//////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2017 EMC Corporation
///
/// 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 EMC Corporation
///
/// @author Andrey Abramov
/// @author Vasiliy Nabatchikov
////////////////////////////////////////////////////////////////////////////////

#include <array>

#include "formats/formats.hpp"
#include "search/all_filter.hpp"
#include "store/memory_directory.hpp"
#include "store/mmap_directory.hpp"
#include "utils/directory_utils.hpp"
#include "utils/memory.hpp"
#include "utils/misc.hpp"

#include "IResearchCommon.h"
#include "IResearchDocument.h"
#include "IResearchFeature.h"
#include "IResearchFilterFactory.h"
#include "IResearchLink.h"
#include "IResearchLinkHelper.h"
#include "IResearchPrimaryKeyFilter.h"

#include "Aql/AstNode.h"
#include "Aql/QueryCache.h"
#include "Logger/LogMacros.h"
#include "StorageEngine/EngineSelectorFeature.h"
#include "StorageEngine/TransactionState.h"
#include "StorageEngine/StorageEngine.h"
#include "RestServer/DatabaseFeature.h"
#include "RestServer/DatabasePathFeature.h"
#include "RestServer/FlushFeature.h"
#include "RestServer/ViewTypesFeature.h"
#include "Transaction/Methods.h"
#include "Transaction/StandaloneContext.h"
#include "Utils/ExecContext.h"
#include "velocypack/Iterator.h"
#include "VocBase/LogicalCollection.h"
#include "VocBase/vocbase.h"

#include "IResearchView.h"

namespace {

////////////////////////////////////////////////////////////////////////////////
/// @brief surrogate root for all queries without a filter
////////////////////////////////////////////////////////////////////////////////
arangodb::aql::AstNode ALL(arangodb::aql::AstNodeValue(true));

////////////////////////////////////////////////////////////////////////////////
/// @brief the storage format used with iResearch writers
////////////////////////////////////////////////////////////////////////////////
const irs::string_ref IRESEARCH_STORE_FORMAT("1_0");

typedef irs::async_utils::read_write_mutex::read_mutex ReadMutex;
typedef irs::async_utils::read_write_mutex::write_mutex WriteMutex;

////////////////////////////////////////////////////////////////////////////////
/// --SECTION--                                               utility constructs
////////////////////////////////////////////////////////////////////////////////

////////////////////////////////////////////////////////////////////////////////
/// @brief index reader implementation over multiple directory readers
////////////////////////////////////////////////////////////////////////////////
class CompoundReader final: public arangodb::iresearch::PrimaryKeyIndexReader {
 public:
  explicit CompoundReader(ReadMutex& viewMutex) noexcept
    : _viewLock(viewMutex) {
  }
  irs::sub_reader const& operator[](
      size_t subReaderId
  ) const noexcept override {
    return *(_subReaders[subReaderId].first);
  }

  void add(irs::directory_reader const& reader);
  virtual reader_iterator begin() const override {
    return reader_iterator(new IteratorImpl(_subReaders.begin()));
  }
  virtual uint64_t docs_count() const override;
  virtual reader_iterator end() const override {
    return reader_iterator(new IteratorImpl(_subReaders.end()));
  }
  virtual uint64_t live_docs_count() const override;

  irs::columnstore_reader::values_reader_f const& pkColumn(
      size_t subReaderId
  ) const noexcept override {
    return _subReaders[subReaderId].second;
  }

  virtual size_t size() const noexcept override { return _subReaders.size(); }

  void clear() noexcept {
    _subReaders.clear();
    _readers.clear();
  }

 private:
  typedef std::vector<
    std::pair<irs::sub_reader*, irs::columnstore_reader::values_reader_f>
  > SubReadersType;

  class IteratorImpl final: public irs::index_reader::reader_iterator_impl {
   public:
    explicit IteratorImpl(SubReadersType::const_iterator const& itr) noexcept
      : _itr(itr) {
    }

    virtual void operator++() noexcept override { ++_itr; }
    virtual reference operator*() noexcept override { return *(_itr->first); }

    virtual const_reference operator*() const noexcept override {
      return *(_itr->first);
    }

    virtual bool operator==(
        const reader_iterator_impl& other
    ) noexcept override {
      return static_cast<IteratorImpl const&>(other)._itr == _itr;
    }

   private:
    SubReadersType::const_iterator _itr;
  };

  std::vector<irs::directory_reader> _readers;
  SubReadersType _subReaders;
  std::lock_guard<ReadMutex> _viewLock; // prevent data-store deallocation (lock @ AsyncSelf)
};

void CompoundReader::add(irs::directory_reader const& reader) {
  _readers.emplace_back(reader);

