////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2014-2016 ArangoDB GmbH, Cologne, Germany
/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
///
/// Licensed under the Apache License, Version 2.0 (the "License");
/// you may not use this file except in compliance with the License.
/// You may obtain a copy of the License at
///
///     http://www.apache.org/licenses/LICENSE-2.0
///
/// Unless required by applicable law or agreed to in writing, software
/// distributed under the License is distributed on an "AS IS" BASIS,
/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
/// See the License for the specific language governing permissions and
/// limitations under the License.
///
/// Copyright holder is ArangoDB GmbH, Cologne, Germany
///
/// @author Jan Steemann
////////////////////////////////////////////////////////////////////////////////

#include "MMFilesWalRecoverState.h"
#include "ApplicationFeatures/ApplicationServer.h"
#include "Basics/Exceptions.h"
#include "Basics/FileUtils.h"
#include "Basics/Result.h"
#include "Basics/VelocyPackHelper.h"
#include "Basics/conversions.h"
#include "Basics/files.h"
#include "Basics/memory-map.h"
#include "Basics/tri-strings.h"
#include "MMFiles/MMFilesCollection.h"
#include "MMFiles/MMFilesDatafileHelper.h"
#include "MMFiles/MMFilesEngine.h"
#include "MMFiles/MMFilesLogfileManager.h"
#include "MMFiles/MMFilesPersistentIndexFeature.h"
#include "MMFiles/MMFilesWalSlots.h"
#include "Rest/Version.h"
#include "RestServer/DatabaseFeature.h"
#include "StorageEngine/EngineSelectorFeature.h"
#include "Transaction/Helpers.h"
#include "Transaction/Hints.h"
#include "Transaction/StandaloneContext.h"
#include "Utils/OperationOptions.h"
#include "Utils/SingleCollectionTransaction.h"
#include "VocBase/LocalDocumentId.h"
#include "VocBase/LogicalCollection.h"
#include "VocBase/LogicalView.h"

#include <velocypack/Collection.h>
#include <velocypack/Parser.h>
#include <velocypack/velocypack-aliases.h>

using namespace arangodb;

namespace {

/// @brief convert a number slice into its numeric equivalent
template <typename T>
static inline T numericValue(VPackSlice const& slice, char const* attribute) {
  if (!slice.isObject()) {
    LOG_TOPIC("4cd95", ERR, arangodb::Logger::ENGINES)
        << "invalid value type when looking for attribute '" << attribute
        << "': expecting object";
    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_BAD_PARAMETER,
                                   "invalid attribute value: expecting object");
  }
  VPackSlice v = slice.get(attribute);
  if (v.isString()) {
    return static_cast<T>(std::stoull(v.copyString()));
  }
  if (v.isNumber()) {
    return v.getNumber<T>();
  }

  LOG_TOPIC("d8702", ERR, arangodb::Logger::ENGINES)
      << "invalid value for attribute '" << attribute << "'";
  THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_BAD_PARAMETER,
                                 "invalid attribute value");
}
}  // namespace

/// @brief creates the recover state
MMFilesWalRecoverState::MMFilesWalRecoverState(bool ignoreRecoveryErrors)
    : databaseFeature(nullptr),
      failedTransactions(),
      lastTick(0),
      logfilesToProcess(),
      openedCollections(),
      openedDatabases(),
      emptyLogfiles(),
      ignoreRecoveryErrors(ignoreRecoveryErrors),
      errorCount(0),
      maxRevisionId(0),
      lastDatabaseId(0),
      lastCollectionId(0) {
  databaseFeature = application_features::ApplicationServer::getFeature<DatabaseFeature>(
      "Database");
}

/// @brief destroys the recover state
MMFilesWalRecoverState::~MMFilesWalRecoverState() { releaseResources(); }

/// @brief release opened collections and databases so they can be shut down
/// etc.
void MMFilesWalRecoverState::releaseResources() {
  // release all collections
  for (auto it = openedCollections.begin(); it != openedCollections.end(); ++it) {
    std::shared_ptr<arangodb::LogicalCollection>& collection = it->second;
    collection->vocbase().releaseCollection(collection.get());
  }

  openedCollections.clear();

  // release all databases
  for (auto it = openedDatabases.begin(); it != openedDatabases.end(); ++it) {
    (*it).second->release();
  }

  openedDatabases.clear();
}

/// @brief gets a database (and inserts it into the cache if not in it)
TRI_vocbase_t* MMFilesWalRecoverState::useDatabase(TRI_voc_tick_t databaseId) {
  auto it = openedDatabases.find(databaseId);

  if (it != openedDatabases.end()) {
    return (*it).second;
  }

  TRI_vocbase_t* vocbase = databaseFeature->useDatabase(databaseId);

  if (vocbase == nullptr) {
    return nullptr;
  }

  openedDatabases.emplace(databaseId, vocbase);
  return vocbase;
}

/// @brief release a database (so it can be dropped)
TRI_vocbase_t* MMFilesWalRecoverState::releaseDatabase(TRI_voc_tick_t databaseId) {
  auto it = openedDatabases.find(databaseId);

  if (it == openedDatabases.end()) {
    return nullptr;
  }

  TRI_vocbase_t* vocbase = (*it).second;

  TRI_ASSERT(vocbase != nullptr);

  // release all collections we ourselves have opened for this database
  auto it2 = openedCollections.begin();

  while (it2 != openedCollections.end()) {
    std::shared_ptr<arangodb::LogicalCollection>& collection = it2->second;

    TRI_ASSERT(collection != nullptr);

    if (collection->vocbase().id() == databaseId) {
      // correct database, now release the collection
      TRI_ASSERT(vocbase == &(collection->vocbase()));
      vocbase->releaseCollection(collection.get());
      // get new iterator position
      it2 = openedCollections.erase(it2);
    } else {
      // collection not found, advance in the loop
      ++it2;
    }
  }

  vocbase->release();
  openedDatabases.erase(databaseId);

  return vocbase;
}

/// @brief release a collection (so it can be dropped)
std::shared_ptr<arangodb::LogicalCollection> MMFilesWalRecoverState::releaseCollection(TRI_voc_cid_t collectionId) {
  auto it = openedCollections.find(collectionId);

  if (it == openedCollections.end()) {
    return nullptr;
  }

  std::shared_ptr<arangodb::LogicalCollection>& collection = it->second;
  TRI_ASSERT(collection != nullptr);
  collection->vocbase().releaseCollection(collection.get());
  openedCollections.erase(collectionId);

  return collection;
}

/// @brief gets a collection (and inserts it into the cache if not in it)
arangodb::LogicalCollection* MMFilesWalRecoverState::useCollection(TRI_vocbase_t* vocbase,
                                                                   TRI_voc_cid_t collectionId,
                                                                   int& res) {
  auto it = openedCollections.find(collectionId);

  if (it != openedCollections.end()) {
    res = TRI_ERROR_NO_ERROR;
    return (*it).second.get();
  }

