Returned an empty remoteId from the creation";
-            } else {
-               syncStore().updateRemoteId(type, uid, remoteId);
-            }
-        } else if (operation == Sink::Operation_Removal) {
-            syncStore().removeRemoteId(type, uid, remoteId);
-        } else {
-            Warning() << "Unkown operation" << operation;
-        }
-
-        mTransaction.abort();
-        mSyncTransaction.commit();
-        mSyncStore.clear();
-        mEntityStore.clear();
-    }, [](int errorCode, const QString &errorMessage) {
-        Warning() << "Failed to replay change: " << errorMessage;
-    });
-}
-
-KAsync::Job<QByteArray> SourceWriteBack::replay(const ApplicationDomain::Mail &, Sink::Operation, const QByteArray &)
-{
-    return KAsync::null<QByteArray>();
-}
-
-KAsync::Job<QByteArray> SourceWriteBack::replay(const ApplicationDomain::Folder &, Sink::Operation, const QByteArray &)
-{
-    return KAsync::null<QByteArray>();
-}
-
-
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wundefined-reinterpret-cast"
 #include "genericresource.moc"
diff --git a/common/genericresource.h b/common/genericresource.h
index 45d5d3a..4ed408d 100644
--- a/common/genericresource.h
+++ b/common/genericresource.h
@@ -82,147 +82,6 @@ private:
     int mError;
     QTimer mCommitQueueTimer;
     qint64 mClientLowerBoundRevision;
-    QHash<QByteArray, DomainTypeAdaptorFactoryInterface::Ptr> mAdaptorFactories;
 };
 
-class SINK_EXPORT SyncStore
-{
-public:
-    SyncStore(Sink::Storage::Transaction &);
-
-    /**
-     * Records a localId to remoteId mapping
-     */
-    void recordRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId);
-    void removeRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId);
-    void updateRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId);
-
-    /**
-     * Tries to find a local id for the remote id, and creates a new local id otherwise.
-     *
-     * The new local id is recorded in the local to remote id mapping.
-     */
-    QByteArray resolveRemoteId(const QByteArray &type, const QByteArray &remoteId);
-
-    /**
-     * Tries to find a remote id for a local id.
-     *
-     * This can fail if the entity hasn't been written back to the server yet.
-     */
-    QByteArray resolveLocalId(const QByteArray &bufferType, const QByteArray &localId);
-
-private:
-    Sink::Storage::Transaction &mTransaction;
-};
-
-class SINK_EXPORT EntityStore
-{
-public:
-    EntityStore(const QByteArray &resourceType, const QByteArray &mResourceInstanceIdentifier, Sink::Storage::Transaction &transaction);
-
-    template<typename T>
-    T read(const QByteArray &identifier) const;
-
-    static QSharedPointer<Sink::ApplicationDomain::BufferAdaptor> getLatest(const Sink::Storage::NamedDatabase &db, const QByteArray &uid, DomainTypeAdaptorFactoryInterface &adaptorFactory);
-private:
-    QByteArray mResourceType;
-    QByteArray mResourceInstanceIdentifier;
-    Sink::Storage::Transaction &mTransaction;
-};
-
-/**
- * Synchronize and add what we don't already have to local queue
- */
-class SINK_EXPORT Synchronizer
-{
-public:
-    Synchronizer(const QByteArray &resourceType, const QByteArray &resourceInstanceIdentifier);
-
-    void setup(const std::function<void(int commandId, const QByteArray &data)> &enqueueCommandCallback);
-    KAsync::Job<void> synchronize();
-
-protected:
-    ///Calls the callback to enqueue the command
-    void enqueueCommand(int commandId, const QByteArray &data);
-
-    static void createEntity(const QByteArray &localId, const QByteArray &bufferType, const Sink::ApplicationDomain::ApplicationDomainType &domainObject,
-        DomainTypeAdaptorFactoryInterface &adaptorFactory, std::function<void(const QByteArray &)> callback);
-    static void modifyEntity(const QByteArray &localId, qint64 revision, const QByteArray &bufferType, const Sink::ApplicationDomain::ApplicationDomainType &domainObject,
-        DomainTypeAdaptorFactoryInterface &adaptorFactory, std::function<void(const QByteArray &)> callback);
-    static void deleteEntity(const QByteArray &localId, qint64 revision, const QByteArray &bufferType, std::function<void(const QByteArray &)> callback);
-
-    /**
-    * A synchronous algorithm to remove entities that are no longer existing.