  for(auto& entry: _readers.back()) {
    const auto* pkColumn =
      entry.column_reader(arangodb::iresearch::DocumentPrimaryKey::PK());

    if (!pkColumn) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "encountered a sub-reader without a primary key column while creating a reader for arangosearch view, ignoring";

      continue;
    }

    _subReaders.emplace_back(&entry, pkColumn->values());
  }
}

uint64_t CompoundReader::docs_count() const {
  uint64_t count = 0;

  for (auto& entry: _subReaders) {
    count += entry.first->docs_count();
  }

  return count;
}

uint64_t CompoundReader::live_docs_count() const {
  uint64_t count = 0;

  for (auto& entry: _subReaders) {
    count += entry.first->live_docs_count();
  }

  return count;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief generates user-friendly description of the specified view
////////////////////////////////////////////////////////////////////////////////
std::string toString(arangodb::iresearch::IResearchView const& view) {
  std::string str(arangodb::iresearch::DATA_SOURCE_TYPE.name());

  str += ":";
  str += std::to_string(view.id());

  return str;
}

////////////////////////////////////////////////////////////////////////////////
/// @returns 'Flush' feature from AppicationServer
////////////////////////////////////////////////////////////////////////////////
inline arangodb::FlushFeature* getFlushFeature() noexcept {
  return arangodb::application_features::ApplicationServer::lookupFeature<
    arangodb::FlushFeature
  >("Flush");
}

// @brief approximate iResearch directory instance size
size_t directoryMemory(irs::directory const& directory, TRI_voc_cid_t viewId) noexcept {
  size_t size = 0;

  try {
    directory.visit([&directory, &size](std::string& file)->bool {
      uint64_t length;

      size += directory.length(length, file) ? length : 0;

      return true;
    });
  } catch (arangodb::basics::Exception& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught error while calculating size of arangosearch view '" << viewId << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();
  } catch (std::exception const& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught error while calculating size of arangosearch view '" << viewId << "': " << e.what();
    IR_LOG_EXCEPTION();
  } catch (...) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught error while calculating size of arangosearch view '" << viewId << "'";
    IR_LOG_EXCEPTION();
  }

  return size;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief compute the data path to user for iresearch persisted-store
///        get base path from DatabaseServerFeature (similar to MMFilesEngine)
///        the path is hardcoded to reside under:
///        <DatabasePath>/<IResearchView::type()>-<view id>
///        similar to the data path calculation for collections
////////////////////////////////////////////////////////////////////////////////
irs::utf8_path getPersistedPath(
    arangodb::DatabasePathFeature const& dbPathFeature,
    TRI_vocbase_t const& vocbase,
    TRI_voc_cid_t id
) {
  irs::utf8_path dataPath(dbPathFeature.directory());
  static const std::string subPath("databases");
  static const std::string dbPath("database-");

  dataPath /= subPath;
  dataPath /= dbPath;
  dataPath += std::to_string(vocbase.id());
  dataPath /= arangodb::iresearch::DATA_SOURCE_TYPE.name();
  dataPath += "-";
  dataPath += std::to_string(id);

  return dataPath;
}

////////////////////////////////////////////////////////////////////////////////
/// @brief inserts ArangoDB document into an IResearch data store
////////////////////////////////////////////////////////////////////////////////
inline void insertDocument(
    irs::segment_writer::document& doc,
    arangodb::iresearch::FieldIterator& body,
    TRI_voc_cid_t cid,
    TRI_voc_rid_t rid) {
  using namespace arangodb::iresearch;

  // reuse the 'Field' instance stored
  // inside the 'FieldIterator' after
  auto& field = const_cast<Field&>(*body);

  // User fields
  while (body.valid()) {
    if (ValueStorage::NONE == field._storeValues) {
      doc.insert(irs::action::index, field);
    } else {
      doc.insert(irs::action::index_store, field);
    }

    ++body;
  }

  // System fields
  DocumentPrimaryKey const primaryKey(cid, rid);

  // Indexed and Stored: CID + RID
  Field::setPkValue(field, primaryKey, Field::init_stream_t());
  doc.insert(irs::action::index_store, field);

  // Indexed: CID
  Field::setCidValue(field, primaryKey.cid());
  doc.insert(irs::action::index, field);
}