  TRI_set_errno(TRI_ERROR_NO_ERROR);
  TRI_vocbase_col_status_e status;  // ignored here
  std::shared_ptr<arangodb::LogicalCollection> collection =
      vocbase->useCollection(collectionId, status);

  if (collection == nullptr) {
    res = TRI_errno();

    if (res == TRI_ERROR_ARANGO_CORRUPTED_COLLECTION) {
      LOG_TOPIC("d765d", WARN, arangodb::Logger::ENGINES)
          << "unable to open collection " << collectionId
          << ". Please check the logs above for errors.";
    }

    return nullptr;
  }

  auto physical = static_cast<MMFilesCollection*>(collection->getPhysical());
  TRI_ASSERT(physical != nullptr);
  // disable secondary indexes for the moment
  physical->useSecondaryIndexes(false);

  openedCollections.emplace(collectionId, collection);
  res = TRI_ERROR_NO_ERROR;
  return collection.get();
}

/// @brief looks up a collection
/// the collection will be opened after this call and inserted into a local
/// cache for faster lookups
/// returns nullptr if the collection does not exist
LogicalCollection* MMFilesWalRecoverState::getCollection(TRI_voc_tick_t databaseId,
                                                         TRI_voc_cid_t collectionId) {
  TRI_vocbase_t* vocbase = useDatabase(databaseId);

  if (vocbase == nullptr) {
    LOG_TOPIC("3dbc4", TRACE, arangodb::Logger::ENGINES) << "database " << databaseId << " not found";
    return nullptr;
  }

  int res;
  arangodb::LogicalCollection* collection = useCollection(vocbase, collectionId, res);

  if (collection == nullptr) {
    LOG_TOPIC("3bb9b", TRACE, arangodb::Logger::ENGINES)
        << "collection " << collectionId << " of database " << databaseId << " not found";
    return nullptr;
  }
  return collection;
}

/// @brief executes a single operation inside a transaction
int MMFilesWalRecoverState::executeSingleOperation(
    TRI_voc_tick_t databaseId, TRI_voc_cid_t collectionId,
    MMFilesMarker const* marker, TRI_voc_fid_t fid,
    std::function<int(SingleCollectionTransaction*, MMFilesMarkerEnvelope*)> func) {
  // first find the correct database
  TRI_vocbase_t* vocbase = useDatabase(databaseId);

  if (vocbase == nullptr) {
    LOG_TOPIC("38c23", TRACE, arangodb::Logger::ENGINES) << "database " << databaseId << " not found";
    return TRI_ERROR_ARANGO_DATABASE_NOT_FOUND;
  }

  Result res;
  int tmpres;
  arangodb::LogicalCollection* collection = useCollection(vocbase, collectionId, tmpres);
  res.reset(tmpres);

  if (collection == nullptr) {
    if (res.errorNumber() == TRI_ERROR_ARANGO_CORRUPTED_COLLECTION) {
      return res.errorNumber();
    }
    return TRI_ERROR_ARANGO_DATA_SOURCE_NOT_FOUND;
  }

  auto mmfiles = static_cast<MMFilesCollection*>(collection->getPhysical());
  TRI_ASSERT(mmfiles);
  TRI_voc_tick_t maxTick = mmfiles->maxTick();
  if (marker->getTick() <= maxTick) {
    // already transferred this marker
    return TRI_ERROR_NO_ERROR;
  }

  res = TRI_ERROR_INTERNAL;

  try {
    auto ctx = transaction::StandaloneContext::Create(*vocbase);
    SingleCollectionTransaction trx(ctx, *collection, AccessMode::Type::WRITE);

    trx.addHint(transaction::Hints::Hint::SINGLE_OPERATION);
    trx.addHint(transaction::Hints::Hint::NO_BEGIN_MARKER);
    trx.addHint(transaction::Hints::Hint::NO_ABORT_MARKER);
    trx.addHint(transaction::Hints::Hint::NO_THROTTLING);
    trx.addHint(transaction::Hints::Hint::LOCK_NEVER);
    trx.addHint(transaction::Hints::Hint::RECOVERY);  // to turn off waitForSync!

    res = trx.begin();

    if (!res.ok()) {
      THROW_ARANGO_EXCEPTION(res);
    }

    MMFilesMarkerEnvelope envelope(marker, fid);

    // execute the operation
    res = func(&trx, &envelope);

    if (!res.ok()) {
      THROW_ARANGO_EXCEPTION(res);
    }

    // commit the operation
    res = trx.commit();
  } catch (arangodb::basics::Exception const& ex) {
    LOG_TOPIC("ceb40", ERR, arangodb::Logger::ENGINES)
        << "caught exception during recovery of marker type "
        << TRI_NameMarkerDatafile(marker) << ": " << ex.what();
    res = ex.code();
  } catch (std::exception const& ex) {
    LOG_TOPIC("9a453", ERR, arangodb::Logger::ENGINES)
        << "caught exception during recovery of marker type "
        << TRI_NameMarkerDatafile(marker) << ": " << ex.what();
    res = TRI_ERROR_INTERNAL;
  } catch (...) {
    LOG_TOPIC("74179", ERR, arangodb::Logger::ENGINES)
        << "caught unknown exception during recovery of marker type "
        << TRI_NameMarkerDatafile(marker);
    res = TRI_ERROR_INTERNAL;
  }

  return res.errorNumber();
}

/// @brief callback to handle one marker during recovery
/// this function only builds up state and does not change any data
bool MMFilesWalRecoverState::InitialScanMarker(MMFilesMarker const* marker, void* data,
                                               MMFilesDatafile* datafile) {
  MMFilesWalRecoverState* state = reinterpret_cast<MMFilesWalRecoverState*>(data);

  TRI_ASSERT(marker != nullptr);

  // note the marker's tick
  TRI_voc_tick_t const tick = marker->getTick();