-    *
-    * A list of entities is generated by @param entryGenerator.
-    * The entiry Generator typically iterates over an index to produce all existing entries.
-    * This algorithm calls @param exists for every entity of type @param type, with its remoteId. For every entity where @param exists returns false,
-    * an entity delete command is enqueued.
-    *
-    * All functions are called synchronously, and both @param entryGenerator and @param exists need to be synchronous.
-    */
-    void scanForRemovals(const QByteArray &bufferType,
-        const std::function<void(const std::function<void(const QByteArray &key)> &callback)> &entryGenerator, std::function<bool(const QByteArray &remoteId)> exists);
-
-    /**
-     * An algorithm to create or modify the entity.
-     *
-     * Depending on whether the entity is locally available, or has changed.
-     */
-    void createOrModify(const QByteArray &bufferType, const QByteArray &remoteId, const Sink::ApplicationDomain::ApplicationDomainType &entity);
-
-    //Read only access to main storage
-    EntityStore &store();
-
-    //Read/Write access to sync storage
-    SyncStore &syncStore();
-
-    virtual KAsync::Job<void> synchronizeWithSource() = 0;
-
-private:
-    QSharedPointer<SyncStore> mSyncStore;
-    QSharedPointer<EntityStore> mEntityStore;
-    Sink::Storage mStorage;
-    Sink::Storage mSyncStorage;
-    QByteArray mResourceType;
-    QByteArray mResourceInstanceIdentifier;
-    Sink::Storage::Transaction mTransaction;
-    Sink::Storage::Transaction mSyncTransaction;
-    std::function<void(int commandId, const QByteArray &data)> mEnqueue;
-};
-
-/**
- * Replay changes to the source
- */
-class SINK_EXPORT SourceWriteBack : public ChangeReplay
-{
-public:
-    SourceWriteBack(const QByteArray &resourceType,const QByteArray &resourceInstanceIdentifier);
-
-protected:
-    ///Base implementation calls the replay$Type calls
-    virtual KAsync::Job<void> replay(const QByteArray &type, const QByteArray &key, const QByteArray &value) Q_DECL_OVERRIDE;
-
-protected:
-    ///Implement to write back changes to the server
-    virtual KAsync::Job<QByteArray> replay(const Sink::ApplicationDomain::Mail &, Sink::Operation, const QByteArray &oldRemoteId);
-    virtual KAsync::Job<QByteArray> replay(const Sink::ApplicationDomain::Folder &, Sink::Operation, const QByteArray &oldRemoteId);
-
-    //Read only access to main storage
-    EntityStore &store();
-
-    //Read/Write access to sync storage
-    SyncStore &syncStore();
-
-private:
-    Sink::Storage mSyncStorage;
-    QSharedPointer<SyncStore> mSyncStore;
-    QSharedPointer<EntityStore> mEntityStore;
-    Sink::Storage::Transaction mTransaction;
-    Sink::Storage::Transaction mSyncTransaction;
-    QByteArray mResourceType;
-    QByteArray mResourceInstanceIdentifier;
-};
-
-
 }
diff --git a/common/remoteidmap.cpp b/common/remoteidmap.cpp
new file mode 100644
index 0000000..f72369d
--- /dev/null
+++ b/common/remoteidmap.cpp
@@ -0,0 +1,75 @@
+/*
+ * Copyright (C) 2016 Christian Mollekopf <mollekopf@kolabsys.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) version 3, or any
+ * later version accepted by the membership of KDE e.V. (or its
+ * successor approved by the membership of KDE e.V.), which shall
+ * act as a proxy defined in Section 6 of version 3 of the license.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include "remoteidmap.h"