////////////////////////////////////////////////////////////////////////////////
/// @brief persist view definition to the storage engine
///        if in-recovery then register a post-recovery lambda for persistence
/// @return success
////////////////////////////////////////////////////////////////////////////////
arangodb::Result persistProperties(
    arangodb::LogicalView const& view,
    arangodb::iresearch::IResearchView::AsyncSelf::ptr asyncSelf
) {
  auto* engine = arangodb::EngineSelectorFeature::ENGINE;

  if (!engine) {
    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("failure to get storage engine while persisting definition for LogicalView '") + view.name() + "'"
    );
  }

  if (!engine->inRecovery()) {
    // change view throws exception on error
    try {
      engine->changeView(view.vocbase(), view, true);
    } catch (arangodb::basics::Exception& e) {
      IR_LOG_EXCEPTION();

      return arangodb::Result(
        e.code(),
        std::string("caught exception during persistance of properties for arangosearch View '") + view.name() + "': " + e.what()
      );
    } catch (std::exception const& e) {
      IR_LOG_EXCEPTION();

      return arangodb::Result(
        TRI_ERROR_INTERNAL,
        std::string("caught exception during persistance of properties for arangosearch View '") + view.name() + "': " + e.what()
      );
    } catch (...) {
      IR_LOG_EXCEPTION();

      return arangodb::Result(
        TRI_ERROR_INTERNAL,
        std::string("caught exception during persistance of properties for arangosearch View '") + view.name() + "'"
      );
    }

    return arangodb::Result();
  }

  auto* feature = arangodb::application_features::ApplicationServer::lookupFeature<
    arangodb::DatabaseFeature
  >("Database");

  if (!feature) {
    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("failure to get 'Database' feature while persisting definition for LogicalView '") + view.name() + "'"
    );
  }

  return feature->registerPostRecoveryCallback(
    [&view, asyncSelf]()->arangodb::Result {
      auto* engine = arangodb::EngineSelectorFeature::ENGINE;

      if (!engine) {
        return arangodb::Result(
          TRI_ERROR_INTERNAL,
          std::string("failure to get storage engine while persisting definition for LogicalView")
        );
      }

      if (!asyncSelf) {
        return arangodb::Result(
          TRI_ERROR_INTERNAL,
          std::string("invalid view instance passed while persisting definition for LogicalView")
        );
      }

      SCOPED_LOCK(asyncSelf->mutex());

      if (!asyncSelf->get()) {
        LOG_TOPIC(INFO, arangodb::iresearch::TOPIC)
          << "no view instance available while persisting definition for LogicalView";

        return arangodb::Result(); // nothing to persist, view allready deallocated
      }

      // change view throws exception on error
      try {
        engine->changeView(view.vocbase(), view, true);
      } catch (arangodb::basics::Exception& e) {
        IR_LOG_EXCEPTION();

        return arangodb::Result(
          e.code(),
          std::string("caught exception during persistance of properties for arangosearch View '") + view.name() + "': " + e.what()
        );
      } catch (std::exception const& e) {
        IR_LOG_EXCEPTION();

        return arangodb::Result(
          TRI_ERROR_INTERNAL,
          std::string("caught exception during persistance of properties for arangosearch View '") + view.name() + "': " + e.what()
        );
      } catch (...) {
        IR_LOG_EXCEPTION();

        return arangodb::Result(
          TRI_ERROR_INTERNAL,
          std::string("caught exception during persistance of properties for arangosearch View '") + view.name() + "'"
        );
      }

      return arangodb::Result();
    }
  );
}

////////////////////////////////////////////////////////////////////////////////
/// @brief consolidates and optionally cleanups an IResearch DataStore if required
/// @return at least consolidation was executed
////////////////////////////////////////////////////////////////////////////////
bool consolidateCleanupStore(
    irs::directory& directory,
    irs::index_writer& writer,
    arangodb::iresearch::IResearchViewMeta::ConsolidationPolicy const& policy,
    irs::merge_writer::flush_progress_t const& progress,
    bool runCleanupAfterConsolidation,
    arangodb::iresearch::IResearchView const& view,
    const char* storeName
) {
  char runId = 0; // value not used

  // ...........................................................................
  // apply consolidation policy
  // ...........................................................................

  // skip if interval not reached or no valid policy to execute
  if (policy.policy()) {
    LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
      << "registering consolidation policy '" << policy.properties().toString() << "' for store '" << storeName << "' with arangosearch view '" << view.name() << "' run id '" << size_t(&runId) << "'";

    try {
      writer.consolidate(policy.policy(), nullptr, progress);
    } catch (arangodb::basics::Exception const& e) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception during registration of consolidation policy '" << policy.properties().toString() << "' for store '" << storeName << "' with arangosearch view '" << view.name() << "': " << e.code() << " " << e.what();
      IR_LOG_EXCEPTION();
    } catch (std::exception const& e) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception during registration of consolidation policy '" << policy.properties().toString() << "' for store '" << storeName << "' with arangosearch view '" << view.name() << "': " << e.what();
      IR_LOG_EXCEPTION();
    } catch (...) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception during registration of consolidation policy '" << policy.properties().toString() << "' for store '" << storeName << "' with arangosearch view '" << view.name() << "'";
      IR_LOG_EXCEPTION();
    }

    LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
      << "finished registering consolidation policy '" << policy.properties().toString() << "' for store '" << storeName << "' with arangosearch view '" << view.name() << "' run id '" << size_t(&runId) << "'";
  }

  if (!runCleanupAfterConsolidation) {
    return true; // commit done
  }

  // ...........................................................................
  // apply cleanup
  // ...........................................................................

  LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
    << "starting '" << storeName << "' store cleanup for arangosearch view '" << view.name() << "' run id '" << size_t(&runId) << "'";

  try {
    irs::directory_utils::remove_all_unreferenced(directory);
  } catch (arangodb::basics::Exception& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception during cleanup of '" << storeName << "' store of arangosearch view '" << view.name() << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();
  } catch (std::exception const& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception during cleanup of '" << storeName << "' store of arangosearch view '" << view.name() << "': " << e.what();
    IR_LOG_EXCEPTION();
  } catch (...) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception during cleanup of '" << storeName << "' of arangosearch view '" << view.name() << "'";
    IR_LOG_EXCEPTION();
  }

  LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
    << "finished '" << storeName << "' store cleanup for arangosearch view '" << view.name() << "' run id '" << size_t(&runId) << "'";

  return true;
}

}

namespace arangodb {
namespace iresearch {

///////////////////////////////////////////////////////////////////////////////
/// --SECTION--                                    IResearchView implementation
///////////////////////////////////////////////////////////////////////////////

IResearchView::DataStore::DataStore(DataStore&& other) noexcept {
  *this = std::move(other);
}

IResearchView::DataStore& IResearchView::DataStore::operator=(
    IResearchView::DataStore&& other
) noexcept {
  if (this != &other) {
    _directory = std::move(other._directory);
    _reader = std::move(other._reader);
    _writer = std::move(other._writer);
  }

  return *this;
}

void IResearchView::DataStore::sync() {
  TRI_ASSERT(_writer && _reader);
  _writer->commit();
  _reader = _reader.reopen(); // update reader
}

IResearchView::PersistedStore::PersistedStore(irs::utf8_path&& path)
  : _path(std::move(path)) {
}

////////////////////////////////////////////////////////////////////////////////
/// @brief IResearchView-specific implementation of a ViewFactory
////////////////////////////////////////////////////////////////////////////////
struct IResearchView::ViewFactory: public arangodb::ViewFactory {
  virtual arangodb::Result create(
      arangodb::LogicalView::ptr& view,
      TRI_vocbase_t& vocbase,
      arangodb::velocypack::Slice const& definition
  ) const override {
    auto* engine = arangodb::EngineSelectorFeature::ENGINE;
    auto& properties = definition.isObject() ? definition : emptyObjectSlice(); // if no 'info' then assume defaults
    auto links = properties.hasKey(StaticStrings::LinksField)
               ? properties.get(StaticStrings::LinksField)
               : arangodb::velocypack::Slice::emptyObjectSlice();
    auto res = engine && engine->inRecovery()
             ? arangodb::Result() // do not validate if in recovery
             : IResearchLinkHelper::validateLinks(vocbase, links);

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

    TRI_set_errno(TRI_ERROR_NO_ERROR); // reset before calling createView(...)
    auto impl = vocbase.createView(definition);

    if (!impl) {
      return arangodb::Result(
        TRI_ERROR_NO_ERROR == TRI_errno() ? TRI_ERROR_INTERNAL : TRI_errno(),
        std::string("failure during instantiation while creating arangosearch View in database '") + vocbase.name() + "'"
      );
    }

    // create links on a best-effor basis
    // link creation failure does not cause view creation failure
    try {
      std::unordered_set<TRI_voc_cid_t> collections;

      res = IResearchLinkHelper::updateLinks(
        collections, vocbase, *impl, links
      );

      if (!res.ok()) {
        LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
          << "failed to create links while creating arangosearch view '" << view->name() <<  "': " << res.errorNumber() << " " <<  res.errorMessage();
      }
    } catch (arangodb::basics::Exception const& e) {
      IR_LOG_EXCEPTION();
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while creating links while creating arangosearch view '" << view->name() << "': " << e.code() << " " << e.what();
    } catch (std::exception const& e) {
      IR_LOG_EXCEPTION();
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while creating links while creating arangosearch view '" << view->name() << "': " << e.what();
    } catch (...) {
      IR_LOG_EXCEPTION();
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while creating links while creating arangosearch view '" << view->name() << "'";
    }

