////////////////////////////////////////////////////////////////////////////////
/// 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 "MMFilesEngine.h"
#include "Basics/FileUtils.h"
#include "Basics/MutexLocker.h"
#include "Basics/StringUtils.h"
#include "Basics/VelocyPackHelper.h"
#include "Basics/WriteLocker.h"
#include "Basics/files.h"
#include "Random/RandomGenerator.h"
#include "RestServer/DatabaseFeature.h"
#include "RestServer/DatabasePathFeature.h"
#include "StorageEngine/EngineSelectorFeature.h"
#include "StorageEngine/MMFilesCleanupThread.h"
#include "StorageEngine/MMFilesCompactorThread.h"
#include "StorageEngine/MMFilesCollection.h"
#include "VocBase/DatafileHelper.h"
#include "VocBase/LogicalCollection.h"
#include "VocBase/datafile.h"
#include "VocBase/ticks.h"
#include "VocBase/vocbase.h"
#include "Wal/LogfileManager.h"

#include "Indexes/RocksDBFeature.h"
#include "Indexes/RocksDBIndex.h"

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

using namespace arangodb;
using namespace arangodb::basics;

namespace {
/// @brief collection meta info filename
static constexpr char const* parametersFilename() { return "parameter.json"; }

/// @brief extract the numeric part from a filename
/// the filename must look like this: /.*type-abc\.ending$/, where abc is
/// a number, and type and ending are arbitrary letters
static uint64_t getNumericFilenamePartFromDatafile(std::string const& filename) {
  char const* pos1 = strrchr(filename.c_str(), '.');

  if (pos1 == nullptr) {
    return 0;
  }

  char const* pos2 = strrchr(filename.c_str(), '-');

  if (pos2 == nullptr || pos2 > pos1) {
    return 0;
  }

  return basics::StringUtils::uint64(pos2 + 1, pos1 - pos2 - 1);
}

/// @brief extract the numeric part from a filename
static uint64_t getNumericFilenamePartFromDatabase(std::string const& filename) {
  char const* pos = strrchr(filename.c_str(), '-');

  if (pos == nullptr) {
    return 0;
  }

  return basics::StringUtils::uint64(pos + 1);
}

static uint64_t getNumericFilenamePartFromDatafile(TRI_datafile_t const* datafile) {
  return getNumericFilenamePartFromDatafile(datafile->getName());
}


struct DatafileComparator {
  bool operator()(TRI_datafile_t const* lhs, TRI_datafile_t const* rhs) const {
    return getNumericFilenamePartFromDatafile(lhs) < getNumericFilenamePartFromDatafile(rhs);
  }
};

/// @brief compare two filenames, based on the numeric part contained in
/// the filename. this is used to sort database filenames on startup
struct DatafileIdStringComparator {
  bool operator()(std::string const& lhs, std::string const& rhs) const {
    return getNumericFilenamePartFromDatafile(lhs) < getNumericFilenamePartFromDatafile(rhs);
  }
};

/// @brief compare two filenames, based on the numeric part contained in
/// the filename. this is used to sort database filenames on startup
struct DatabaseIdStringComparator {
  bool operator()(std::string const& lhs, std::string const& rhs) const {
    return getNumericFilenamePartFromDatabase(lhs) < getNumericFilenamePartFromDatabase(rhs);
  }
};

}

std::string const MMFilesEngine::EngineName("mmfiles");
std::string const MMFilesEngine::FeatureName("MMFilesEngine");

// create the storage engine
MMFilesEngine::MMFilesEngine(application_features::ApplicationServer* server)
    : StorageEngine(server, EngineName, FeatureName),
      _isUpgrade(false),
      _maxTick(0) {
}

MMFilesEngine::~MMFilesEngine() {
}

// add the storage engine's specifc options to the global list of options
void MMFilesEngine::collectOptions(std::shared_ptr<options::ProgramOptions>) {
}
  
// validate the storage engine's specific options
void MMFilesEngine::validateOptions(std::shared_ptr<options::ProgramOptions>) {
}

// preparation phase for storage engine. can be used for internal setup.
// the storage engine must not start any threads here or write any files
void MMFilesEngine::prepare() {
  TRI_ASSERT(EngineSelectorFeature::ENGINE = this);

  // get base path from DatabaseServerFeature 
  auto databasePathFeature = application_features::ApplicationServer::getFeature<DatabasePathFeature>("DatabasePath");
  _basePath = databasePathFeature->directory();
  _databasePath += databasePathFeature->subdirectoryName("databases") + TRI_DIR_SEPARATOR_CHAR;
  
  TRI_ASSERT(!_basePath.empty());
  TRI_ASSERT(!_databasePath.empty());
}

// initialize engine
void MMFilesEngine::start() {
  TRI_ASSERT(EngineSelectorFeature::ENGINE = this);
  
  // test if the "databases" directory is present and writable
  verifyDirectories();
  
  // get names of all databases
  std::vector<std::string> names(getDatabaseNames());

  if (names.empty()) {
    // no databases found, i.e. there is no system database!
    // create a database for the system database
    int res = createDatabaseDirectory(TRI_NewTickServer(), TRI_VOC_SYSTEM_DATABASE);

    if (res != TRI_ERROR_NO_ERROR) {
      LOG(ERR) << "unable to initialize databases: " << TRI_errno_string(res);
      THROW_ARANGO_EXCEPTION(res);
    }
  } 
}

// stop the storage engine. this can be used to flush all data to disk,
// shutdown threads etc. it is guaranteed that there will be no read and
// write requests to the storage engine after this call
void MMFilesEngine::stop() {
  TRI_ASSERT(EngineSelectorFeature::ENGINE = this);
}

// create storage-engine specific collection
PhysicalCollection* MMFilesEngine::createPhysicalCollection(LogicalCollection* collection) {
  TRI_ASSERT(EngineSelectorFeature::ENGINE = this);
  return new MMFilesCollection(collection);
}

void MMFilesEngine::recoveryDone(TRI_vocbase_t* vocbase) {    
  DatabaseFeature* databaseFeature = application_features::ApplicationServer::getFeature<DatabaseFeature>("Database");

  if (!databaseFeature->checkVersion() && !databaseFeature->upgrade()) {
    // start compactor thread
    LOG(TRACE) << "starting compactor for database '" << vocbase->name() << "'";

    startCompactor(vocbase);
  }

  // delete all collection files from collections marked as deleted
  for (auto& it : _deleted) {
    std::string const& name = it.first;
    std::string const& file = it.second;

    LOG(DEBUG) << "collection '" << name << "' was deleted, wiping it";

    int res = TRI_RemoveDirectory(file.c_str());

    if (res != TRI_ERROR_NO_ERROR) {
      LOG(WARN) << "cannot wipe deleted collection '" << name << "': " << TRI_errno_string(res);
    }
  }
  _deleted.clear();
}

// fill the Builder object with an array of databases that were detected
// by the storage engine. this method must sort out databases that were not
// fully created (see "createDatabase" below). called at server start only
void MMFilesEngine::getDatabases(arangodb::velocypack::Builder& result) {
  result.openArray();

  // open databases in defined order
  std::vector<std::string> files = TRI_FilesDirectory(_databasePath.c_str());
  std::sort(files.begin(), files.end(), DatabaseIdStringComparator());

  for (auto const& name : files) {
    TRI_ASSERT(!name.empty());
    
    TRI_voc_tick_t id = getNumericFilenamePartFromDatabase(name);

    if (id == 0) {
      // invalid id
      continue;
    }

    // construct and validate path
    std::string const directory(basics::FileUtils::buildFilename(_databasePath, name));

    if (!TRI_IsDirectory(directory.c_str())) {
      continue;
    }

    if (!basics::StringUtils::isPrefix(name, "database-") ||
        basics::StringUtils::isSuffix(name, ".tmp")) {
      LOG_TOPIC(TRACE, Logger::DATAFILES) << "ignoring file '" << name << "'";
      continue;
    }

    // we have a directory...

    if (!TRI_IsWritable(directory.c_str())) {
      // the database directory we found is not writable for the current user
      // this can cause serious trouble so we will abort the server start if we
      // encounter this situation
      LOG(ERR) << "database directory '" << directory
               << "' is not writable for current user";
      THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_DATADIR_NOT_WRITABLE);
    }

    // we have a writable directory...
    std::string const tmpfile(basics::FileUtils::buildFilename(directory, ".tmp"));

    if (TRI_ExistsFile(tmpfile.c_str())) {
      // still a temporary... must ignore
      LOG(TRACE) << "ignoring temporary directory '" << tmpfile << "'";
      continue;
    }