+
+#include <QUuid>
+#include "index.h"
+#include "log.h"
+
+using namespace Sink;
+
+RemoteIdMap::RemoteIdMap(Sink::Storage::Transaction &transaction)
+    : mTransaction(transaction)
+{
+
+}
+
+void RemoteIdMap::recordRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId)
+{
+    Index("rid.mapping." + bufferType, mTransaction).add(remoteId, localId);
+    Index("localid.mapping." + bufferType, mTransaction).add(localId, remoteId);
+}
+
+void RemoteIdMap::removeRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId)
+{
+    Index("rid.mapping." + bufferType, mTransaction).remove(remoteId, localId);
+    Index("localid.mapping." + bufferType, mTransaction).remove(localId, remoteId);
+}
+
+void RemoteIdMap::updateRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId)
+{
+    const auto oldRemoteId = Index("localid.mapping." + bufferType, mTransaction).lookup(localId);
+    removeRemoteId(bufferType, localId, oldRemoteId);
+    recordRemoteId(bufferType, localId, remoteId);
+}
+
+QByteArray RemoteIdMap::resolveRemoteId(const QByteArray &bufferType, const QByteArray &remoteId)
+{
+    // Lookup local id for remote id, or insert a new pair otherwise
+    Index index("rid.mapping." + bufferType, mTransaction);
+    QByteArray sinkId = index.lookup(remoteId);
+    if (sinkId.isEmpty()) {
+        sinkId = QUuid::createUuid().toString().toUtf8();
+        index.add(remoteId, sinkId);
+        Index("localid.mapping." + bufferType, mTransaction).add(sinkId, remoteId);
+    }
+    return sinkId;
+}
+
+QByteArray RemoteIdMap::resolveLocalId(const QByteArray &bufferType, const QByteArray &localId)
+{
+    QByteArray remoteId = Index("localid.mapping." + bufferType, mTransaction).lookup(localId);
+    if (remoteId.isEmpty()) {
+        Warning() << "Couldn't find the remote id for " << localId;
+        return QByteArray();
+    }
+    return remoteId;
+}
+
diff --git a/common/remoteidmap.h b/common/remoteidmap.h
new file mode 100644
index 0000000..12891dc
--- /dev/null
+++ b/common/remoteidmap.h
@@ -0,0 +1,61 @@
+/*
+ * Copyright (C) 2016 Christian Mollekopf <mollekopf@kolabsys.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) version 3, or any
+ * later version accepted by the membership of KDE e.V. (or its
+ * successor approved by the membership of KDE e.V.), which shall
+ * act as a proxy defined in Section 6 of version 3 of the license.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include "sink_export.h"
+
+#include "storage.h"
+
+namespace Sink {
+
+/**
+ * A remoteId mapping
+ */
+class SINK_EXPORT RemoteIdMap
+{
+public:
+    RemoteIdMap(Sink::Storage::Transaction &);
+
+    /**
+     * Records a localId to remoteId mapping
+     */
+    void recordRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId);
+    void removeRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId);
+    void updateRemoteId(const QByteArray &bufferType, const QByteArray &localId, const QByteArray &remoteId);
+
+    /**
+     * Tries to find a local id for the remote id, and creates a new local id otherwise.
+     *
+     * The new local id is recorded in the local to remote id mapping.
+     */
+    QByteArray resolveRemoteId(const QByteArray &type, const QByteArray &remoteId);
+
+    /**
+     * Tries to find a remote id for a local id.
+     *
+     * This can fail if the entity hasn't been written back to the server yet.