    view = impl;

    return arangodb::Result();
  }

  virtual arangodb::Result instantiate(
      arangodb::LogicalView::ptr& view,
      TRI_vocbase_t& vocbase,
      arangodb::velocypack::Slice const& definition,
      uint64_t planVersion
  ) const override {
    auto* feature = arangodb::application_features::ApplicationServer::lookupFeature<
      arangodb::DatabasePathFeature
    >("DatabasePath");

    if (!feature) {
      return arangodb::Result(
        TRI_ERROR_INTERNAL,
        std::string("failure to find feature 'DatabasePath' while constructing arangosearch View in database '") + vocbase.name() + "'"
      );
    }

    std::string error;
    auto meta = std::make_shared<AsyncMeta>();
    auto impl = std::shared_ptr<IResearchView>(
      new IResearchView(vocbase, definition, *feature, planVersion)
    );

    if (!meta->init(definition, error)
        || !impl->_metaState.init(definition, error)) {
      return arangodb::Result(
        TRI_ERROR_BAD_PARAMETER,
        error.empty()
        ? (std::string("failed to initialize arangosearch View '") + impl->name() + "' from definition: " + definition.toString())
        : (std::string("failed to initialize arangosearch View '") + impl->name() + "' from definition, error in attribute '" + error + "': " + definition.toString())
      );
    }

    auto res = impl->updateProperties(meta); // update separately since per-instance async jobs already started

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

    view = impl;

    return arangodb::Result();
  }
};

////////////////////////////////////////////////////////////////////////////////
/// @brief container storing the view 'read' state for a given TransactionState
////////////////////////////////////////////////////////////////////////////////
struct IResearchView::ViewStateRead final
    : public arangodb::TransactionState::Cookie {
  CompoundReader _snapshot;

  explicit ViewStateRead(ReadMutex& mutex) noexcept
    : _snapshot(mutex) {
  }
};

////////////////////////////////////////////////////////////////////////////////
/// @brief container storing the view 'write' state for a given TransactionState
////////////////////////////////////////////////////////////////////////////////
class IResearchView::ViewStateWrite final
  : public arangodb::TransactionState::Cookie {
 public:
  std::lock_guard<ReadMutex> _viewLock; // prevent data-store deallocation (lock @ AsyncSelf)
  irs::index_writer::documents_context _ctx;
  PrimaryKeyFilterContainer _removals; // list of document removals

  explicit ViewStateWrite(
      ReadMutex& viewMutex,
      irs::index_writer& writer
  ) noexcept
    : _viewLock(viewMutex),
      _ctx(writer.documents()) {
  }

  virtual ~ViewStateWrite() noexcept {
    if (_removals.empty()) {
      // nothing to do
      return;
    }

    try {
      // hold references even after transaction
      _ctx.remove(
        irs::filter::make<PrimaryKeyFilterContainer>(std::move(_removals))
      );
    } catch (std::exception const& e) {
      LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
        << "Failed to apply accumulated removals, error '" << e.what() << "'";
    } catch (...) {
      // NOOP
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "Failed to apply accumulated removals";
    }
  }

  operator irs::index_writer::documents_context&() noexcept {
    return _ctx;
  }

  void remove(TRI_voc_cid_t cid, TRI_voc_rid_t rid) {
    _ctx.remove(_removals.emplace(cid, rid));
  }

  void reset() noexcept {
    _removals.clear();
    _ctx.reset();
  }
};

////////////////////////////////////////////////////////////////////////////////
/// @brief helper class for retrieving/setting view transaction states
////////////////////////////////////////////////////////////////////////////////
class IResearchView::ViewStateHelper {
 public:
  static IResearchView::ViewStateRead* read(
      arangodb::TransactionState& state,
      IResearchView const& view
  ) {
    static_assert(sizeof(IResearchView) > Reader, "'Reader' offset >= sizeof(IResearchView)");
    auto* key = &view + Reader;