  TRI_ASSERT(tick >= state->lastTick);

  if (tick > state->lastTick) {
    state->lastTick = tick;
  }

  MMFilesMarkerType const type = marker->getType();

  switch (type) {
    case TRI_DF_MARKER_VPACK_DOCUMENT: {
      VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                    MMFilesDatafileHelper::VPackOffset(type));
      if (payloadSlice.isObject()) {
        TRI_voc_rid_t revisionId =
            transaction::helpers::extractRevFromDocument(payloadSlice);
        if (revisionId != UINT64_MAX && revisionId > state->maxRevisionId) {
          state->maxRevisionId = revisionId;
        }
      }
      if (marker->getSize() ==
          MMFilesDatafileHelper::VPackOffset(TRI_DF_MARKER_VPACK_DOCUMENT) +
              payloadSlice.byteSize() + sizeof(LocalDocumentId::BaseType)) {
        // we do have a LocalDocumentId stored at the end of the marker
        uint8_t const* ptr = payloadSlice.begin() + payloadSlice.byteSize();
        LocalDocumentId localDocumentId{
            encoding::readNumber<LocalDocumentId::BaseType>(ptr, sizeof(LocalDocumentId::BaseType))};
        if (!state->maxLocalDocumentId.isSet() || localDocumentId > state->maxLocalDocumentId) {
          state->maxLocalDocumentId = localDocumentId;
        }
      }
      break;
    }

    case TRI_DF_MARKER_VPACK_BEGIN_TRANSACTION: {
      // insert this transaction into the list of failed transactions
      // we do this because if we don't find a commit marker for this
      // transaction,
      // we'll have it in the failed list at the end of the scan and can ignore
      // it
      TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
      TRI_voc_tid_t const tid = MMFilesDatafileHelper::TransactionId(marker);
      state->failedTransactions.emplace(tid, std::make_pair(databaseId, false));
      break;
    }

    case TRI_DF_MARKER_VPACK_COMMIT_TRANSACTION: {
      // remove this transaction from the list of failed transactions
      TRI_voc_tid_t const tid = MMFilesDatafileHelper::TransactionId(marker);
      state->failedTransactions.erase(tid);
      break;
    }

    case TRI_DF_MARKER_VPACK_ABORT_TRANSACTION: {
      // insert this transaction into the list of failed transactions
      TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
      TRI_voc_tid_t const tid = MMFilesDatafileHelper::TransactionId(marker);
      state->failedTransactions[tid] = std::make_pair(databaseId, true);
      break;
    }

    case TRI_DF_MARKER_VPACK_DROP_DATABASE: {
      // note that the database was dropped and doesn't need to be recovered
      TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
      state->totalDroppedDatabases.emplace(databaseId);
      break;
    }

    case TRI_DF_MARKER_VPACK_DROP_COLLECTION: {
      // note that the collection was dropped and doesn't need to be recovered
      TRI_voc_cid_t const collectionId = MMFilesDatafileHelper::CollectionId(marker);
      state->totalDroppedCollections.emplace(collectionId);
      break;
    }

    case TRI_DF_MARKER_VPACK_DROP_VIEW: {
      // note that the view was dropped and doesn't need to be recovered
      TRI_voc_cid_t const viewId = MMFilesDatafileHelper::ViewId(marker);
      state->totalDroppedViews.emplace(viewId);
      break;
    }

    default: {
      // do nothing
    }
  }

  return true;
}

/// @brief callback to replay one marker during recovery
/// this function modifies indexes etc.
bool MMFilesWalRecoverState::ReplayMarker(MMFilesMarker const* marker,
                                          void* data, MMFilesDatafile* datafile) {
  MMFilesWalRecoverState* state = reinterpret_cast<MMFilesWalRecoverState*>(data);
  auto visitRecoveryHelpers = arangodb::scopeGuard([marker, state]()->void { // ensure recovery helpers are called
    if (!state || (!state->canContinue() && state->errorCount) || !marker) {
        return; // ignore invalid state or unset marker
      }

      auto visitor = [marker](MMFilesRecoveryHelper const& helper)->bool {
        auto res = helper.replayMarker(*marker);

        if (!res.ok()) {
          LOG_TOPIC("4fbfe", WARN, arangodb::Logger::ENGINES)
            << "failure during recovery helper invocation: " << res.errorNumber() << " " << res.errorMessage();

          return false;
        }

        return true;
      };

      try {
        if (!MMFilesEngine::visitRecoveryHelpers(visitor)) {
          ++state->errorCount;
        }
      } catch(...) {
        ++state->errorCount;
      }
  });

#ifdef ARANGODB_ENABLE_FAILURE_TESTS
  LOG_TOPIC("4c1c9", TRACE, arangodb::Logger::ENGINES)
      << "replaying marker of type " << TRI_NameMarkerDatafile(marker);
#endif

  MMFilesMarkerType const type = marker->getType();

  try {
    switch (type) {
      case TRI_DF_MARKER_PROLOGUE: {
        // simply note the last state
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const collectionId = MMFilesDatafileHelper::CollectionId(marker);

        LOG_TOPIC("80b3d", TRACE, arangodb::Logger::ENGINES)
            << "found prologue marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId;
        state->resetCollection(databaseId, collectionId);
        return true;
      }

        // -----------------------------------------------------------------------------
        // crud operations
        // -----------------------------------------------------------------------------

      case TRI_DF_MARKER_VPACK_DOCUMENT: {
        // re-insert the document/edge into the collection
        TRI_voc_tick_t const databaseId = state->lastDatabaseId;  // from prologue
        TRI_voc_cid_t const collectionId = state->lastCollectionId;  // from prologue

        if (state->isDropped(databaseId, collectionId)) {
          return true;
        }

        TRI_voc_tid_t const tid = MMFilesDatafileHelper::TransactionId(marker);

        if (state->ignoreTransaction(tid)) {
          // transaction was aborted
          return true;
        }

        LOG_TOPIC("2978f", TRACE, arangodb::Logger::ENGINES)
            << "found document marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId << ", transactionId: " << tid;

        int res = state->executeSingleOperation(
            databaseId, collectionId, marker, datafile->fid(),
            [&](SingleCollectionTransaction* trx, MMFilesMarkerEnvelope* envelope) -> int {
              if (arangodb::MMFilesCollection::toMMFilesCollection(trx->documentCollection())
                      ->isVolatile()) {
                return TRI_ERROR_NO_ERROR;
              }

              MMFilesMarker const* marker =
                  static_cast<MMFilesMarker const*>(envelope->mem());

              std::string const collectionName = trx->documentCollection()->name();
              uint8_t const* ptr = reinterpret_cast<uint8_t const*>(marker) +
                                   MMFilesDatafileHelper::VPackOffset(type);

              OperationOptions options;
              options.silent = true;
              options.recoveryData = static_cast<void*>(envelope);
              options.isRestore = true;
              options.waitForSync = false;
              options.ignoreRevs = true;