    // a valid database directory

    // now read data from parameter.json file
    std::string const file = databaseParametersFilename(id);

    if (!TRI_ExistsFile(file.c_str())) {
      // no parameter.json file
      
      if (TRI_FilesDirectory(directory.c_str()).empty()) {
        // directory is otherwise empty, continue!
        LOG(WARN) << "ignoring empty database directory '" << directory
                  << "' without parameters file";
        continue;
      } 
        
      // abort
      LOG(ERR) << "database directory '" << directory
               << "' does not contain parameters file or parameters file cannot be read";
      THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_ILLEGAL_PARAMETER_FILE);
    }

    LOG(DEBUG) << "reading database parameters from file '" << file << "'";
    std::shared_ptr<VPackBuilder> builder;
    try {
      builder = arangodb::basics::VelocyPackHelper::velocyPackFromFile(file);
    } catch (...) {
      LOG(ERR) << "database directory '" << directory
               << "' does not contain a valid parameters file";

      // abort
      THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_ILLEGAL_PARAMETER_FILE);
    }

    VPackSlice parameters = builder->slice();
    std::string const parametersString = parameters.toJson();

    LOG(DEBUG) << "database parameters: " << parametersString;
      
    VPackSlice idSlice = parameters.get("id");
    
    if (!idSlice.isString() ||
        id != static_cast<TRI_voc_tick_t>(basics::StringUtils::uint64(idSlice.copyString()))) {
      LOG(ERR) << "database directory '" << directory
               << "' does not contain a valid parameters file";
      THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_ILLEGAL_PARAMETER_FILE);
    }
    
    if (arangodb::basics::VelocyPackHelper::getBooleanValue(parameters, "deleted", false)) {
      // database is deleted, skip it!
      LOG(DEBUG) << "found dropped database in directory '" << directory << "'";
      LOG(DEBUG) << "removing superfluous database directory '" << directory << "'";

      // delete persistent indexes for this database
      TRI_voc_tick_t id = static_cast<TRI_voc_tick_t>(
          basics::StringUtils::uint64(idSlice.copyString()));
      RocksDBFeature::dropDatabase(id);

      dropDatabaseDirectory(directory);
      continue;
    }

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

    if (!nameSlice.isString()) {
      LOG(ERR) << "database directory '" << directory << "' does not contain a valid parameters file";
      THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_ILLEGAL_PARAMETER_FILE);
    }

    result.add(parameters);
  }

  result.close();
}

// fills the provided builder with information about the collection 
void MMFilesEngine::getCollectionInfo(TRI_vocbase_t* vocbase, TRI_voc_cid_t id, 
                                      arangodb::velocypack::Builder& builder, 
                                      bool includeIndexes, TRI_voc_tick_t maxTick) {

  std::string const path = collectionDirectory(vocbase->id(), id);

  builder.openObject();

  std::shared_ptr<VPackBuilder> fileInfoBuilder =
      arangodb::basics::VelocyPackHelper::velocyPackFromFile(basics::FileUtils::buildFilename(path, parametersFilename()));
  builder.add("parameters", fileInfoBuilder->slice());

  if (includeIndexes) {
    // dump index information
    builder.add("indexes", VPackValue(VPackValueType::Array));

    std::vector<std::string> files = TRI_FilesDirectory(path.c_str());

    // sort by index id
    std::sort(files.begin(), files.end(), DatafileIdStringComparator());

    for (auto const& file : files) {
      if (StringUtils::isPrefix(file, "index-") &&
          StringUtils::isSuffix(file, ".json")) {
        std::string const filename = basics::FileUtils::buildFilename(path, file);
        std::shared_ptr<VPackBuilder> indexVPack =
            arangodb::basics::VelocyPackHelper::velocyPackFromFile(filename);

        VPackSlice const indexSlice = indexVPack->slice();
        VPackSlice const id = indexSlice.get("id");

        if (id.isNumber()) {
          uint64_t iid = id.getNumericValue<uint64_t>();
          if (iid <= static_cast<uint64_t>(maxTick)) {
            // convert "id" to string
            VPackBuilder toMerge;
            {
              VPackObjectBuilder b(&toMerge);
              toMerge.add("id", VPackValue(std::to_string(iid)));
            }
            VPackBuilder mergedBuilder =
                VPackCollection::merge(indexSlice, toMerge.slice(), false);
            builder.add(mergedBuilder.slice());
          }
        } else if (id.isString()) {
          std::string data = id.copyString();
          uint64_t iid = StringUtils::uint64(data);
          if (iid <= static_cast<uint64_t>(maxTick)) {
            builder.add(indexSlice);
          }
        }
      }
    }
    builder.close();
  }

  builder.close();
}

// fill the Builder object with an array of collections (and their corresponding
// indexes) that were detected by the storage engine. called at server start only
int MMFilesEngine::getCollectionsAndIndexes(TRI_vocbase_t* vocbase, 
                                            arangodb::velocypack::Builder& result,
                                            bool wasCleanShutdown,
                                            bool isUpgrade) {
  result.openArray();

  std::string const path = databaseDirectory(vocbase->id());
  std::vector<std::string> files = TRI_FilesDirectory(path.c_str());

  for (auto const& name : files) {
    TRI_ASSERT(!name.empty());

    if (!StringUtils::isPrefix(name, "collection-") ||
        StringUtils::isSuffix(name, ".tmp")) {
      // no match, ignore this file
      continue;
    }

    std::string const directory = FileUtils::buildFilename(path, name);

    if (!TRI_IsDirectory(directory.c_str())) {
      LOG(DEBUG) << "ignoring non-directory '" << directory << "'";
      continue;
    }

    if (!TRI_IsWritable(directory.c_str())) {
      // the collection directory we found is not writable for the current
      // user
      // this can cause serious trouble so we will abort the server start if
      // we
      // encounter this situation
      LOG(ERR) << "database subdirectory '" << directory
                << "' is not writable for current user";

      return TRI_ERROR_ARANGO_DATADIR_NOT_WRITABLE;
    }

    int res = TRI_ERROR_NO_ERROR;

    try {
      std::unique_ptr<LogicalCollection> collection(loadCollectionInfo(vocbase, directory));
     
      if (collection->deleted()) {
        _deleted.emplace_back(std::make_pair(collection->name(), directory));
        continue;
      }

      // add collection info
      collection->toVelocyPack(result, true);
    } catch (arangodb::basics::Exception const& e) {
      std::string tmpfile = FileUtils::buildFilename(directory, ".tmp");

      if (TRI_ExistsFile(tmpfile.c_str())) {
        LOG(TRACE) << "ignoring temporary directory '" << tmpfile << "'";
        // temp file still exists. this means the collection was not created
        // fully and needs to be ignored
        continue;  // ignore this directory
      }

      res = e.code();

      LOG(ERR) << "cannot read collection info file in directory '"
                << directory << "': " << TRI_errno_string(res);

      return res;
    }
  }

  result.close();

  return TRI_ERROR_NO_ERROR;
}

TRI_vocbase_t* MMFilesEngine::openDatabase(VPackSlice const& parameters, bool isUpgrade) {
  VPackSlice idSlice = parameters.get("id");
  TRI_voc_tick_t id = static_cast<TRI_voc_tick_t>(basics::StringUtils::uint64(idSlice.copyString()));
  std::string const name = parameters.get("name").copyString();
  
  bool const wasCleanShutdown = arangodb::wal::LogfileManager::instance()->hasFoundLastTick();
  return openExistingDatabase(id, name, wasCleanShutdown, isUpgrade);
}

// asks the storage engine to create a database as specified in the VPack
// Slice object and persist the creation info. It is guaranteed by the server that 
// no other active database with the same name and id exists when this function
// is called. If this operation fails somewhere in the middle, the storage 
// engine is required to fully clean up the creation and throw only then, 
// so that subsequent database creation requests will not fail.
// the WAL entry for the database creation will be written *after* the call
// to "createDatabase" returns
TRI_vocbase_t* MMFilesEngine::createDatabase(TRI_voc_tick_t id, arangodb::velocypack::Slice const& data) {
  std::string const name = data.get("name").copyString();

  waitUntilDeletion(id, true);
  
  int res = createDatabaseDirectory(id, name);

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

  return openExistingDatabase(id, name, true, false);
}

// asks the storage engine to drop the specified database and persist the 
// deletion info. Note that physical deletion of the database data must not 
// be carried out by this call, as there may still be readers of the database's data. 
// It is recommended that this operation only sets a deletion flag for the database 
// but let's an async task perform the actual deletion. 
// the WAL entry for database deletion will be written *after* the call
// to "prepareDropDatabase" returns
int MMFilesEngine::prepareDropDatabase(TRI_vocbase_t* vocbase) {
  // signal the compactor thread to finish
  beginShutdownCompactor(vocbase);

  return saveDatabaseParameters(vocbase->id(), vocbase->name(), true);
}

// perform a physical deletion of the database      
int MMFilesEngine::dropDatabase(TRI_vocbase_t* vocbase) {
  // stop compactor thread
  shutdownDatabase(vocbase);