+     */
+    QByteArray resolveLocalId(const QByteArray &bufferType, const QByteArray &localId);
+
+private:
+    Sink::Storage::Transaction &mTransaction;
+};
+
+}
diff --git a/common/sourcewriteback.cpp b/common/sourcewriteback.cpp
new file mode 100644
index 0000000..1ef20d2
--- /dev/null
+++ b/common/sourcewriteback.cpp
@@ -0,0 +1,124 @@
+/*
+ * Copyright (C) 2016 Christian Mollekopf <mollekopf@kolabsys.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) version 3, or any
+ * later version accepted by the membership of KDE e.V. (or its
+ * successor approved by the membership of KDE e.V.), which shall
+ * act as a proxy defined in Section 6 of version 3 of the license.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include "sourcewriteback.h"
+
+#include "definitions.h"
+#include "log.h"
+
+#define ENTITY_TYPE_MAIL "mail"
+#define ENTITY_TYPE_FOLDER "folder"
+
+using namespace Sink;
+
+SourceWriteBack::SourceWriteBack(const QByteArray &resourceType, const QByteArray &resourceInstanceIdentifier)
+    : ChangeReplay(resourceInstanceIdentifier),
+    mSyncStorage(Sink::storageLocation(), resourceInstanceIdentifier + ".synchronization", Sink::Storage::ReadWrite),
+    mResourceType(resourceType),
+    mResourceInstanceIdentifier(resourceInstanceIdentifier)
+{
+
+}
+
+EntityStore &SourceWriteBack::store()
+{
+    if (!mEntityStore) {
+        mEntityStore = QSharedPointer<EntityStore>::create(mResourceType, mResourceInstanceIdentifier, mTransaction);
+    }
+    return *mEntityStore;
+}
+
+RemoteIdMap &SourceWriteBack::syncStore()
+{
+    if (!mSyncStore) {
+        mSyncStore = QSharedPointer<RemoteIdMap>::create(mSyncTransaction);
+    }
+    return *mSyncStore;
+}
+
+KAsync::Job<void> SourceWriteBack::replay(const QByteArray &type, const QByteArray &key, const QByteArray &value)
+{
+    mTransaction = mStorage.createTransaction(Sink::Storage::ReadOnly);
+    mSyncTransaction = mSyncStorage.createTransaction(Sink::Storage::ReadWrite);
+
+    Sink::EntityBuffer buffer(value);
+    const Sink::Entity &entity = buffer.entity();
+    const auto metadataBuffer = Sink::EntityBuffer::readBuffer<Sink::Metadata>(entity.metadata());
+    Q_ASSERT(metadataBuffer);
+    if (!metadataBuffer->replayToSource()) {
+        Trace() << "Change is coming from the source";
+        return KAsync::null<void>();
+    }
+    const qint64 revision = metadataBuffer ? metadataBuffer->revision() : -1;
+    const auto operation = metadataBuffer ? metadataBuffer->operation() : Sink::Operation_Creation;
+    const auto uid = Sink::Storage::uidFromKey(key);
+    QByteArray oldRemoteId;
+
+    if (operation != Sink::Operation_Creation) {
+        oldRemoteId = syncStore().resolveLocalId(type, uid);
+    }
+    Trace() << "Replaying " << key << type;
+
+    KAsync::Job<QByteArray> job = KAsync::null<QByteArray>();
+    if (type == ENTITY_TYPE_FOLDER) {
+        auto folder = store().read<ApplicationDomain::Folder>(uid);
+        job = replay(folder, operation, oldRemoteId);
+    } else if (type == ENTITY_TYPE_MAIL) {
+        auto mail = store().read<ApplicationDomain::Mail>(uid);
+        job = replay(mail, operation, oldRemoteId);
+    }
+
+    return job.then<void, QByteArray>([this, operation, type, uid](const QByteArray &remoteId) {
+        Trace() << "Replayed change with remote id: " << remoteId;
+        if (operation == Sink::Operation_Creation) {
+            if (remoteId.isEmpty()) {
+                Warning() << "Returned an empty remoteId from the creation";
+            } else {
+                syncStore().recordRemoteId(type, uid, remoteId);
+            }
+        } else if (operation == Sink::Operation_Modification) {
+            if (remoteId.isEmpty()) {