              // try an insert first
              TRI_ASSERT(VPackSlice(ptr).isObject());
              OperationResult opRes = trx->insert(collectionName, VPackSlice(ptr), options);
              int res = opRes.errorNumber();

              if (opRes.is(TRI_ERROR_ARANGO_UNIQUE_CONSTRAINT_VIOLATED)) {
                // document/edge already exists, now make it a replace
                opRes = trx->replace(collectionName, VPackSlice(ptr), options);
                res = opRes.errorNumber();
              }

              return res;
            });

        if (res != TRI_ERROR_NO_ERROR && res != TRI_ERROR_ARANGO_CONFLICT &&
            res != TRI_ERROR_ARANGO_DATABASE_NOT_FOUND &&
            res != TRI_ERROR_ARANGO_DATA_SOURCE_NOT_FOUND) {
          LOG_TOPIC("42c11", WARN, arangodb::Logger::ENGINES)
              << "unable to insert document in collection " << collectionId
              << " of database " << databaseId << ": " << TRI_errno_string(res);
          ++state->errorCount;
          return state->canContinue();
        }
        break;
      }

      case TRI_DF_MARKER_VPACK_REMOVE: {
        // re-apply the remove operation
        TRI_voc_tick_t const databaseId = state->lastDatabaseId;  // from prologue
        TRI_voc_cid_t const collectionId = state->lastCollectionId;  // from prologue

        TRI_ASSERT(databaseId > 0);
        TRI_ASSERT(collectionId > 0);

        if (state->isDropped(databaseId, collectionId)) {
          return true;
        }

        TRI_voc_tid_t const tid = MMFilesDatafileHelper::TransactionId(marker);

        if (state->ignoreTransaction(tid)) {
          return true;
        }

        LOG_TOPIC("8bc4d", TRACE, arangodb::Logger::ENGINES)
            << "found remove marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId << ", transactionId: " << tid;

        int res = state->executeSingleOperation(
            databaseId, collectionId, marker, datafile->fid(),
            [&](SingleCollectionTransaction* trx, MMFilesMarkerEnvelope* envelope) -> int {
              if (arangodb::MMFilesCollection::toMMFilesCollection(trx->documentCollection())
                      ->isVolatile()) {
                return TRI_ERROR_NO_ERROR;
              }

              MMFilesMarker const* marker =
                  static_cast<MMFilesMarker const*>(envelope->mem());

              std::string const collectionName = trx->documentCollection()->name();
              uint8_t const* ptr = reinterpret_cast<uint8_t const*>(marker) +
                                   MMFilesDatafileHelper::VPackOffset(type);

              OperationOptions options;
              options.silent = true;
              options.recoveryData = static_cast<void*>(envelope);
              options.waitForSync = false;
              options.ignoreRevs = true;

              try {
                OperationResult opRes =
                    trx->remove(collectionName, VPackSlice(ptr), options);
                if (opRes.is(TRI_ERROR_ARANGO_DOCUMENT_NOT_FOUND)) {
                  // document to delete is not present. this error can be
                  // ignored
                  return TRI_ERROR_NO_ERROR;
                }
                return opRes.errorNumber();
              } catch (arangodb::basics::Exception const& ex) {
                if (ex.code() == TRI_ERROR_ARANGO_DOCUMENT_NOT_FOUND) {
                  // document to delete is not present. this error can be
                  // ignored
                  return TRI_ERROR_NO_ERROR;
                }
                return ex.code();
              }
              // should not get here...
              return TRI_ERROR_INTERNAL;
            });

        if (res != TRI_ERROR_NO_ERROR && res != TRI_ERROR_ARANGO_CONFLICT &&
            res != TRI_ERROR_ARANGO_DATABASE_NOT_FOUND &&
            res != TRI_ERROR_ARANGO_DATA_SOURCE_NOT_FOUND &&
            res != TRI_ERROR_ARANGO_DOCUMENT_NOT_FOUND) {
          LOG_TOPIC("5369d", WARN, arangodb::Logger::ENGINES)
              << "unable to remove document in collection " << collectionId
              << " of database " << databaseId << ": " << TRI_errno_string(res);
          ++state->errorCount;
          return state->canContinue();
        }
        break;
      }

        // -----------------------------------------------------------------------------
        // ddl
        // -----------------------------------------------------------------------------

      case TRI_DF_MARKER_VPACK_RENAME_COLLECTION: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const collectionId = MMFilesDatafileHelper::CollectionId(marker);
        VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                      MMFilesDatafileHelper::VPackOffset(type));

        if (!payloadSlice.isObject()) {
          LOG_TOPIC("df77c", WARN, arangodb::Logger::ENGINES)
              << "cannot rename collection: invalid marker";
          ++state->errorCount;
          return state->canContinue();
        }

        if (state->isDropped(databaseId)) {
          return true;
        }

        LOG_TOPIC("c199a", TRACE, arangodb::Logger::ENGINES)
            << "found collection rename marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId;

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // if the underlying database is gone, we can go on
          LOG_TOPIC("4c182", TRACE, arangodb::Logger::ENGINES) << "cannot open database " << databaseId;
          return true;
        }

        auto collection = state->releaseCollection(collectionId);
        if (!collection) {
          collection = vocbase->lookupCollection(collectionId);
        }

        if (collection == nullptr) {
          // if the underlying collection is gone, we can go on
          LOG_TOPIC("7802d", TRACE, arangodb::Logger::ENGINES)
              << "cannot open collection " << collectionId;
          return true;
        }

        VPackSlice nameSlice = payloadSlice.get("name");
        if (!nameSlice.isString()) {
          LOG_TOPIC("e5b48", WARN, arangodb::Logger::ENGINES)
              << "cannot rename collection " << collectionId << " in database "
              << databaseId << ": name attribute is no string";
          ++state->errorCount;
          return state->canContinue();
        }
        std::string name = nameSlice.copyString();