  _collectionPaths.erase(vocbase->id());

  return dropDatabaseDirectory(databaseDirectory(vocbase->id()));
}

/// @brief wait until a database directory disappears
int MMFilesEngine::waitUntilDeletion(TRI_voc_tick_t id, bool force) {
  std::string const path = databaseDirectory(id);
  
  int iterations = 0;
  // wait for at most 30 seconds for the directory to be removed
  while (TRI_IsDirectory(path.c_str())) {
    if (iterations == 0) {
      LOG(TRACE) << "waiting for deletion of database directory '" << path << "'";
    } else if (iterations >= 30 * 20) {
      LOG(WARN) << "unable to remove database directory '" << path << "'";

      if (force) {
        LOG(WARN) << "forcefully deleting database directory '" << path << "'";
        return dropDatabaseDirectory(path);
      }
      return TRI_ERROR_INTERNAL;
    }

    if (iterations == 5 * 20) {
      LOG(INFO) << "waiting for deletion of database directory '" << path << "'";
    }

    ++iterations;
    usleep(50000);
  }

  return TRI_ERROR_NO_ERROR;
}


// asks the storage engine to create a collection as specified in the VPack
// Slice object and persist the creation info. It is guaranteed by the server 
// that no other active collection with the same name and id exists in the same
// database when this function is called. If this operation fails somewhere in 
// the middle, the storage engine is required to fully clean up the creation 
// and throw only then, so that subsequent collection creation requests will not fail.
// the WAL entry for the collection creation will be written *after* the call
// to "createCollection" returns
std::string MMFilesEngine::createCollection(TRI_vocbase_t* vocbase, TRI_voc_cid_t id,
                                            arangodb::LogicalCollection const* parameters) {
  std::string const path = databasePath(vocbase);

  // sanity check
  if (sizeof(TRI_df_header_marker_t) + sizeof(TRI_df_footer_marker_t) > parameters->journalSize()) {
    LOG(ERR) << "cannot create datafile '" << parameters->name() << "' in '"
             << path << "', maximal size '"
             << parameters->journalSize() << "' is too small";
    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_DATAFILE_FULL);
  }

  if (!TRI_IsDirectory(path.c_str())) {
    LOG(ERR) << "cannot create collection '" << path << "', database path is not a directory";
    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_DATADIR_INVALID);
  }

  TRI_ASSERT(id != 0);
  std::string const dirname = createCollectionDirectoryName(path, id);

  registerCollectionPath(vocbase->id(), id, dirname);

  // directory must not exist
  if (TRI_ExistsFile(dirname.c_str())) {
    LOG(ERR) << "cannot create collection '" << parameters->name()
             << "' in directory '" << dirname << "': directory already exists";
    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_COLLECTION_DIRECTORY_ALREADY_EXISTS);
  }

  // use a temporary directory first. this saves us from leaving an empty
  // directory behind, and the server refusing to start
  std::string const tmpname = dirname + ".tmp";

  // create directory
  std::string errorMessage;
  long systemError;
  int res = TRI_CreateDirectory(tmpname.c_str(), systemError, errorMessage);

  if (res != TRI_ERROR_NO_ERROR) {
    LOG(ERR) << "cannot create collection '" << parameters->name()
             << "' in directory '" << path << "': " << TRI_errno_string(res)
             << " - " << systemError << " - " << errorMessage;
    THROW_ARANGO_EXCEPTION(res);
  }

  TRI_IF_FAILURE("CreateCollection::tempDirectory") { THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG); }

  // create a temporary file (.tmp)
  std::string const tmpfile(
      arangodb::basics::FileUtils::buildFilename(tmpname.c_str(), ".tmp"));
  res = TRI_WriteFile(tmpfile.c_str(), "", 0);

  // this file will be renamed to this filename later...
  std::string const tmpfile2(
      arangodb::basics::FileUtils::buildFilename(dirname.c_str(), ".tmp"));

  TRI_IF_FAILURE("CreateCollection::tempFile") { THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG); }

  if (res != TRI_ERROR_NO_ERROR) {
    LOG(ERR) << "cannot create collection '" << parameters->name()
             << "' in directory '" << path << "': " << TRI_errno_string(res)
             << " - " << systemError << " - " << errorMessage;
    TRI_RemoveDirectory(tmpname.c_str());
    THROW_ARANGO_EXCEPTION(res);
  }

  TRI_IF_FAILURE("CreateCollection::renameDirectory") { THROW_ARANGO_EXCEPTION(TRI_ERROR_DEBUG); }

  res = TRI_RenameFile(tmpname.c_str(), dirname.c_str());

  if (res != TRI_ERROR_NO_ERROR) {
    LOG(ERR) << "cannot create collection '" << parameters->name()
             << "' in directory '" << path << "': " << TRI_errno_string(res)
             << " - " << systemError << " - " << errorMessage;
    TRI_RemoveDirectory(tmpname.c_str());
    THROW_ARANGO_EXCEPTION(res);
  }

  // now we have the collection directory in place with the correct name and a
  // .tmp file in it

  // delete .tmp file
  TRI_UnlinkFile(tmpfile2.c_str());

  // save the parameters file
  bool const doSync = application_features::ApplicationServer::getFeature<DatabaseFeature>("Database")->forceSyncProperties();
  saveCollectionInfo(vocbase, id, parameters, doSync);

  return dirname;
}
 
// asks the storage engine to drop the specified collection and persist the 
// deletion info. Note that physical deletion of the collection data must not 
// be carried out by this call, as there may still be readers of the collection's 
// data. It is recommended that this operation
// only sets a deletion flag for the collection but let's an async task perform
// the actual deletion.
// the WAL entry for collection deletion will be written *after* the call
// to "dropCollection" returns
void MMFilesEngine::prepareDropCollection(TRI_vocbase_t*, arangodb::LogicalCollection*) {
  // nothing to do here
}

// perform a physical deletion of the collection
void MMFilesEngine::dropCollection(TRI_vocbase_t* vocbase, arangodb::LogicalCollection* collection) {
  std::string const name(collection->name());
  unregisterCollectionPath(vocbase->id(), collection->cid());
  
  // delete persistent indexes    
  RocksDBFeature::dropCollection(vocbase->id(), collection->cid());

  // rename collection directory
  if (!collection->path().empty()) {
    std::string const collectionPath = collection->path();

#ifdef _WIN32
    size_t pos = collectionPath.find_last_of('\\');
#else
    size_t pos = collectionPath.find_last_of('/');
#endif

    bool invalid = false;

    if (pos == std::string::npos || pos + 1 >= collectionPath.size()) {
      invalid = true;
    }

    std::string path;
    std::string relName;
    if (!invalid) {
      // extract path part
      if (pos > 0) {
        path = collectionPath.substr(0, pos); 
      }

      // extract relative filename
      relName = collectionPath.substr(pos + 1);

      if (!StringUtils::isPrefix(relName, "collection-") || 
          StringUtils::isSuffix(relName, ".tmp")) {
        invalid = true;
      }
    }

    if (invalid) {
      LOG(ERR) << "cannot rename dropped collection '" << name
               << "': unknown path '" << collection->path() << "'";
    } else {
      // prefix the collection name with "deleted-"

      std::string const newFilename = 
        FileUtils::buildFilename(path, "deleted-" + relName.substr(std::string("collection-").size()));

      // check if target directory already exists
      if (TRI_IsDirectory(newFilename.c_str())) {
        // remove existing target directory
        TRI_RemoveDirectory(newFilename.c_str());
      }

      // perform the rename
      int res = TRI_RenameFile(collection->path().c_str(), newFilename.c_str());

      LOG(TRACE) << "renaming collection directory from '"
                 << collection->path() << "' to '" << newFilename << "'";

      if (res != TRI_ERROR_NO_ERROR) {
        LOG(ERR) << "cannot rename dropped collection '" << name
                 << "' from '" << collection->path() << "' to '"
                 << newFilename << "': " << TRI_errno_string(res);
      } else {
        LOG(DEBUG) << "wiping dropped collection '" << name
                   << "' from disk";

        res = TRI_RemoveDirectory(newFilename.c_str());

        if (res != TRI_ERROR_NO_ERROR) {
          LOG(ERR) << "cannot wipe dropped collection '" << name
                   << "' from disk: " << TRI_errno_string(res);
        }
      }
    }
  }
}

// asks the storage engine to change properties of the collection as specified in 
// the VPack Slice object and persist them. If this operation fails 
// somewhere in the middle, the storage engine is required to fully revert the 
// property changes and throw only then, so that subsequent operations will not fail.
// the WAL entry for the propery change will be written *after* the call
// to "changeCollection" returns
void MMFilesEngine::changeCollection(TRI_vocbase_t* vocbase, TRI_voc_cid_t id,
                                     arangodb::LogicalCollection const* parameters,
                                     bool doSync) {
  saveCollectionInfo(vocbase, id, parameters, doSync);
}