+                Warning() << "Returned an empty remoteId from the creation";
+            } else {
+               syncStore().updateRemoteId(type, uid, remoteId);
+            }
+        } else if (operation == Sink::Operation_Removal) {
+            syncStore().removeRemoteId(type, uid, remoteId);
+        } else {
+            Warning() << "Unkown operation" << operation;
+        }
+
+        mTransaction.abort();
+        mSyncTransaction.commit();
+        mSyncStore.clear();
+        mEntityStore.clear();
+    }, [](int errorCode, const QString &errorMessage) {
+        Warning() << "Failed to replay change: " << errorMessage;
+    });
+}
+
+KAsync::Job<QByteArray> SourceWriteBack::replay(const ApplicationDomain::Mail &, Sink::Operation, const QByteArray &)
+{
+    return KAsync::null<QByteArray>();
+}
+
+KAsync::Job<QByteArray> SourceWriteBack::replay(const ApplicationDomain::Folder &, Sink::Operation, const QByteArray &)
+{
+    return KAsync::null<QByteArray>();
+}
diff --git a/common/sourcewriteback.h b/common/sourcewriteback.h
new file mode 100644
index 0000000..8470e00
--- /dev/null
+++ b/common/sourcewriteback.h
@@ -0,0 +1,64 @@
+/*
+ * Copyright (C) 2016 Christian Mollekopf <mollekopf@kolabsys.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) version 3, or any
+ * later version accepted by the membership of KDE e.V. (or its
+ * successor approved by the membership of KDE e.V.), which shall
+ * act as a proxy defined in Section 6 of version 3 of the license.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include "sink_export.h"
+
+#include "changereplay.h"
+#include "storage.h"
+#include "entitystore.h"
+#include "remoteidmap.h"
+
+namespace Sink {
+
+/**
+ * Replay changes to the source
+ */
+class SINK_EXPORT SourceWriteBack : public ChangeReplay
+{
+public:
+    SourceWriteBack(const QByteArray &resourceType,const QByteArray &resourceInstanceIdentifier);
+
+protected:
+    ///Base implementation calls the replay$Type calls
+    virtual KAsync::Job<void> replay(const QByteArray &type, const QByteArray &key, const QByteArray &value) Q_DECL_OVERRIDE;
+
+protected:
+    ///Implement to write back changes to the server
+    virtual KAsync::Job<QByteArray> replay(const Sink::ApplicationDomain::Mail &, Sink::Operation, const QByteArray &oldRemoteId);
+    virtual KAsync::Job<QByteArray> replay(const Sink::ApplicationDomain::Folder &, Sink::Operation, const QByteArray &oldRemoteId);
+
+    //Read only access to main storage
+    EntityStore &store();
+
+    //Read/Write access to sync storage
+    RemoteIdMap &syncStore();
+
+private:
+    Sink::Storage mSyncStorage;
+    QSharedPointer<RemoteIdMap> mSyncStore;
+    QSharedPointer<EntityStore> mEntityStore;
+    Sink::Storage::Transaction mTransaction;
+    Sink::Storage::Transaction mSyncTransaction;
+    QByteArray mResourceType;
+    QByteArray mResourceInstanceIdentifier;
+};
+
+}
diff --git a/common/synchronizer.cpp b/common/synchronizer.cpp
new file mode 100644
index 0000000..fb0baaa
--- /dev/null
+++ b/common/synchronizer.cpp
@@ -0,0 +1,176 @@
+/*
+ * Copyright (C) 2016 Christian Mollekopf <mollekopf@kolabsys.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) version 3, or any
+ * later version accepted by the membership of KDE e.V. (or its
+ * successor approved by the membership of KDE e.V.), which shall
+ * act as a proxy defined in Section 6 of version 3 of the license.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#include "synchronizer.h"
+
+#include "definitions.h"
+#include "commands.h"
+#include "bufferutils.h"
+#include "entitystore.h"
+#include "remoteidmap.h"
+#include "adaptorfactoryregistry.h"
+#include "createentity_generated.h"
+#include "modifyentity_generated.h"