    // TODO FIXME find a better way to look up a ViewState
    #ifdef ARANGODB_ENABLE_MAINTAINER_MODE
      return dynamic_cast<IResearchView::ViewStateRead*>(state.cookie(key));
    #else
      return static_cast<IResearchView::ViewStateRead*>(state.cookie(key));
    #endif
  }

  static bool read(
      arangodb::TransactionState& state,
      IResearchView const& view,
      std::unique_ptr<IResearchView::ViewStateRead>&& value
  ) {
    static_assert(sizeof(IResearchView) > Reader, "'Reader' offset >= sizeof(IResearchView)");
    auto* key = &view + Reader;
    auto prev = state.cookie(key, std::move(value));

    if (!prev) {
      return true;
    }

    state.cookie(key, std::move(prev)); // put back original value

    return false;
  }

  static IResearchView::ViewStateWrite* write(
      arangodb::TransactionState& state,
      IResearchView const& view
  ) {
    static_assert(sizeof(IResearchView) > Writer, "'Writer' offset >= sizeof(IResearchView)");
    auto* key = &view + Writer;

    // TODO FIXME find a better way to look up a ViewState
    #ifdef ARANGODB_ENABLE_MAINTAINER_MODE
      return dynamic_cast<IResearchView::ViewStateWrite*>(state.cookie(key));
    #else
      return static_cast<IResearchView::ViewStateWrite*>(state.cookie(key));
    #endif
  }

  static bool write(
      arangodb::TransactionState& state,
      IResearchView const& view,
      std::unique_ptr<IResearchView::ViewStateWrite>&& value
  ) {
    static_assert(sizeof(IResearchView) > Writer, "'Writer' offset >= sizeof(IResearchView)");
    auto* key = &view + Writer;
    auto prev = state.cookie(key, std::move(value));

    if (!prev) {
      return true;
    }

    state.cookie(key, std::move(prev)); // put back original value

    return false;
  }

  static void commitWrite(
      arangodb::TransactionState& state,
      IResearchView const& view,
      bool rollback
  ) noexcept {
    static_assert(sizeof(IResearchView) > Writer, "'Writer' offset >= sizeof(IResearchView)");
    auto* key = &view + Writer;
    auto prev = state.cookie(key, nullptr); // get existing cookie

    if (rollback && prev) {
      // TODO FIXME find a better way to look up a ViewState
      #ifdef ARANGODB_ENABLE_MAINTAINER_MODE
        auto& ctx = dynamic_cast<IResearchView::ViewStateWrite&>(*prev);
      #else
        auto& ctx = static_cast<IResearchView::ViewStateWrite&>(*prev);
      #endif

      ctx.reset();
    }

    prev.reset();
  }

 private:
  enum offsets { Reader, Writer }; // offsets from key
};

IResearchView::IResearchView(
    TRI_vocbase_t& vocbase,
    arangodb::velocypack::Slice const& info,
    arangodb::DatabasePathFeature const& dbPathFeature,
    uint64_t planVersion
): LogicalViewStorageEngine(vocbase, info, planVersion),
   FlushTransaction(toString(*this)),
   _asyncFeature(nullptr),
   _asyncSelf(irs::memory::make_unique<AsyncSelf>(this)),
   _meta(std::make_shared<AsyncMeta>()),
   _storePersisted(getPersistedPath(dbPathFeature, vocbase, id())),
   _asyncTerminate(false),
   _inRecovery(false) {
  // set up in-recovery insertion hooks
  auto* feature = arangodb::application_features::ApplicationServer::lookupFeature<
    arangodb::DatabaseFeature
  >("Database");

  if (feature) {
    auto view = _asyncSelf; // create copy for lambda

    feature->registerPostRecoveryCallback([view]()->arangodb::Result {
      auto& viewMutex = view->mutex();
      SCOPED_LOCK(viewMutex); // ensure view does not get deallocated before call back finishes
      auto* viewPtr = view->get();

      if (!viewPtr) {
        LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
          << "Invalid call to post-recovery callback of arangosearch view";

        return arangodb::Result(); // view no longer in recovery state
      }

      viewPtr->verifyKnownCollections();

      if (viewPtr->_storePersisted) {
        LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
          << "starting persisted-sync sync for arangosearch view '" << viewPtr->name() << "'";

        try {
          viewPtr->_storePersisted.sync();
        } catch (arangodb::basics::Exception& e) {
          LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
            << "caught exception while committing persisted store for arangosearch view '" << viewPtr->name()
            << "': " << e.code() << " " << e.what();
          IR_LOG_EXCEPTION();

          return arangodb::Result(
            e.code(),
            std::string("error committing persisted store for arangosearch view '") + viewPtr->name() + "'"
          );
        } catch (std::exception const& e) {
          LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
            << "caught exception while committing persisted store for arangosearch view '" << viewPtr->name()
            << "': " << e.what();
          IR_LOG_EXCEPTION();

          return arangodb::Result(
            TRI_ERROR_INTERNAL,
            std::string("error committing persisted store for arangosearch view '") + viewPtr->name() + "'"
          );
        } catch (...) {
          LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
            << "caught exception while committing persisted store for arangosearch view '" << viewPtr->name() << "'";
          IR_LOG_EXCEPTION();

          return arangodb::Result(
            TRI_ERROR_INTERNAL,
            std::string("error committing persisted store for arangosearch view '") + viewPtr->name() + "'"
          );
        }

        LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
          << "finished persisted-sync sync for arangosearch view '" << viewPtr->name() << "'";
      }

      viewPtr->_inRecovery = false;

      return arangodb::Result();
    });
  }

  _asyncFeature = arangodb::application_features::ApplicationServer::lookupFeature<
    arangodb::iresearch::IResearchFeature
  >();

  // add asynchronous commit tasks
  if (_asyncFeature) {
    struct State : public IResearchViewMeta {
      size_t _cleanupIntervalCount{ 0 };
      std::chrono::system_clock::time_point _last{ std::chrono::system_clock::now() };
      irs::merge_writer::flush_progress_t _progress;
    } state;

    state._progress = [this]()->bool { return !_asyncTerminate.load(); };
    _asyncFeature->async(
      self(),
      [this, state](size_t& timeoutMsec, bool) mutable ->bool {
        if (_asyncTerminate.load()) {
          return false; // termination requested
        }

        // reload meta
        {
          auto meta = std::atomic_load(&_meta);
          SCOPED_LOCK(meta->read());

          if (state != *meta) {
            static_cast<IResearchViewMeta&>(state) = *meta;
          }
        }

        if (!state._consolidationIntervalMsec) {
          timeoutMsec = 0; // task not enabled

          return true; // reschedule
        }

        size_t usedMsec = std::chrono::duration_cast<std::chrono::milliseconds>(
          std::chrono::system_clock::now() - state._last
        ).count();

        if (usedMsec < state._consolidationIntervalMsec) {
          timeoutMsec = state._consolidationIntervalMsec - usedMsec; // still need to sleep

          return true; // reschedule (with possibly updated '_consolidationIntervalMsec')
        }

        state._last = std::chrono::system_clock::now(); // remember last task start time
        timeoutMsec = state._consolidationIntervalMsec;

        auto const runCleanupAfterConsolidation =
          state._cleanupIntervalCount > state._cleanupIntervalStep;

        if (_storePersisted
            && consolidateCleanupStore(
                 *_storePersisted._directory,
                 *_storePersisted._writer,
                 state._consolidationPolicy,
                 state._progress,
                 runCleanupAfterConsolidation,
                 *this,
                 "persistent store")
            && state._cleanupIntervalStep
            && state._cleanupIntervalCount++ > state._cleanupIntervalStep) {
          state._cleanupIntervalCount = 0;
        };

        return true; // reschedule
    });
  }

  auto& viewRef = *this;

  // initialize transaction read callback
  _trxReadCallback = [&viewRef](
      arangodb::transaction::Methods& trx,
      arangodb::transaction::Status status
  )->void {
    if (arangodb::transaction::Status::RUNNING != status) {
      return; // NOOP
    }

    viewRef.snapshot(trx, IResearchView::Snapshot::FindOrCreate);
  };

  // initialize transaction write callback
  _trxWriteCallback = [&viewRef](
      arangodb::transaction::Methods& trx,
      arangodb::transaction::Status status
  )->void {
    auto* state = trx.state();

    // check state of the top-most transaction only
    if (!state) {
      return; // NOOP
    }

    ViewStateHelper::commitWrite(
      *state, viewRef, arangodb::transaction::Status::COMMITTED != status
    );
  };
}

IResearchView::~IResearchView() {
  _asyncTerminate.store(true); // mark long-running async jobs for terminatation
  updateProperties(_meta); // trigger reload of settings for async jobs
  _asyncSelf->reset(); // the view is being deallocated, its use is no longer valid (wait for all the view users to finish)
  _flushCallback.reset(); // unregister flush callback from flush thread

  {
    WriteMutex mutex(_mutex); // '_meta' can be asynchronously read
    SCOPED_LOCK(mutex);

    if (_storePersisted) {
      try {
        // NOTE: do not commit writer so as not to go out-of-sync with the WAL (i.e. flush thread)
        _storePersisted._writer->close();
        _storePersisted._writer.reset();
        _storePersisted._directory->close();
        _storePersisted._directory.reset();
      } catch (...) {
        // must not propagate exception out of destructor
        LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
          << "caught exception while closing persisted store for arangosearch view '" << name() << "'";
        IR_LOG_EXCEPTION();
      }
    }
  }
}

arangodb::Result IResearchView::appendVelocyPackDetailed(
    arangodb::velocypack::Builder& builder,
    bool forPersistence
) const {
  if (!builder.isOpenObject()) {
    return arangodb::Result(TRI_ERROR_BAD_PARAMETER);
  }

  std::vector<std::string> collections;