// asks the storage engine to create an index as specified in the VPack
// Slice object and persist the creation info. The database id, collection id 
// and index data are passed in the Slice object. Note that this function
// is not responsible for inserting the individual documents into the index.
// If this operation fails somewhere in the middle, the storage engine is required 
// to fully clean up the creation and throw only then, so that subsequent index 
// creation requests will not fail.
// the WAL entry for the index creation will be written *after* the call
// to "createIndex" returns
void MMFilesEngine::createIndex(TRI_vocbase_t* vocbase, TRI_voc_cid_t collectionId,
                                TRI_idx_iid_t id, arangodb::velocypack::Slice const& data) {
  // construct filename
  std::string const filename = indexFilename(vocbase->id(), collectionId, id);

  // and save
  bool const doSync = application_features::ApplicationServer::getFeature<DatabaseFeature>("Database")->forceSyncProperties();
  bool ok = arangodb::basics::VelocyPackHelper::velocyPackToFile(filename, data, doSync);

  if (!ok) {
    LOG(ERR) << "cannot save index definition: " << TRI_last_error();
    THROW_ARANGO_EXCEPTION(TRI_errno());
  }
}

// asks the storage engine to drop the specified index and persist the deletion 
// info. Note that physical deletion of the index must not be carried out by this call, 
// as there may still be users of the index. It is recommended that this operation
// only sets a deletion flag for the index but let's an async task perform
// the actual deletion.
// the WAL entry for index deletion will be written *after* the call
// to "dropIndex" returns
void MMFilesEngine::dropIndex(TRI_vocbase_t* vocbase, TRI_voc_cid_t collectionId,
                              TRI_idx_iid_t id) {
  // construct filename
  std::string const filename = indexFilename(vocbase->id(), collectionId, id);

  int res = TRI_UnlinkFile(filename.c_str());

  if (res != TRI_ERROR_NO_ERROR) {
    LOG(ERR) << "cannot remove index definition in file '" << filename << "': " << TRI_errno_string(res);
  }
}
  
void MMFilesEngine::unloadCollection(TRI_vocbase_t* vocbase, TRI_voc_cid_t collectionId) {
  signalCleanup(vocbase);
}

void MMFilesEngine::signalCleanup(TRI_vocbase_t* vocbase) {
  MUTEX_LOCKER(locker, _threadsLock);

  auto it = _cleanupThreads.find(vocbase);
  
  if (it == _cleanupThreads.end()) {
    return;
  }

  (*it).second->signal();
}

// iterate all documents of the underlying collection
// this is called when a collection is openend, and all its documents need to be added to
// indexes etc.
void MMFilesEngine::iterateDocuments(TRI_voc_tick_t databaseId, TRI_voc_cid_t collectionId,
                                     std::function<void(arangodb::velocypack::Slice const&)> const& cb) {
}

// adds a document to the storage engine
// this will be called by the WAL collector when surviving documents are being moved
// into the storage engine's realm
void MMFilesEngine::addDocumentRevision(TRI_voc_tick_t databaseId, TRI_voc_cid_t collectionId,
                                        arangodb::velocypack::Slice const& document) {
}

// removes a document from the storage engine
// this will be called by the WAL collector when non-surviving documents are being removed
// from the storage engine's realm
void MMFilesEngine::removeDocumentRevision(TRI_voc_tick_t databaseId, TRI_voc_cid_t collectionId,
                                           arangodb::velocypack::Slice const& document) {
}

/// @brief scans a collection and locates all files
MMFilesEngineCollectionFiles MMFilesEngine::scanCollectionDirectory(std::string const& path) {
  LOG_TOPIC(TRACE, Logger::DATAFILES) << "scanning collection directory '"
                                      << path << "'";

  MMFilesEngineCollectionFiles structure;

  // check files within the directory
  std::vector<std::string> files = TRI_FilesDirectory(path.c_str());

  for (auto const& file : files) {
    std::vector<std::string> parts = StringUtils::split(file, '.');

    if (parts.size() < 2 || parts.size() > 3 || parts[0].empty()) {
      LOG_TOPIC(DEBUG, Logger::DATAFILES)
          << "ignoring file '" << file
          << "' because it does not look like a datafile";
      continue;
    }

    std::string filename = FileUtils::buildFilename(path, file);
    std::string extension = parts[1];
    std::string isDead = (parts.size() > 2) ? parts[2] : "";

    std::vector<std::string> next = StringUtils::split(parts[0], "-");

    if (next.size() < 2) {
      LOG_TOPIC(DEBUG, Logger::DATAFILES)
          << "ignoring file '" << file
          << "' because it does not look like a datafile";
      continue;
    }

    std::string filetype = next[0];
    next.erase(next.begin());
    std::string qualifier = StringUtils::join(next, '-');

    // file is dead
    if (!isDead.empty()) {
      if (isDead == "dead") {
        FileUtils::remove(filename);
      } else {
        LOG_TOPIC(DEBUG, Logger::DATAFILES)
            << "ignoring file '" << file
            << "' because it does not look like a datafile";
      }

      continue;
    }

    // file is an index
    if (filetype == "index" && extension == "json") {
      structure.indexes.emplace_back(filename);
      continue;
    }

    // file is a journal or datafile
    if (extension == "db") {
      // file is a journal
      if (filetype == "journal") {
        structure.journals.emplace_back(filename);
      }

      // file is a datafile
      else if (filetype == "datafile") {
        structure.datafiles.emplace_back(filename);
      }

      // file is a left-over compaction file. rename it back
      else if (filetype == "compaction") {
        std::string relName = "datafile-" + qualifier + "." + extension;
        std::string newName = FileUtils::buildFilename(path, relName);

        if (FileUtils::exists(newName)) {
          // we have a compaction-xxxx and a datafile-xxxx file. we'll keep
          // the datafile

          FileUtils::remove(filename);

          LOG_TOPIC(WARN, Logger::DATAFILES)
              << "removing left-over compaction file '" << filename << "'";

          continue;
        } else {
          // this should fail, but shouldn't do any harm either...
          FileUtils::remove(newName);

          // rename the compactor to a datafile
          int res = TRI_RenameFile(filename.c_str(), newName.c_str());

          if (res != TRI_ERROR_NO_ERROR) {
            LOG_TOPIC(ERR, Logger::DATAFILES)
                << "unable to rename compaction file '" << filename << "'";
            continue;
          }
        }

        structure.datafiles.emplace_back(filename);
      }

      // temporary file, we can delete it!
      else if (filetype == "temp") {
        LOG_TOPIC(WARN, Logger::DATAFILES)
            << "found temporary file '" << filename
            << "', which is probably a left-over. deleting it";
        FileUtils::remove(filename);
      }

      // ups, what kind of file is that
      else {
        LOG_TOPIC(ERR, Logger::DATAFILES) << "unknown datafile type '" << file
                                          << "'";
      }
    }
  }

  // now sort the files in the structures that we created.
  // the sorting allows us to iterate the files in the correct order
  std::sort(structure.journals.begin(), structure.journals.end(), DatafileIdStringComparator());
  std::sort(structure.compactors.begin(), structure.compactors.end(), DatafileIdStringComparator());
  std::sort(structure.datafiles.begin(), structure.datafiles.end(), DatafileIdStringComparator());
  std::sort(structure.indexes.begin(), structure.indexes.end(), DatafileIdStringComparator());

  return structure;
}
  
void MMFilesEngine::verifyDirectories() {
  if (!TRI_IsDirectory(_basePath.c_str())) {
    LOG(ERR) << "database path '" << _basePath << "' is not a directory";

    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_DATADIR_INVALID);
  }

  if (!TRI_IsWritable(_basePath.c_str())) {
    // database directory is not writable for the current user... bad luck
    LOG(ERR) << "database directory '" << _basePath
             << "' is not writable for current user";

    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_DATADIR_NOT_WRITABLE);
  }

  // verify existence of "databases" subdirectory
  if (!TRI_IsDirectory(_databasePath.c_str())) {
    long systemError;
    std::string errorMessage;
    int res = TRI_CreateDirectory(_databasePath.c_str(), systemError, errorMessage);

    if (res != TRI_ERROR_NO_ERROR) {
      LOG(ERR) << "unable to create database directory '"
               << _databasePath << "': " << errorMessage;

      THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_DATADIR_NOT_WRITABLE);
    }
  }

  if (!TRI_IsWritable(_databasePath.c_str())) {
    LOG(ERR) << "database directory '" << _databasePath << "' is not writable";

    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_DATADIR_NOT_WRITABLE);
  }
}