        // check if other collection exist with target name
        auto other = vocbase->lookupCollection(name);

        if (other != nullptr) {
          if (other->id() == collection->id()) {
            LOG_TOPIC("f68b3", TRACE, arangodb::Logger::ENGINES)
                << "collection " << collectionId << " in database "
                << databaseId << " already renamed; moving on";
            break;
          } else {
            auto otherCid = other->id();

            state->releaseCollection(otherCid);
            vocbase->dropCollection(otherCid, true, -1.0);
          }
        }

        auto res = vocbase->renameCollection(collection->id(), name);

        if (!res.ok()) {
          LOG_TOPIC("1806c", WARN, arangodb::Logger::ENGINES)
              << "cannot rename collection " << collectionId << " in database "
              << databaseId << " to '" << name << "': " << res.errorMessage();
          ++state->errorCount;

          return state->canContinue();
        }

        break;
      }

      case TRI_DF_MARKER_VPACK_CHANGE_COLLECTION: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const collectionId = MMFilesDatafileHelper::CollectionId(marker);
        VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                      MMFilesDatafileHelper::VPackOffset(type));

        if (!payloadSlice.isObject()) {
          LOG_TOPIC("15a24", WARN, arangodb::Logger::ENGINES)
              << "cannot change properties of collection: invalid marker";
          ++state->errorCount;
          return state->canContinue();
        }

        if (state->isDropped(databaseId)) {
          return true;
        }

        LOG_TOPIC("07e35", TRACE, arangodb::Logger::ENGINES)
            << "found collection change marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId;

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // if the underlying database is gone, we can go on
          LOG_TOPIC("416f2", TRACE, arangodb::Logger::ENGINES) << "cannot open database " << databaseId;
          return true;
        }

        LogicalCollection* collection = state->getCollection(databaseId, collectionId);

        if (collection == nullptr) {
          // if the underlying collection is gone, we can go on
          LOG_TOPIC("2dc09", TRACE, arangodb::Logger::ENGINES)
              << "cannot change properties of collection " << collectionId
              << " in database " << databaseId << ": "
              << TRI_errno_string(TRI_ERROR_ARANGO_DATA_SOURCE_NOT_FOUND);
          return true;
        }

        auto res = collection->properties(payloadSlice,
                                          false);  // always a full-update

        if (!res.ok()) {
          LOG_TOPIC("ece63", WARN, arangodb::Logger::ENGINES)
              << "cannot change properties for collection " << collectionId
              << " in database " << databaseId << ": " << res.errorMessage();
          ++state->errorCount;
          return state->canContinue();
        }

        break;
      }

      case TRI_DF_MARKER_VPACK_CHANGE_VIEW: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const viewId = MMFilesDatafileHelper::ViewId(marker);
        VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                      MMFilesDatafileHelper::VPackOffset(type));

        if (!payloadSlice.isObject()) {
          LOG_TOPIC("e8972", WARN, arangodb::Logger::ENGINES)
              << "cannot change properties of view: invalid marker";
          ++state->errorCount;
          return state->canContinue();
        }

        if (state->isDropped(databaseId)) {
          return true;
        }

        LOG_TOPIC("0306d", TRACE, arangodb::Logger::ENGINES)
            << "found view change marker. databaseId: " << databaseId
            << ", viewId: " << viewId;

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // if the underlying database is gone, we can go on
          LOG_TOPIC("ec730", TRACE, arangodb::Logger::ENGINES) << "cannot open database " << databaseId;
          return true;
        }

        std::shared_ptr<arangodb::LogicalView> view = vocbase->lookupView(viewId);

        if (view == nullptr) {
          // if the underlying collection is gone, we can go on
          LOG_TOPIC("20f79", TRACE, arangodb::Logger::ENGINES)
              << "cannot change properties of view " << viewId
              << " in database " << databaseId << ": "
              << TRI_errno_string(TRI_ERROR_ARANGO_DATA_SOURCE_NOT_FOUND);
          return true;
        }

        VPackSlice nameSlice = payloadSlice.get("name");

        if (nameSlice.isString() && !nameSlice.isEqualString(view->name())) {
          std::string name = nameSlice.copyString();
          // check if other view exists with target name
          std::shared_ptr<arangodb::LogicalView> other = vocbase->lookupView(name);

          if (other != nullptr) {
            if (other->id() == view->id()) {
              LOG_TOPIC("2709b", TRACE, arangodb::Logger::ENGINES)
                  << "view " << viewId << " in database " << databaseId
                  << " was already renamed; moving on";
              break;
            }
            other->drop();
          }

          auto res = view->rename(std::string(name));

          if (!res.ok()) {
            LOG_TOPIC("11470", WARN, arangodb::Logger::ENGINES)
                << "cannot rename view " << viewId << " in database "
                << databaseId << " to '" << name << "': " << res.errorMessage();
            ++state->errorCount;

            return state->canContinue();
          }
        }

        auto res = view->properties(payloadSlice, false);  // always a full-update

        if (!res.ok()) {
          LOG_TOPIC("f68a6", WARN, arangodb::Logger::ENGINES)
              << "cannot change properties for view " << viewId
              << " in database " << databaseId << ": " << res.errorMessage();
          ++state->errorCount;
          return state->canContinue();
        }

        break;
      }

      case TRI_DF_MARKER_VPACK_CREATE_INDEX: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const collectionId = MMFilesDatafileHelper::CollectionId(marker);
        VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                      MMFilesDatafileHelper::VPackOffset(type));

        if (!payloadSlice.isObject()) {
          LOG_TOPIC("8f0b8", WARN, arangodb::Logger::ENGINES)
              << "cannot create index for collection: invalid marker";
          ++state->errorCount;
          return state->canContinue();
        }

        TRI_idx_iid_t indexId = numericValue<TRI_idx_iid_t>(payloadSlice, "id");

        if (state->isDropped(databaseId, collectionId)) {
          return true;
        }

        LOG_TOPIC("b55bc", TRACE, arangodb::Logger::ENGINES)
            << "found create index marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId;

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // if the underlying database is gone, we can go on
          LOG_TOPIC("2e4f0", TRACE, arangodb::Logger::ENGINES)
              << "cannot create index for collection " << collectionId
              << " in database " << databaseId << ": "
              << TRI_errno_string(TRI_ERROR_ARANGO_DATABASE_NOT_FOUND);
          return true;
        }

        auto col = vocbase->lookupCollection(collectionId);

        if (col == nullptr) {
          // if the underlying collection gone, we can go on
          LOG_TOPIC("404f6", TRACE, arangodb::Logger::ENGINES)
              << "cannot create index for collection " << collectionId
              << " in database " << databaseId << ": "
              << TRI_errno_string(TRI_ERROR_ARANGO_DATA_SOURCE_NOT_FOUND);
          return true;
        }

        auto physical = static_cast<MMFilesCollection*>(col->getPhysical());
        TRI_ASSERT(physical != nullptr);
        MMFilesPersistentIndexFeature::dropIndex(databaseId, collectionId, indexId);

        std::string const indexName("index-" + std::to_string(indexId) +
                                    ".json");
        std::string const filename(
            arangodb::basics::FileUtils::buildFilename(physical->path(), indexName));