+#include "deleteentity_generated.h"
+
+using namespace Sink;
+
+Synchronizer::Synchronizer(const QByteArray &resourceType, const QByteArray &resourceInstanceIdentifier)
+    : mStorage(Sink::storageLocation(), resourceInstanceIdentifier, Sink::Storage::ReadOnly),
+    mSyncStorage(Sink::storageLocation(), resourceInstanceIdentifier + ".synchronization", Sink::Storage::ReadWrite),
+    mResourceType(resourceType),
+    mResourceInstanceIdentifier(resourceInstanceIdentifier)
+{
+    Trace() << "Starting synchronizer: " << resourceType << resourceInstanceIdentifier;
+}
+
+void Synchronizer::setup(const std::function<void(int commandId, const QByteArray &data)> &enqueueCommandCallback)
+{
+    mEnqueue = enqueueCommandCallback;
+}
+
+void Synchronizer::enqueueCommand(int commandId, const QByteArray &data)
+{
+    Q_ASSERT(mEnqueue);
+    mEnqueue(commandId, data);
+}
+
+EntityStore &Synchronizer::store()
+{
+    if (!mEntityStore) {
+        mEntityStore = QSharedPointer<EntityStore>::create(mResourceType, mResourceInstanceIdentifier, mTransaction);
+    }
+    return *mEntityStore;
+}
+
+RemoteIdMap &Synchronizer::syncStore()
+{
+    if (!mSyncStore) {
+        mSyncStore = QSharedPointer<RemoteIdMap>::create(mSyncTransaction);
+    }
+    return *mSyncStore;
+}
+
+void Synchronizer::createEntity(const QByteArray &sinkId, const QByteArray &bufferType, const Sink::ApplicationDomain::ApplicationDomainType &domainObject,
+    DomainTypeAdaptorFactoryInterface &adaptorFactory, std::function<void(const QByteArray &)> callback)
+{
+    // These changes are coming from the source
+    const auto replayToSource = false;
+    flatbuffers::FlatBufferBuilder entityFbb;
+    adaptorFactory.createBuffer(domainObject, entityFbb);
+    flatbuffers::FlatBufferBuilder fbb;
+    // This is the resource type and not the domain type
+    auto entityId = fbb.CreateString(sinkId.toStdString());
+    auto type = fbb.CreateString(bufferType.toStdString());
+    auto delta = Sink::EntityBuffer::appendAsVector(fbb, entityFbb.GetBufferPointer(), entityFbb.GetSize());
+    auto location = Sink::Commands::CreateCreateEntity(fbb, entityId, type, delta, replayToSource);
+    Sink::Commands::FinishCreateEntityBuffer(fbb, location);
+    callback(BufferUtils::extractBuffer(fbb));
+}
+
+void Synchronizer::modifyEntity(const QByteArray &sinkId, qint64 revision, const QByteArray &bufferType, const Sink::ApplicationDomain::ApplicationDomainType &domainObject,
+    DomainTypeAdaptorFactoryInterface &adaptorFactory, std::function<void(const QByteArray &)> callback)
+{
+    // These changes are coming from the source
+    const auto replayToSource = false;
+    flatbuffers::FlatBufferBuilder entityFbb;
+    adaptorFactory.createBuffer(domainObject, entityFbb);
+    flatbuffers::FlatBufferBuilder fbb;
+    auto entityId = fbb.CreateString(sinkId.toStdString());
+    // This is the resource type and not the domain type
+    auto type = fbb.CreateString(bufferType.toStdString());
+    auto delta = Sink::EntityBuffer::appendAsVector(fbb, entityFbb.GetBufferPointer(), entityFbb.GetSize());
+    // FIXME removals
+    auto location = Sink::Commands::CreateModifyEntity(fbb, revision, entityId, 0, type, delta, replayToSource);
+    Sink::Commands::FinishModifyEntityBuffer(fbb, location);
+    callback(BufferUtils::extractBuffer(fbb));
+}
+
+void Synchronizer::deleteEntity(const QByteArray &sinkId, qint64 revision, const QByteArray &bufferType, std::function<void(const QByteArray &)> callback)
+{
+    // These changes are coming from the source
+    const auto replayToSource = false;
+    flatbuffers::FlatBufferBuilder fbb;
+    auto entityId = fbb.CreateString(sinkId.toStdString());
+    // This is the resource type and not the domain type