/// @brief get the names of all databases 
std::vector<std::string> MMFilesEngine::getDatabaseNames() const {
  std::vector<std::string> databases;

  for (auto const& name : TRI_FilesDirectory(_databasePath.c_str())) {
    TRI_ASSERT(!name.empty());

    if (!basics::StringUtils::isPrefix(name, "database-")) {
      // found some other file
      continue;
    }

    // found a database name
    std::string const dname(arangodb::basics::FileUtils::buildFilename(_databasePath, name));

    if (TRI_IsDirectory(dname.c_str())) {
      databases.emplace_back(name);
    }
  }

  // sort by id
  std::sort(databases.begin(), databases.end(), DatabaseIdStringComparator());

  return databases;
}


/// @brief create a new database directory 
int MMFilesEngine::createDatabaseDirectory(TRI_voc_tick_t id,
                                           std::string const& name) {
  std::string const dirname = databaseDirectory(id);

  // use a temporary directory first. otherwise, if creation fails, the server
  // might be left with an empty database directory at restart, and abort.

  std::string const tmpname(dirname + ".tmp");

  if (TRI_IsDirectory(tmpname.c_str())) {
    TRI_RemoveDirectory(tmpname.c_str());
  }

  std::string errorMessage;
  long systemError;

  int res = TRI_CreateDirectory(tmpname.c_str(), systemError, errorMessage);

  if (res != TRI_ERROR_NO_ERROR) {
    if (res != TRI_ERROR_FILE_EXISTS) {
      LOG(ERR) << "failed to create database directory: " << errorMessage;
    }
    return res;
  }

  TRI_IF_FAILURE("CreateDatabase::tempDirectory") { return TRI_ERROR_DEBUG; }

  std::string const tmpfile(
      arangodb::basics::FileUtils::buildFilename(tmpname, ".tmp"));
  res = TRI_WriteFile(tmpfile.c_str(), "", 0);

  TRI_IF_FAILURE("CreateDatabase::tempFile") { return TRI_ERROR_DEBUG; }

  if (res != TRI_ERROR_NO_ERROR) {
    TRI_RemoveDirectory(tmpname.c_str());
    return res;
  }

  // finally rename
  res = TRI_RenameFile(tmpname.c_str(), dirname.c_str());

  TRI_IF_FAILURE("CreateDatabase::renameDirectory") { return TRI_ERROR_DEBUG; }

  if (res != TRI_ERROR_NO_ERROR) {
    TRI_RemoveDirectory(tmpname.c_str());  // clean up
    return res;
  }

  // now everything is valid

  res = saveDatabaseParameters(id, name, false);

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

  // finally remove the .tmp file
  {
    std::string const tmpfile(arangodb::basics::FileUtils::buildFilename(dirname, ".tmp"));
    TRI_UnlinkFile(tmpfile.c_str());
  }

  return TRI_ERROR_NO_ERROR;
}

/// @brief save a parameter.json file for a database
int MMFilesEngine::saveDatabaseParameters(TRI_voc_tick_t id, 
                                          std::string const& name,
                                          bool deleted) {
  TRI_ASSERT(id > 0);
  TRI_ASSERT(!name.empty());

  VPackBuilder builder = databaseToVelocyPack(id, name, deleted);
  std::string const file = databaseParametersFilename(id);

  if (!arangodb::basics::VelocyPackHelper::velocyPackToFile(
          file, builder.slice(), true)) {
    LOG(ERR) << "cannot save database information in file '" << file << "'";
    return TRI_ERROR_INTERNAL;
  }

  return TRI_ERROR_NO_ERROR;
}
  
VPackBuilder MMFilesEngine::databaseToVelocyPack(TRI_voc_tick_t id, 
                                                 std::string const& name, 
                                                 bool deleted) const {
  TRI_ASSERT(id > 0);
  TRI_ASSERT(!name.empty());

  VPackBuilder builder;
  builder.openObject();
  builder.add("id", VPackValue(std::to_string(id)));
  builder.add("name", VPackValue(name));
  builder.add("deleted", VPackValue(deleted));
  builder.close();

  return builder;
}

std::string MMFilesEngine::databaseDirectory(TRI_voc_tick_t id) const {
  return _databasePath + "database-" + std::to_string(id);
}

std::string MMFilesEngine::databaseParametersFilename(TRI_voc_tick_t id) const {
  return basics::FileUtils::buildFilename(databaseDirectory(id), parametersFilename());
}

std::string MMFilesEngine::collectionDirectory(TRI_voc_tick_t databaseId, TRI_voc_cid_t id) const {
  auto it = _collectionPaths.find(databaseId);

  if (it == _collectionPaths.end()) {
    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, "trying to determine directory for unknown database"); 
  }

  auto it2 = (*it).second.find(id);

  if (it2 == (*it).second.end()) {
    THROW_ARANGO_EXCEPTION_MESSAGE(TRI_ERROR_INTERNAL, "trying to determine directory for unknown collection"); 
  }
  return (*it2).second;
}

/// @brief build a parameters filename (absolute path)
std::string MMFilesEngine::collectionParametersFilename(TRI_voc_tick_t databaseId, TRI_voc_cid_t id) const {
  return basics::FileUtils::buildFilename(collectionDirectory(databaseId, id), parametersFilename());
}

/// @brief build an index filename (absolute path)
std::string MMFilesEngine::indexFilename(TRI_voc_tick_t databaseId, TRI_voc_cid_t collectionId, TRI_idx_iid_t id) const {
  return basics::FileUtils::buildFilename(collectionDirectory(databaseId, collectionId), indexFilename(id));
}

/// @brief build an index filename (relative path)
std::string MMFilesEngine::indexFilename(TRI_idx_iid_t id) const {
  return std::string("index-") + std::to_string(id) + ".json";
}

/// @brief open an existing database. internal function
TRI_vocbase_t* MMFilesEngine::openExistingDatabase(TRI_voc_tick_t id, std::string const& name, 
                                                   bool wasCleanShutdown, bool isUpgrade) {
  auto vocbase = std::make_unique<TRI_vocbase_t>(TRI_VOCBASE_TYPE_NORMAL, id, name);

  // scan the database path for collections
  try {
    VPackBuilder builder;
    int res = getCollectionsAndIndexes(vocbase.get(), builder, wasCleanShutdown, isUpgrade);

    if (res != TRI_ERROR_NO_ERROR) {
      THROW_ARANGO_EXCEPTION(res);
    }
    
    VPackSlice slice = builder.slice();
    TRI_ASSERT(slice.isArray());

    for (auto const& it : VPackArrayIterator(slice)) {
      // we found a collection that is still active
      TRI_ASSERT(!it.get("id").isNone() || !it.get("cid").isNone());
      arangodb::LogicalCollection* collection = StorageEngine::registerCollection(ConditionalWriteLocker<ReadWriteLock>::DoLock(), vocbase.get(), it);

      registerCollectionPath(vocbase->id(), collection->cid(), collection->path());

      if (!wasCleanShutdown) {
        // iterating markers may be time-consuming. we'll only do it if
        // we have to
        LOG(WARN) << "no shutdown info found. scanning all collection markers in collection '" << collection->name() << "', database '" << vocbase->name() << "'";
        findMaxTickInJournals(collection->path());
      }

      LOG(DEBUG) << "added document collection '" << collection->name() << "'";
    }

    // start cleanup thread
    startCleanup(vocbase.get());
      
    return vocbase.release();
  } catch (std::exception const& ex) {
    LOG(ERR) << "error while opening database: " << ex.what();
    throw;
  } catch (...) {
    LOG(ERR) << "error while opening database: unknown exception";
    throw;
  }
}
      
/// @brief physically erases the database directory
int MMFilesEngine::dropDatabaseDirectory(std::string const& path) {
  return TRI_RemoveDirectory(path.c_str());
}

/// @brief iterate over a set of datafiles, identified by filenames
/// note: the files will be opened and closed
bool MMFilesEngine::iterateFiles(std::vector<std::string> const& files) {
  /// @brief this iterator is called on startup for journal and compactor file
  /// of a collection
  /// it will check the ticks of all markers and update the internal tick
  /// counter accordingly. this is done so we'll not re-assign an already used
  /// tick value
  auto cb = [this](TRI_df_marker_t const* marker, TRI_datafile_t* datafile) -> bool {
    TRI_voc_tick_t markerTick = marker->getTick();
  
    if (markerTick > _maxTick) {
      _maxTick = markerTick;
    }
    return true;
  };

  for (auto const& filename : files) {
    LOG(DEBUG) << "iterating over collection journal file '" << filename << "'";

    std::unique_ptr<TRI_datafile_t> datafile(TRI_datafile_t::open(filename, true));

    if (datafile != nullptr) {
      TRI_IterateDatafile(datafile.get(), cb);
    }
  }

  return true;
}

/// @brief iterate over the markers in the collection's journals
/// this function is called on server startup for all collections. we do this
/// to get the last tick used in a collection
bool MMFilesEngine::findMaxTickInJournals(std::string const& path) {
  LOG(TRACE) << "iterating ticks of journal '" << path << "'";
  MMFilesEngineCollectionFiles structure = scanCollectionDirectory(path);

  if (structure.journals.empty()) {
    // no journal found for collection. should not happen normally, but if
    // it does, we need to grab the ticks from the datafiles, too
    return iterateFiles(structure.datafiles);
  }