        bool const forceSync = !state->willBeDropped(databaseId, collectionId);
        bool ok = arangodb::basics::VelocyPackHelper::velocyPackToFile(filename, payloadSlice,
                                                                       forceSync);

        if (!ok) {
          LOG_TOPIC("3a1aa", WARN, arangodb::Logger::ENGINES)
              << "cannot create index " << indexId << ", collection "
              << collectionId << " in database " << databaseId;
          ++state->errorCount;

          return state->canContinue();
        } else {
          try {
            bool created;
            auto unused = physical->createIndex(payloadSlice, /*restore*/ true, created);
            TRI_ASSERT(unused != nullptr);
          } catch (basics::Exception const&) {
            LOG_TOPIC("92fdf", WARN, arangodb::Logger::ENGINES)
                << "cannot create index " << indexId << ", collection "
                << collectionId << " in database " << databaseId;
            ++state->errorCount;
            return state->canContinue();
          }
        }

        break;
      }

      case TRI_DF_MARKER_VPACK_CREATE_COLLECTION: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const collectionId = MMFilesDatafileHelper::CollectionId(marker);
        VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                      MMFilesDatafileHelper::VPackOffset(type));

        if (!payloadSlice.isObject()) {
          LOG_TOPIC("ba343", WARN, arangodb::Logger::ENGINES)
              << "cannot create collection: invalid marker";
          ++state->errorCount;
          return state->canContinue();
        }

        LOG_TOPIC("a3063", TRACE, arangodb::Logger::ENGINES)
            << "found create collection marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId;

        // remove the drop marker
        state->droppedCollections.erase(collectionId);

        if (state->isDropped(databaseId)) {
          return true;
        }

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // if the underlying database is gone, we can go on
          LOG_TOPIC("5881d", TRACE, arangodb::Logger::ENGINES) << "cannot open database " << databaseId;
          return true;
        }

        auto collection = state->releaseCollection(collectionId);
        if (!collection) {
          collection = vocbase->lookupCollection(collectionId);
        }

        if (collection != nullptr) {
          // drop an existing collection
          vocbase->dropCollection(collection->id(), true, -1.0);
        }

        MMFilesPersistentIndexFeature::dropCollection(databaseId, collectionId);

        // check if there is another collection with the same name as the one
        // that
        // we attempt to create
        VPackSlice const nameSlice = payloadSlice.get("name");
        std::string name = "";

        if (nameSlice.isString()) {
          name = nameSlice.copyString();
          collection = vocbase->lookupCollection(name);

          if (collection != nullptr) {
            auto otherCid = collection->id();

            state->releaseCollection(otherCid);
            vocbase->dropCollection(otherCid, true, -1.0);
          }
        } else {
          LOG_TOPIC("e1cb1", WARN, arangodb::Logger::ENGINES)
              << "empty name attribute in create collection marker for "
                 "collection "
              << collectionId << " and database " << databaseId;
          ++state->errorCount;
          return state->canContinue();
        }

        // fiddle "isSystem" value, which is not contained in the JSON file
        bool isSystemValue = false;
        if (!name.empty()) {
          isSystemValue = name[0] == '_';
        }

        VPackBuilder bx;
        bx.openObject();
        bx.add("isSystem", VPackValue(isSystemValue));
        bx.close();
        VPackSlice isSystem = bx.slice();
        VPackBuilder b2 = VPackCollection::merge(payloadSlice, isSystem, false);

        int res = TRI_ERROR_NO_ERROR;
        try {
          if (state->willBeDropped(collectionId)) {
            // in case we detect that this collection is going to be deleted
            // anyway,
            // set the sync properties to false temporarily
            bool oldSync = state->databaseFeature->forceSyncProperties();
            state->databaseFeature->forceSyncProperties(false);
            // restore the old behavior afterwards
            TRI_DEFER(state->databaseFeature->forceSyncProperties(oldSync));

            collection = vocbase->createCollection(b2.slice());
          } else {
            // collection will be kept
            collection = vocbase->createCollection(b2.slice());
          }
          TRI_ASSERT(collection != nullptr);
        } catch (basics::Exception const& ex) {
          res = ex.code();
        } catch (...) {
          res = TRI_ERROR_INTERNAL;
        }

        if (res != TRI_ERROR_NO_ERROR) {
          LOG_TOPIC("52711", WARN, arangodb::Logger::ENGINES)
              << "cannot create collection " << collectionId << " in database "
              << databaseId << ": " << TRI_errno_string(res);
          ++state->errorCount;
          return state->canContinue();
        }
        break;
      }

      case TRI_DF_MARKER_VPACK_CREATE_VIEW: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const viewId = MMFilesDatafileHelper::ViewId(marker);
        VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                      MMFilesDatafileHelper::VPackOffset(type));

        if (!payloadSlice.isObject()) {
          LOG_TOPIC("ba248", WARN, arangodb::Logger::ENGINES)
              << "cannot create view: invalid marker";
          ++state->errorCount;
          return state->canContinue();
        }

        LOG_TOPIC("bb257", TRACE, arangodb::Logger::ENGINES)
            << "found create view marker. databaseId: " << databaseId
            << ", viewId: " << viewId;

        // remove the drop marker
        state->droppedViews.erase(viewId);

        if (state->isDropped(databaseId)) {
          return true;
        }

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // if the underlying database is gone, we can go on
          LOG_TOPIC("b317f", TRACE, arangodb::Logger::ENGINES) << "cannot open database " << databaseId;
          return true;
        }

        std::shared_ptr<arangodb::LogicalView> view = vocbase->lookupView(viewId);

        if (view != nullptr) {
          view->drop();  // drop an existing view
        }