+    auto type = fbb.CreateString(bufferType.toStdString());
+    auto location = Sink::Commands::CreateDeleteEntity(fbb, revision, entityId, type, replayToSource);
+    Sink::Commands::FinishDeleteEntityBuffer(fbb, location);
+    callback(BufferUtils::extractBuffer(fbb));
+}
+
+void Synchronizer::scanForRemovals(const QByteArray &bufferType, const std::function<void(const std::function<void(const QByteArray &key)> &callback)> &entryGenerator, std::function<bool(const QByteArray &remoteId)> exists)
+{
+    entryGenerator([this, bufferType, &exists](const QByteArray &key) {
+        auto sinkId = Sink::Storage::uidFromKey(key);
+        Trace() << "Checking for removal " << key;
+        const auto remoteId = syncStore().resolveLocalId(bufferType, sinkId);
+        // If we have no remoteId, the entity hasn't been replayed to the source yet
+        if (!remoteId.isEmpty()) {
+            if (!exists(remoteId)) {
+                Trace() << "Found a removed entity: " << sinkId;
+                deleteEntity(sinkId, Sink::Storage::maxRevision(mTransaction), bufferType,
+                    [this](const QByteArray &buffer) { enqueueCommand(Sink::Commands::DeleteEntityCommand, buffer); });
+            }
+        }
+    });
+}
+
+void Synchronizer::createOrModify(const QByteArray &bufferType, const QByteArray &remoteId, const Sink::ApplicationDomain::ApplicationDomainType &entity)
+{
+    Trace() << "Create or modify" << bufferType << remoteId;
+    auto mainDatabase = Storage::mainDatabase(mTransaction, bufferType);
+    const auto sinkId = syncStore().resolveRemoteId(bufferType, remoteId);
+    const auto found = mainDatabase.contains(sinkId);
+    auto adaptorFactory = Sink::AdaptorFactoryRegistry::instance().getFactory(mResourceType, bufferType);
+    if (!found) {
+        Trace() << "Found a new entity: " << remoteId;
+        createEntity(
+            sinkId, bufferType, entity, *adaptorFactory, [this](const QByteArray &buffer) { enqueueCommand(Sink::Commands::CreateEntityCommand, buffer); });
+    } else { // modification
+        if (auto current = store().getLatest(mainDatabase, sinkId, *adaptorFactory)) {
+            bool changed = false;
+            for (const auto &property : entity.changedProperties()) {
+                if (entity.getProperty(property) != current->getProperty(property)) {
+                    Trace() << "Property changed " << sinkId << property;
+                    changed = true;
+                }
+            }
+            if (changed) {
+                Trace() << "Found a modified entity: " << remoteId;
+                modifyEntity(sinkId, Sink::Storage::maxRevision(mTransaction), bufferType, entity, *adaptorFactory,
+                    [this](const QByteArray &buffer) { enqueueCommand(Sink::Commands::ModifyEntityCommand, buffer); });
+            }
+        } else {
+            Warning() << "Failed to get current entity";
+        }
+    }
+}
+
+KAsync::Job<void> Synchronizer::synchronize()
+{
+    mTransaction = mStorage.createTransaction(Sink::Storage::ReadOnly);
+    mSyncTransaction = mSyncStorage.createTransaction(Sink::Storage::ReadWrite);
+    return synchronizeWithSource().then<void>([this]() {
+        mTransaction.abort();
+        mSyncTransaction.commit();
+        mSyncStore.clear();
+        mEntityStore.clear();
+    });
+}
diff --git a/common/synchronizer.h b/common/synchronizer.h
new file mode 100644
index 0000000..61bca7d
--- /dev/null
+++ b/common/synchronizer.h
@@ -0,0 +1,95 @@
+/*
+ * Copyright (C) 2016 Christian Mollekopf <mollekopf@kolabsys.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) version 3, or any
+ * later version accepted by the membership of KDE e.V. (or its