  // compactor files don't need to be iterated... they just contain data
  // copied from other files, so their tick values will never be any higher
  return iterateFiles(structure.journals);
}

/// @brief create a full directory name for a collection
std::string MMFilesEngine::createCollectionDirectoryName(std::string const& basePath, TRI_voc_cid_t cid) {
  std::string filename("collection-");
  filename.append(std::to_string(cid));
  filename.push_back('-');
  filename.append(std::to_string(RandomGenerator::interval(UINT32_MAX)));

  return arangodb::basics::FileUtils::buildFilename(basePath, filename);
}

void MMFilesEngine::registerCollectionPath(TRI_voc_tick_t databaseId, TRI_voc_cid_t id, std::string const& path) {
  auto it = _collectionPaths.find(databaseId);

  if (it == _collectionPaths.end()) {
    it = _collectionPaths.emplace(databaseId, std::unordered_map<TRI_voc_cid_t, std::string>()).first;
  }
  (*it).second[id] = path;
}

void MMFilesEngine::unregisterCollectionPath(TRI_voc_tick_t databaseId, TRI_voc_cid_t id) {
  auto it = _collectionPaths.find(databaseId);

  if (it == _collectionPaths.end()) {
    return;
  }
//  (*it).second.erase(id);
}

void MMFilesEngine::saveCollectionInfo(TRI_vocbase_t* vocbase,
                                       TRI_voc_cid_t id,
                                       arangodb::LogicalCollection const* parameters,
                                       bool forceSync) const {
  std::string const filename = collectionParametersFilename(vocbase->id(), id);

  VPackBuilder builder;
  parameters->toVelocyPack(builder, false);

  TRI_ASSERT(id != 0);

  bool ok = VelocyPackHelper::velocyPackToFile(filename,
                                               builder.slice(), forceSync);

  if (!ok) {
    int res = TRI_errno();
    THROW_ARANGO_EXCEPTION_MESSAGE(res, std::string("cannot save collection properties file '") + 
                                   filename + "': " + TRI_errno_string(res));
  }
}

LogicalCollection* MMFilesEngine::loadCollectionInfo(TRI_vocbase_t* vocbase, std::string const& path) {
  // find parameter file
  std::string filename =
      arangodb::basics::FileUtils::buildFilename(path, parametersFilename());

  if (!TRI_ExistsFile(filename.c_str())) {
    filename += ".tmp";  // try file with .tmp extension
    if (!TRI_ExistsFile(filename.c_str())) {
      THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_ILLEGAL_PARAMETER_FILE);
    }
  }

  std::shared_ptr<VPackBuilder> content =
      arangodb::basics::VelocyPackHelper::velocyPackFromFile(filename);
  VPackSlice slice = content->slice();
  if (!slice.isObject()) {
    LOG(ERR) << "cannot open '" << filename
             << "', collection parameters are not readable";
    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_ILLEGAL_PARAMETER_FILE);
  }

  if (filename.substr(filename.size() - 4, 4) == ".tmp") {
    // we got a tmp file. Now try saving the original file
    arangodb::basics::VelocyPackHelper::velocyPackToFile(filename.substr(0, filename.size() - 4),
                                                         slice, true);
  }

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

  VPackBuilder patch;
  patch.openObject();
  patch.add("isSystem", VPackValue(isSystemValue));
  patch.add("path", VPackValue(path));

  // auto-magic version detection to disambiguate collections from 3.0 and from 3.1
  if (slice.hasKey("version") && slice.get("version").isNumber() && 
      slice.get("version").getNumber<int>() == LogicalCollection::VERSION_30 &&
      slice.hasKey("allowUserKeys") && slice.hasKey("replicationFactor") && slice.hasKey("numberOfShards")) {
    // these attributes were added to parameter.json in 3.1. so this is a 3.1 collection already
    // fix version number
    patch.add("version", VPackValue(LogicalCollection::VERSION_31));
  }

  patch.close();
  VPackBuilder b2 = VPackCollection::merge(slice, patch.slice(), false);
  slice = b2.slice();

  return new LogicalCollection(vocbase, slice, true);
}

/// @brief remove data of expired compaction blockers
bool MMFilesEngine::cleanupCompactionBlockers(TRI_vocbase_t* vocbase) {
  // check if we can instantly acquire the lock
  TRY_WRITE_LOCKER(locker, _compactionBlockersLock);

  if (!locker.isLocked()) {
    // couldn't acquire lock
    return false;
  }

  auto it = _compactionBlockers.find(vocbase);

  if (it == _compactionBlockers.end()) {
    // no entry for this database
    return true;
  }

  // we are now holding the write lock
  double now = TRI_microtime();

  size_t n = (*it).second.size();

  for (size_t i = 0; i < n; /* no hoisting */) {
    auto& blocker = (*it).second[i];

    if (blocker._expires < now) {
      (*it).second.erase((*it).second.begin() + i);
      n--;
    } else {
      i++;
    }
  }

  if ((*it).second.empty()) {
    // remove last element
    _compactionBlockers.erase(it);
  }

  return true;
}

/// @brief insert a compaction blocker
int MMFilesEngine::insertCompactionBlocker(TRI_vocbase_t* vocbase, double ttl,
                                           TRI_voc_tick_t& id) {
  id = 0;

  if (ttl <= 0.0) {
    return TRI_ERROR_BAD_PARAMETER;
  }

  CompactionBlocker blocker(TRI_NewTickServer(), TRI_microtime() + ttl);

  {
    WRITE_LOCKER_EVENTUAL(locker, _compactionBlockersLock, 1000); 

    auto it = _compactionBlockers.find(vocbase);

    if (it == _compactionBlockers.end()) {
      it = _compactionBlockers.emplace(vocbase, std::vector<CompactionBlocker>()).first;
    }

    (*it).second.emplace_back(blocker);
  }

  id = blocker._id;

  return TRI_ERROR_NO_ERROR;
}

/// @brief touch an existing compaction blocker
int MMFilesEngine::extendCompactionBlocker(TRI_vocbase_t* vocbase, TRI_voc_tick_t id,
                                           double ttl) {
  if (ttl <= 0.0) {
    return TRI_ERROR_BAD_PARAMETER;
  }

  WRITE_LOCKER_EVENTUAL(locker, _compactionBlockersLock, 1000); 

  auto it = _compactionBlockers.find(vocbase);

  if (it == _compactionBlockers.end()) {
    return TRI_ERROR_ARANGO_DOCUMENT_NOT_FOUND;
  }

  for (auto& blocker : (*it).second) {
    if (blocker._id == id) {
      blocker._expires = TRI_microtime() + ttl;
      return TRI_ERROR_NO_ERROR;
    }
  }

  return TRI_ERROR_ARANGO_DOCUMENT_NOT_FOUND;
}

/// @brief remove an existing compaction blocker
int MMFilesEngine::removeCompactionBlocker(TRI_vocbase_t* vocbase,
                                           TRI_voc_tick_t id) {
  WRITE_LOCKER_EVENTUAL(locker, _compactionBlockersLock, 1000); 
  
  auto it = _compactionBlockers.find(vocbase);

  if (it == _compactionBlockers.end()) {
    return TRI_ERROR_ARANGO_DOCUMENT_NOT_FOUND;
  }

  size_t const n = (*it).second.size();

  for (size_t i = 0; i < n; ++i) {
    auto& blocker = (*it).second[i];
    if (blocker._id == id) {
      (*it).second.erase((*it).second.begin() + i);

      if ((*it).second.empty()) {
        // remove last item
        _compactionBlockers.erase(it);
      }
      return TRI_ERROR_NO_ERROR;
    }
  }

  return TRI_ERROR_ARANGO_DOCUMENT_NOT_FOUND;
}

void MMFilesEngine::preventCompaction(TRI_vocbase_t* vocbase,
                                      std::function<void(TRI_vocbase_t*)> const& callback) {
  WRITE_LOCKER_EVENTUAL(locker, _compactionBlockersLock, 5000);
  callback(vocbase);
}

bool MMFilesEngine::tryPreventCompaction(TRI_vocbase_t* vocbase,
                                         std::function<void(TRI_vocbase_t*)> const& callback,
                                         bool checkForActiveBlockers) {
  TRY_WRITE_LOCKER(locker, _compactionBlockersLock);

  if (locker.isLocked()) {
    if (checkForActiveBlockers) {
      double const now = TRI_microtime();