        // check if there is another view with the same name as the one that
        // we attempt to create
        VPackSlice const nameSlice = payloadSlice.get("name");
        std::string name = "";

        if (nameSlice.isString()) {
          name = nameSlice.copyString();
          view = vocbase->lookupView(name);

          if (view != nullptr) {
            view->drop();
          }
        } else {
          LOG_TOPIC("8d8d4", WARN, arangodb::Logger::ENGINES)
              << "empty name attribute in create view marker for "
                 "view "
              << viewId << " and database " << databaseId;
          ++state->errorCount;
          return state->canContinue();
        }

        int res = TRI_ERROR_NO_ERROR;

        try {
          if (state->willViewBeDropped(viewId)) {
            // in case we detect that this view is going to be deleted
            // anyway,
            // set the sync properties to false temporarily
            bool oldSync = state->databaseFeature->forceSyncProperties();
            state->databaseFeature->forceSyncProperties(false);
            // restore the old behavior afterwards
            TRI_DEFER(state->databaseFeature->forceSyncProperties(oldSync));
          }

          auto res = arangodb::LogicalView::create(view, *vocbase, payloadSlice);
          TRI_ASSERT(res.ok());
          TRI_ASSERT(view != nullptr);
          TRI_ASSERT(view->id() == viewId);  // otherwise this a corrupt marker
        } catch (basics::Exception const& ex) {
          res = ex.code();
        } catch (...) {
          res = TRI_ERROR_INTERNAL;
        }

        if (res != TRI_ERROR_NO_ERROR) {
          LOG_TOPIC("925e1", WARN, arangodb::Logger::ENGINES)
              << "cannot create view " << viewId << " in database "
              << databaseId << ": " << TRI_errno_string(res);
          ++state->errorCount;
          return state->canContinue();
        }
        break;
      }

      case TRI_DF_MARKER_VPACK_CREATE_DATABASE: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                      MMFilesDatafileHelper::VPackOffset(type));

        if (!payloadSlice.isObject()) {
          LOG_TOPIC("502aa", WARN, arangodb::Logger::ENGINES)
              << "cannot create database: invalid marker";
          ++state->errorCount;
          return state->canContinue();
        }

        LOG_TOPIC("55653", TRACE, arangodb::Logger::ENGINES)
            << "found create database marker. databaseId: " << databaseId;

        // remove the drop marker
        state->droppedDatabases.erase(databaseId);
        TRI_vocbase_t* vocbase = state->releaseDatabase(databaseId);

        if (vocbase != nullptr) {
          // remove already existing database
          // TODO: how to signal a dropDatabase failure here?
          state->databaseFeature->dropDatabase(databaseId, true, false);
        }

        VPackSlice const nameSlice = payloadSlice.get("name");

        if (!nameSlice.isString()) {
          LOG_TOPIC("9f541", WARN, arangodb::Logger::ENGINES)
              << "cannot unpack database properties for database " << databaseId;
          ++state->errorCount;
          return state->canContinue();
        }

        std::string nameString = nameSlice.copyString();

        MMFilesEngine* engine = static_cast<MMFilesEngine*>(EngineSelectorFeature::ENGINE);
        std::string const versionFile = engine->versionFilename(databaseId);
        std::string const versionFileContent =
            std::string("{\"version\":") +
            std::to_string(rest::Version::getNumericServerVersion()) +
            ",\"tasks\":{}}";

        // remove already existing database with same name
        vocbase = state->databaseFeature->lookupDatabase(nameString);

        if (vocbase != nullptr) {
          TRI_voc_tick_t otherId = vocbase->id();

          state->releaseDatabase(otherId);
          // TODO: how to signal a dropDatabase failure here?
          state->databaseFeature->dropDatabase(nameString, true, false);
        }

        MMFilesPersistentIndexFeature::dropDatabase(databaseId);

        vocbase = nullptr;
        int res = state->databaseFeature->createDatabase(databaseId, nameString, vocbase);

        if (res != TRI_ERROR_NO_ERROR) {
          LOG_TOPIC("9c045", WARN, arangodb::Logger::ENGINES)
              << "cannot create database " << databaseId << ": "
              << TRI_errno_string(res);
          ++state->errorCount;
          return state->canContinue();
        }

        try {
          basics::FileUtils::spit(versionFile, versionFileContent);
        } catch (...) {
          LOG_TOPIC("c30a1", WARN, arangodb::Logger::ENGINES)
              << "unable to store version file '" << versionFile
              << "' for database " << databaseId;
          ++state->errorCount;
          return state->canContinue();
        }

        break;
      }

      case TRI_DF_MARKER_VPACK_DROP_INDEX: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const collectionId = MMFilesDatafileHelper::CollectionId(marker);
        VPackSlice const payloadSlice(reinterpret_cast<uint8_t const*>(marker) +
                                      MMFilesDatafileHelper::VPackOffset(type));

        if (!payloadSlice.isObject()) {
          LOG_TOPIC("9c0e8", WARN, arangodb::Logger::ENGINES)
              << "cannot drop index for collection: invalid marker";
          ++state->errorCount;
          return state->canContinue();
        }

        TRI_idx_iid_t indexId = numericValue<TRI_idx_iid_t>(payloadSlice, "id");

        LOG_TOPIC("bb61e", TRACE, arangodb::Logger::ENGINES)
            << "found drop index marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId << ", indexId: " << indexId;

        if (state->isDropped(databaseId, collectionId)) {
          return true;
        }

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // if the underlying database is gone, we can go on
          LOG_TOPIC("43f06", TRACE, arangodb::Logger::ENGINES) << "cannot open database " << databaseId;
          return true;
        }

        auto col = vocbase->lookupCollection(collectionId);

        if (col == nullptr) {
          // if the underlying collection gone, we can go on
          return true;
        }

        // ignore any potential error returned by this call
        auto physical = static_cast<MMFilesCollection*>(col->getPhysical());
        TRI_ASSERT(physical != nullptr);
        col->dropIndex(indexId);

        MMFilesPersistentIndexFeature::dropIndex(databaseId, collectionId, indexId);

        // additionally remove the index file
        std::string const indexName("index-" + std::to_string(indexId) +
                                    ".json");
        std::string const filename(
            arangodb::basics::FileUtils::buildFilename(physical->path(), indexName));

        TRI_UnlinkFile(filename.c_str());
        break;
      }

      case TRI_DF_MARKER_VPACK_DROP_COLLECTION: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const collectionId = MMFilesDatafileHelper::CollectionId(marker);

        // insert the drop marker
        state->droppedCollections.emplace(collectionId);

        LOG_TOPIC("e3cfc", TRACE, arangodb::Logger::ENGINES)
            << "found drop collection marker. databaseId: " << databaseId
            << ", collectionId: " << collectionId;

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // database already deleted - do nothing
          return true;
        }

        // ignore any potential error returned by this call
        auto collection = state->releaseCollection(collectionId);

        if (collection == nullptr) {
          collection = vocbase->lookupCollection(collectionId);
        }

        if (collection != nullptr) {
          vocbase->dropCollection(collection->id(), true, -1.0);
        }
        MMFilesPersistentIndexFeature::dropCollection(databaseId, collectionId);
        break;
      }

      case TRI_DF_MARKER_VPACK_DROP_VIEW: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);
        TRI_voc_cid_t const viewId = MMFilesDatafileHelper::ViewId(marker);

        // insert the drop marker
        state->droppedViews.emplace(viewId);