+ * successor approved by the membership of KDE e.V.), which shall
+ * act as a proxy defined in Section 6 of version 3 of the license.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include "sink_export.h"
+#include <QObject>
+#include <Async/Async>
+#include <domainadaptor.h>
+
+#include "storage.h"
+
+namespace Sink {
+class EntityStore;
+class RemoteIdMap;
+
+/**
+ * Synchronize and add what we don't already have to local queue
+ */
+class SINK_EXPORT Synchronizer
+{
+public:
+    Synchronizer(const QByteArray &resourceType, const QByteArray &resourceInstanceIdentifier);
+
+    void setup(const std::function<void(int commandId, const QByteArray &data)> &enqueueCommandCallback);
+    KAsync::Job<void> synchronize();
+
+protected:
+    ///Calls the callback to enqueue the command
+    void enqueueCommand(int commandId, const QByteArray &data);
+
+    static void createEntity(const QByteArray &localId, const QByteArray &bufferType, const Sink::ApplicationDomain::ApplicationDomainType &domainObject,
+        DomainTypeAdaptorFactoryInterface &adaptorFactory, std::function<void(const QByteArray &)> callback);
+    static void modifyEntity(const QByteArray &localId, qint64 revision, const QByteArray &bufferType, const Sink::ApplicationDomain::ApplicationDomainType &domainObject,
+        DomainTypeAdaptorFactoryInterface &adaptorFactory, std::function<void(const QByteArray &)> callback);
+    static void deleteEntity(const QByteArray &localId, qint64 revision, const QByteArray &bufferType, std::function<void(const QByteArray &)> callback);
+
+    /**
+    * A synchronous algorithm to remove entities that are no longer existing.
+    *
+    * A list of entities is generated by @param entryGenerator.
+    * The entiry Generator typically iterates over an index to produce all existing entries.
+    * This algorithm calls @param exists for every entity of type @param type, with its remoteId. For every entity where @param exists returns false,
+    * an entity delete command is enqueued.
+    *
+    * All functions are called synchronously, and both @param entryGenerator and @param exists need to be synchronous.
+    */
+    void scanForRemovals(const QByteArray &bufferType,
+        const std::function<void(const std::function<void(const QByteArray &key)> &callback)> &entryGenerator, std::function<bool(const QByteArray &remoteId)> exists);
+
+    /**
+     * An algorithm to create or modify the entity.
+     *
+     * Depending on whether the entity is locally available, or has changed.
+     */
+    void createOrModify(const QByteArray &bufferType, const QByteArray &remoteId, const Sink::ApplicationDomain::ApplicationDomainType &entity);
+
+    //Read only access to main storage
+    EntityStore &store();
+
+    //Read/Write access to sync storage
+    RemoteIdMap &syncStore();
+
+    virtual KAsync::Job<void> synchronizeWithSource() = 0;
+
+private:
+    QSharedPointer<RemoteIdMap> mSyncStore;
+    QSharedPointer<EntityStore> mEntityStore;
+    Sink::Storage mStorage;
+    Sink::Storage mSyncStorage;
+    QByteArray mResourceType;
+    QByteArray mResourceInstanceIdentifier;
+    Sink::Storage::Transaction mTransaction;
+    Sink::Storage::Transaction mSyncTransaction;
+    std::function<void(int commandId, const QByteArray &data)> mEnqueue;
+};
+
+}
+
diff --git a/examples/dummyresource/resourcefactory.cpp b/examples/dummyresource/resourcefactory.cpp
index 609d23e..1708cc5 100644
--- a/examples/dummyresource/resourcefactory.cpp
+++ b/examples/dummyresource/resourcefactory.cpp
@@ -36,6 +36,8 @@
 #include "facadefactory.h"
 #include "indexupdater.h"
 #include "adaptorfactoryregistry.h"
+#include "synchronizer.h"
+#include "remoteidmap.h"
 #include <QDate>
 #include <QUuid>