      // check if we have a still-valid compaction blocker
      auto it = _compactionBlockers.find(vocbase);

      if (it != _compactionBlockers.end()) {
        for (auto const& blocker : (*it).second) {
          if (blocker._expires > now) {
            // found a compaction blocker
            return false;
          }
        }
      }
    }
    callback(vocbase);
    return true;
  }
  return false;
}

int MMFilesEngine::shutdownDatabase(TRI_vocbase_t* vocbase) {
  try {
    stopCompactor(vocbase);
    return stopCleanup(vocbase);
  } catch (basics::Exception const& ex) {
    return ex.code();
  } catch (...) {
    return TRI_ERROR_INTERNAL;
  }
}

// start the cleanup thread for the database 
int MMFilesEngine::startCleanup(TRI_vocbase_t* vocbase) {
  std::unique_ptr<MMFilesCleanupThread> thread;

  {
    MUTEX_LOCKER(locker, _threadsLock);

    thread.reset(new MMFilesCleanupThread(vocbase));
    _cleanupThreads.emplace(vocbase, thread.get());
  }
  
  TRI_ASSERT(thread != nullptr);

  if (!thread->start()) {
    LOG(ERR) << "could not start cleanup thread";
    THROW_ARANGO_EXCEPTION(TRI_ERROR_OUT_OF_MEMORY);
  }
  
  thread.release();
  return TRI_ERROR_NO_ERROR;
}

// stop and delete the cleanup thread for the database
int MMFilesEngine::stopCleanup(TRI_vocbase_t* vocbase) {
  MMFilesCleanupThread* thread = nullptr;
  
  {
    MUTEX_LOCKER(locker, _threadsLock);

    auto it = _cleanupThreads.find(vocbase);
    
    if (it == _cleanupThreads.end()) {
      // already stopped
      return TRI_ERROR_NO_ERROR;
    }

    thread = (*it).second;
    _cleanupThreads.erase(it);
  }

  TRI_ASSERT(thread != nullptr);

  thread->beginShutdown();
  thread->signal();

  while (thread->isRunning()) {
    usleep(5000);
  }

  delete thread;

  return TRI_ERROR_NO_ERROR;
}

// start the compactor thread for the database 
int MMFilesEngine::startCompactor(TRI_vocbase_t* vocbase) {
  std::unique_ptr<MMFilesCompactorThread> thread;

  {
    MUTEX_LOCKER(locker, _threadsLock);
    auto it = _compactorThreads.find(vocbase);

    if (it != _compactorThreads.end()) {
      return TRI_ERROR_INTERNAL;
    }

    thread.reset(new MMFilesCompactorThread(vocbase));
    _compactorThreads.emplace(vocbase, thread.get());
  }

  TRI_ASSERT(thread != nullptr);

  if (!thread->start()) {
    LOG(ERR) << "could not start compactor thread";
    THROW_ARANGO_EXCEPTION(TRI_ERROR_OUT_OF_MEMORY);
  }

  thread.release();
  return TRI_ERROR_NO_ERROR;
}

// signal the compactor thread to stop
int MMFilesEngine::beginShutdownCompactor(TRI_vocbase_t* vocbase) {
  MMFilesCompactorThread* thread = nullptr;
  
  {
    MUTEX_LOCKER(locker, _threadsLock);

    auto it = _compactorThreads.find(vocbase);
    
    if (it == _compactorThreads.end()) {
      // already stopped
      return TRI_ERROR_NO_ERROR;
    }

    thread = (*it).second;
  }

  TRI_ASSERT(thread != nullptr);

  thread->beginShutdown();
  thread->signal();
  
  return TRI_ERROR_NO_ERROR;
}

// stop and delete the compactor thread for the database
int MMFilesEngine::stopCompactor(TRI_vocbase_t* vocbase) {
  MMFilesCompactorThread* thread = nullptr;
  
  {
    MUTEX_LOCKER(locker, _threadsLock);

    auto it = _compactorThreads.find(vocbase);
    
    if (it == _compactorThreads.end()) {
      // already stopped
      return TRI_ERROR_NO_ERROR;
    }

    thread = (*it).second;
    _compactorThreads.erase(it);
  }

  TRI_ASSERT(thread != nullptr);

  thread->beginShutdown();
  thread->signal();

  while (thread->isRunning()) {
    usleep(5000);
  }

  delete thread;
  
  return TRI_ERROR_NO_ERROR;
}

/// @brief checks a collection
int MMFilesEngine::openCollection(TRI_vocbase_t* vocbase, LogicalCollection* collection, bool ignoreErrors) {
  LOG_TOPIC(TRACE, Logger::DATAFILES) << "check collection directory '"
                                      << collection->path() << "'";

  std::vector<TRI_datafile_t*> all;
  std::vector<TRI_datafile_t*> compactors;
  std::vector<TRI_datafile_t*> datafiles;
  std::vector<TRI_datafile_t*> journals;
  std::vector<TRI_datafile_t*> sealed;
  bool stop = false;

  TRI_ASSERT(collection->cid() != 0);

  // check files within the directory
  std::vector<std::string> files = TRI_FilesDirectory(collection->path().c_str());

  for (auto const& file : files) {
    std::vector<std::string> parts = StringUtils::split(file, '.');

    if (parts.size() < 2 || parts.size() > 3 || parts[0].empty()) {
      LOG_TOPIC(DEBUG, Logger::DATAFILES)
          << "ignoring file '" << file
          << "' because it does not look like a datafile";
      continue;
    }

    std::string extension = parts[1];
    std::string isDead = (parts.size() > 2) ? parts[2] : "";

    std::vector<std::string> next = StringUtils::split(parts[0], "-");

    if (next.size() < 2) {
      LOG_TOPIC(DEBUG, Logger::DATAFILES)
          << "ignoring file '" << file
          << "' because it does not look like a datafile";
      continue;
    }

    std::string filename =
        FileUtils::buildFilename(collection->path(), file);
    std::string filetype = next[0];
    next.erase(next.begin());
    std::string qualifier = StringUtils::join(next, '-');

    // .............................................................................
    // file is dead
    // .............................................................................

    if (!isDead.empty() || filetype == "temp") {
      if (isDead == "dead" || filetype == "temp") {
        LOG_TOPIC(TRACE, Logger::DATAFILES)
            << "found temporary file '" << filename
            << "', which is probably a left-over. deleting it";
        FileUtils::remove(filename);
        continue;
      } else {
        LOG_TOPIC(DEBUG, Logger::DATAFILES)
            << "ignoring file '" << file
            << "' because it does not look like a datafile";
        continue;
      }
    }

    // file is an index. indexes are handled elsewhere
    if (filetype == "index" && extension == "json") {
      continue;
    }

    // file is a journal or datafile, open the datafile
    if (extension == "db") {
      // found a compaction file. now rename it back
      if (filetype == "compaction") {
        std::string relName = "datafile-" + qualifier + "." + extension;
        std::string newName =
            FileUtils::buildFilename(collection->path(), relName);

        if (FileUtils::exists(newName)) {
          // we have a compaction-xxxx and a datafile-xxxx file. we'll keep
          // the datafile
          FileUtils::remove(filename);

          LOG_TOPIC(WARN, Logger::DATAFILES)
              << "removing unfinished compaction file '" << filename << "'";
          continue;
        } else {
          // this should fail, but shouldn't do any harm either...
          FileUtils::remove(newName);

          int res = TRI_RenameFile(filename.c_str(), newName.c_str());

          if (res != TRI_ERROR_NO_ERROR) {
            LOG_TOPIC(ERR, Logger::DATAFILES)
                << "unable to rename compaction file '" << filename << "' to '"
                << newName << "'";
            stop = true;
            break;
          }
        }

        // reuse newName
        filename = newName;
      }

      std::unique_ptr<TRI_datafile_t> df(TRI_datafile_t::open(filename, ignoreErrors));

      if (df == nullptr) {
        LOG_TOPIC(ERR, Logger::DATAFILES) << "cannot open datafile '"
                                          << filename
                                          << "': " << TRI_last_error();

        stop = true;
        break;
      }

      all.emplace_back(df.get());
      TRI_datafile_t* datafile = df.release();

      // check the document header
      char const* ptr = datafile->_data;