        LOG_TOPIC("06d71", TRACE, arangodb::Logger::ENGINES)
            << "found drop view marker. databaseId: " << databaseId
            << ", viewId: " << viewId;

        TRI_vocbase_t* vocbase = state->useDatabase(databaseId);

        if (vocbase == nullptr) {
          // database already deleted - do nothing
          return true;
        }

        // ignore any potential error returned by this call
        std::shared_ptr<arangodb::LogicalView> view = vocbase->lookupView(viewId);
        if (view != nullptr) {
          view->drop();
        }

        break;
      }

      case TRI_DF_MARKER_VPACK_DROP_DATABASE: {
        TRI_voc_tick_t const databaseId = MMFilesDatafileHelper::DatabaseId(marker);

        // insert the drop marker
        state->droppedDatabases.emplace(databaseId);

        LOG_TOPIC("68c18", TRACE, arangodb::Logger::ENGINES)
            << "found drop database marker. databaseId: " << databaseId;

        /*TRI_vocbase_t* vocbase = */ state->releaseDatabase(databaseId);

        // ignore any potential error returned by this call
        state->databaseFeature->dropDatabase(databaseId, true, state->isDropped(databaseId));

        MMFilesPersistentIndexFeature::dropDatabase(databaseId);
        break;
      }

      case TRI_DF_MARKER_HEADER:
      case TRI_DF_MARKER_COL_HEADER:
      case TRI_DF_MARKER_FOOTER: {
        // new datafile or end of datafile. forget state!
        state->resetCollection();
        return true;
      }

      default: {
        // do nothing
      }
    }

    return true;
  } catch (std::exception const& ex) {
    LOG_TOPIC("f79d5", WARN, arangodb::Logger::ENGINES) << "cannot replay marker: " << ex.what();
    ++state->errorCount;
    return state->canContinue();
  } catch (...) {
    LOG_TOPIC("774ed", WARN, arangodb::Logger::ENGINES) << "cannot replay marker";
    ++state->errorCount;
    return state->canContinue();
  }
}

/// @brief replay a single logfile
int MMFilesWalRecoverState::replayLogfile(MMFilesWalLogfile* logfile, int number) {
  std::string const logfileName = logfile->filename();

  int const n = static_cast<int>(logfilesToProcess.size());

  LOG_TOPIC("b1bb8", INFO, arangodb::Logger::ENGINES)
      << "replaying WAL logfile '" << logfileName << "' (" << (number + 1)
      << " of " << n << ")";

  MMFilesDatafile* df = logfile->df();

  // Advise on sequential use:
  df->sequentialAccess();
  df->willNeed();

  if (!TRI_IterateDatafile(df, &MMFilesWalRecoverState::ReplayMarker,
                           static_cast<void*>(this))) {
    LOG_TOPIC("b3dd6", WARN, arangodb::Logger::ENGINES)
        << "WAL inspection failed when scanning logfile '" << logfileName << "'";
    return TRI_ERROR_ARANGO_RECOVERY;
  }

  // Advise on random access use:
  df->randomAccess();

  return TRI_ERROR_NO_ERROR;
}

/// @brief replay all logfiles
int MMFilesWalRecoverState::replayLogfiles() {
  droppedCollections.clear();
  droppedDatabases.clear();

  int i = 0;
  for (auto& it : logfilesToProcess) {
    TRI_ASSERT(it != nullptr);
    int res = replayLogfile(it, i++);

    if (res != TRI_ERROR_NO_ERROR) {
      return res;
    }
  }

  return TRI_ERROR_NO_ERROR;
}

/// @brief abort open transactions
int MMFilesWalRecoverState::abortOpenTransactions() {
  if (failedTransactions.empty()) {
    // nothing to do
    return TRI_ERROR_NO_ERROR;
  }

  LOG_TOPIC("5480d", TRACE, arangodb::Logger::ENGINES)
      << "writing abort markers for still open transactions";
  int res = TRI_ERROR_NO_ERROR;

  VPackBuilder builder;

  try {
    // write abort markers for all transactions
    for (auto it = failedTransactions.begin(); it != failedTransactions.end(); ++it) {
      TRI_voc_tid_t transactionId = (*it).first;

      if ((*it).second.second) {
        // already handled
        continue;
      }

      TRI_voc_tick_t databaseId = (*it).second.first;

      MMFilesTransactionMarker marker(TRI_DF_MARKER_VPACK_ABORT_TRANSACTION,
                                      databaseId, transactionId);
      MMFilesWalSlotInfoCopy slotInfo =
          MMFilesLogfileManager::instance()->allocateAndWrite(marker, false);

      if (slotInfo.errorCode != TRI_ERROR_NO_ERROR) {
        THROW_ARANGO_EXCEPTION(slotInfo.errorCode);
      }

      // recycle builder
      builder.clear();
    }
  } catch (arangodb::basics::Exception const& ex) {
    res = ex.code();
  } catch (...) {
    res = TRI_ERROR_INTERNAL;
  }

  return res;
}

/// @brief remove all empty logfiles found during logfile inspection
int MMFilesWalRecoverState::removeEmptyLogfiles() {
  if (emptyLogfiles.empty()) {
    return TRI_ERROR_NO_ERROR;
  }

  LOG_TOPIC("561e7", TRACE, arangodb::Logger::ENGINES) << "removing empty WAL logfiles";

  for (auto it = emptyLogfiles.begin(); it != emptyLogfiles.end(); ++it) {
    auto filename = (*it);

    if (basics::FileUtils::remove(filename, nullptr)) {
      LOG_TOPIC("6245c", TRACE, arangodb::Logger::ENGINES)
          << "removing empty WAL logfile '" << filename << "'";
    }
  }

  emptyLogfiles.clear();

  return TRI_ERROR_NO_ERROR;
}

/// @brief fill the secondary indexes of all collections used in recovery
int MMFilesWalRecoverState::fillIndexes() {
  // release all collections
  for (auto it = openedCollections.begin(); it != openedCollections.end(); ++it) {
    std::shared_ptr<arangodb::LogicalCollection>& collection = (*it).second;
    auto physical = static_cast<MMFilesCollection*>(collection->getPhysical());

    TRI_ASSERT(physical != nullptr);
    // activate secondary indexes
    physical->useSecondaryIndexes(true);

    auto ctx = transaction::StandaloneContext::Create(collection->vocbase());
    arangodb::SingleCollectionTransaction trx(ctx, *collection, AccessMode::Type::WRITE);
    int res = physical->fillAllIndexes(trx);

    if (res != TRI_ERROR_NO_ERROR) {
      return res;
    }
  }

  return TRI_ERROR_NO_ERROR;
}