      // skip the datafile header
      ptr +=
          DatafileHelper::AlignedSize<size_t>(sizeof(TRI_df_header_marker_t));
      TRI_col_header_marker_t const* cm =
          reinterpret_cast<TRI_col_header_marker_t const*>(ptr);

      if (cm->base.getType() != TRI_DF_MARKER_COL_HEADER) {
        LOG(ERR) << "collection header mismatch in file '" << filename
                 << "', expected TRI_DF_MARKER_COL_HEADER, found "
                 << cm->base.getType();

        stop = true;
        break;
      }

      if (cm->_cid != collection->cid()) {
        LOG(ERR) << "collection identifier mismatch, expected "
                 << collection->cid() << ", found " << cm->_cid;

        stop = true;
        break;
      }

      // file is a journal
      if (filetype == "journal") {
        if (datafile->_isSealed) {
          if (datafile->state() != TRI_DF_STATE_READ) {
            LOG_TOPIC(WARN, Logger::DATAFILES)
                << "strange, journal '" << filename
                << "' is already sealed; must be a left over; will use "
                   "it as datafile";
          }

          sealed.emplace_back(datafile);
        } else {
          journals.emplace_back(datafile);
        }
      }

      // file is a compactor
      else if (filetype == "compactor") {
        // ignore
      }

      // file is a datafile (or was a compaction file)
      else if (filetype == "datafile" || filetype == "compaction") {
        if (!datafile->_isSealed) {
          LOG_TOPIC(ERR, Logger::DATAFILES)
              << "datafile '" << filename
              << "' is not sealed, this should never happen";
          stop = true;
          break;
        } else {
          datafiles.emplace_back(datafile);
        }
      }

      else {
        LOG_TOPIC(ERR, Logger::DATAFILES) << "unknown datafile '" << file
                                          << "'";
      }
    } else {
      LOG_TOPIC(ERR, Logger::DATAFILES) << "unknown datafile '" << file << "'";
    }
  }

  // convert the sealed journals into datafiles
  if (!stop) {
    for (auto& datafile : sealed) {
      std::string dname("datafile-" + std::to_string(datafile->fid()) + ".db");
      std::string filename = arangodb::basics::FileUtils::buildFilename(collection->path(), dname);

      int res = datafile->rename(filename);

      if (res == TRI_ERROR_NO_ERROR) {
        datafiles.emplace_back(datafile);
        LOG(DEBUG) << "renamed sealed journal to '" << filename << "'";
      } else {
        stop = true;
        LOG(ERR) << "cannot rename sealed log-file to " << filename
                 << ", this should not happen: " << TRI_errno_string(res);
        break;
      }
    }
  }

  // stop if necessary
  if (stop) {
    for (auto& datafile : all) {
      LOG(TRACE) << "closing datafile '" << datafile->getName() << "'";
      delete datafile;
    }

    return TRI_ERROR_INTERNAL;
  }

  // sort the datafiles
  // this allows us to iterate them in the correct order
  std::sort(datafiles.begin(), datafiles.end(), DatafileComparator());
  std::sort(journals.begin(), journals.end(), DatafileComparator());
  std::sort(compactors.begin(), compactors.end(), DatafileComparator());

  MMFilesCollection* physical = static_cast<MMFilesCollection*>(collection->getPhysical());
  // add the datafiles and journals
  physical->_datafiles = datafiles;
  physical->_journals = journals;
  physical->_compactors = compactors;

  return TRI_ERROR_NO_ERROR;
}

/// @brief transfer markers into a collection, actual work
/// the collection must have been prepared to call this function
int MMFilesEngine::transferMarkers(LogicalCollection* collection,
                                   wal::CollectorCache* cache,
                                   wal::OperationsType const& operations) {
  int res = transferMarkersWorker(collection, cache, operations);
    
  TRI_IF_FAILURE("transferMarkersCrash") {
    // intentionally kill the server
    TRI_SegfaultDebugging("CollectorThreadTransfer");
  }
    
  if (res == TRI_ERROR_NO_ERROR && !cache->operations->empty()) {
    // now sync the datafile
    res = syncJournalCollection(collection);

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

  return res;
}

/// @brief transfer markers into a collection, actual work
/// the collection must have been prepared to call this function
int MMFilesEngine::transferMarkersWorker(LogicalCollection* collection,
                                         wal::CollectorCache* cache,
                                         wal::OperationsType const& operations) {
  // used only for crash / recovery tests
  int numMarkers = 0;

  TRI_voc_tick_t const minTransferTick = collection->maxTick();
  TRI_ASSERT(!operations.empty());

  for (auto it2 = operations.begin(); it2 != operations.end(); ++it2) {
    TRI_df_marker_t const* source = (*it2);
    TRI_voc_tick_t const tick = source->getTick();

    if (tick <= minTransferTick) {
      // we have already transferred this marker in a previous run, nothing to
      // do
      continue;
    }

    TRI_IF_FAILURE("CollectorThreadTransfer") {
      if (++numMarkers > 5) {
        // intentionally kill the server
        TRI_SegfaultDebugging("CollectorThreadTransfer");
      }
    }

    TRI_df_marker_type_t const type = source->getType();

    if (type == TRI_DF_MARKER_VPACK_DOCUMENT ||
        type == TRI_DF_MARKER_VPACK_REMOVE) {
      TRI_voc_size_t const size = source->getSize();

      char* dst = nextFreeMarkerPosition(collection, tick, type, size, cache);

      if (dst == nullptr) {
        return TRI_ERROR_OUT_OF_MEMORY;
      }

      auto& dfi = cache->getDfi(cache->lastFid);
      dfi.numberUncollected++;

      memcpy(dst, source, size);

      finishMarker(reinterpret_cast<char const*>(source), dst, collection, tick, cache);
    }
  }

  TRI_IF_FAILURE("CollectorThreadTransferFinal") {
    // intentionally kill the server
    TRI_SegfaultDebugging("CollectorThreadTransferFinal");
  }

  return TRI_ERROR_NO_ERROR;
}

/// @brief get the next position for a marker of the specified size
char* MMFilesEngine::nextFreeMarkerPosition(
    LogicalCollection* collection, TRI_voc_tick_t tick,
    TRI_df_marker_type_t type, TRI_voc_size_t size, wal::CollectorCache* cache) {
  
  // align the specified size
  size = DatafileHelper::AlignedSize<TRI_voc_size_t>(size);

  char* dst = nullptr; // will be modified by reserveJournalSpace()
  TRI_datafile_t* datafile = nullptr; // will be modified by reserveJournalSpace()
  int res = static_cast<MMFilesCollection*>(collection->getPhysical())->reserveJournalSpace(tick, size, dst, datafile);

  if (res != TRI_ERROR_NO_ERROR) {
    // could not reserve space, for whatever reason
    THROW_ARANGO_EXCEPTION(TRI_ERROR_ARANGO_NO_JOURNAL);
  }

  // if we get here, we successfully reserved space in the datafile

  TRI_ASSERT(datafile != nullptr);

  if (cache->lastFid != datafile->fid()) {
    if (cache->lastFid > 0) {
      // rotated the existing journal... now update the old journal's stats
      auto& dfi = cache->createDfi(cache->lastFid);
      static_cast<MMFilesCollection*>(collection->getPhysical())->_datafileStatistics.increaseUncollected(cache->lastFid, dfi.numberUncollected);
      // and reset them afterwards
      dfi.numberUncollected = 0;
    }
 
    // reset datafile in cache   
    cache->lastDatafile = datafile;
    cache->lastFid = datafile->fid();
    
    // create a local datafile info struct
    cache->createDfi(datafile->fid());

    // we only need the ditches when we are outside the recovery
    // the compactor will not run during recovery
    auto ditch =
        collection->ditches()->createDocumentDitch(false, __FILE__, __LINE__);

    if (ditch == nullptr) {
      THROW_ARANGO_EXCEPTION(TRI_ERROR_OUT_OF_MEMORY);
    }

    cache->addDitch(ditch);
  }
  
  TRI_ASSERT(dst != nullptr);
  
  DatafileHelper::InitMarker(reinterpret_cast<TRI_df_marker_t*>(dst), type, size);

  return dst;
}

/// @brief set the tick of a marker and calculate its CRC value
void MMFilesEngine::finishMarker(char const* walPosition,
                                 char* datafilePosition,
                                 LogicalCollection* collection,
                                 TRI_voc_tick_t tick, wal::CollectorCache* cache) {
  TRI_df_marker_t* marker =
      reinterpret_cast<TRI_df_marker_t*>(datafilePosition);

  TRI_datafile_t* datafile = cache->lastDatafile;
  TRI_ASSERT(datafile != nullptr);

  // update ticks
  TRI_UpdateTicksDatafile(datafile, marker);

  TRI_ASSERT(collection->maxTick() < tick);
  collection->maxTick(tick);

  cache->operations->emplace_back(wal::CollectorOperation(
      datafilePosition, marker->getSize(), walPosition, cache->lastFid));
}

/// @brief sync all journals of a collection
int MMFilesEngine::syncJournalCollection(LogicalCollection* collection) {
  TRI_IF_FAILURE("CollectorThread::syncDatafileCollection") {
    return TRI_ERROR_DEBUG;
  }

  return static_cast<MMFilesCollection*>(collection->getPhysical())->syncActiveJournal();
}