  {
    ReadMutex mutex(_mutex);
    SCOPED_LOCK(mutex); // '_metaState' can be asynchronously updated

    auto meta = std::atomic_load(&_meta);
    SCOPED_LOCK(meta->read()); // '_meta' can be asynchronously updated

    if (!meta->json(builder)) {
      return arangodb::Result(
        TRI_ERROR_INTERNAL,
        std::string("failure to generate definition while generating properties jSON for arangosearch View in database '") + vocbase().name() + "'"
      );
    }

    if (forPersistence) {
      _metaState.json(builder);

      return arangodb::Result(); // nothing more to output (persistent configuration does not need links)
    }

    // add CIDs of known collections to list
    for (auto& entry: _metaState._collections) {
      // skip collections missing from vocbase or UserTransaction constructor will throw an exception
      if (vocbase().lookupCollection(entry)) {
        collections.emplace_back(std::to_string(entry));
      }
    }
  }

  // open up a read transaction and add all linked collections to verify that
  // the current user has access

  arangodb::velocypack::Builder linksBuilder;
  static std::vector<std::string> const EMPTY;

  // use default lock timeout
  arangodb::transaction::Options options;

  options.waitForSync = false;
  options.allowImplicitCollections = false;

  try {
    arangodb::transaction::Methods trx(
      transaction::StandaloneContext::Create(vocbase()),
      collections, // readCollections
      EMPTY, // writeCollections
      EMPTY, // exclusiveCollections
      options
    );
    auto res = trx.begin();

    if (!res.ok()) {
      return res; // nothing more to output
    }

    auto* state = trx.state();

    if (!state) {
      return arangodb::Result(
        TRI_ERROR_INTERNAL,
        std::string("failed to get transaction state while generating json for arangosearch view '") + name() + "'"
      );
    }

    arangodb::velocypack::ObjectBuilder linksBuilderWrapper(&linksBuilder);

    for (auto& collectionName: state->collectionNames()) {
      for (auto& index: trx.indexesForCollection(collectionName)) {
        if (index && arangodb::Index::IndexType::TRI_IDX_TYPE_IRESEARCH_LINK == index->type()) {
          // TODO FIXME find a better way to retrieve an IResearch Link
          // cannot use static_cast/reinterpret_cast since Index is not related to IResearchLink
          auto* ptr = dynamic_cast<IResearchLink*>(index.get());

          if (!ptr || *ptr != *this) {
            continue; // the index is not a link for the current view
          }

          arangodb::velocypack::Builder linkBuilder;

          linkBuilder.openObject();

          if (!ptr->json(linkBuilder)) {
            LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
              << "failed to generate json for arangosearch link '" << ptr->id()
              << "' while generating json for arangosearch view '" << id() << "'";
            continue; // skip invalid link definitions
          }

          linkBuilder.close();
          linksBuilderWrapper->add(collectionName, linkBuilder.slice());
        }
      }
    }

    trx.commit();
  } catch (arangodb::basics::Exception& e) {
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      e.code(),
      std::string("caught exception while generating json for arangosearch view '") + name() + "': " + e.what()
    );
  } catch (std::exception const& e) {
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("caught exception while generating json for arangosearch view '") + name() + "': " + e.what()
    );
  } catch (...) {
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("caught exception while generating json for arangosearch view '") + name() + "'"
    );
  }

  builder.add(StaticStrings::LinksField, linksBuilder.slice());

  return arangodb::Result();
}

bool IResearchView::apply(arangodb::transaction::Methods& trx) {
  // called from IResearchView when this view is added to a transaction
  return trx.addStatusChangeCallback(&_trxReadCallback); // add shapshot
}

arangodb::Result IResearchView::drop(
    TRI_voc_cid_t cid,
    bool unlink /*= true*/
) {
  auto filter = iresearch::FilterFactory::filter(cid);

  WriteMutex rmutex(_mutex); // '_meta' and '_storeByTid' can be asynchronously updated
  WriteMutex wmutex(_mutex); // '_meta' and '_storeByTid' can be asynchronously updated
  DEFER_SCOPED_LOCK_NAMED(rmutex, rlock);
  DEFER_SCOPED_LOCK_NAMED(wmutex, wlock);

  if (!unlink) {
    rlock.lock(); // aquire only a read-lock since do not need a write-lock
  } else {
    wlock.lock(); // aquire a write-lock that will be downgraded to a read-lock

    auto cid_itr = _metaState._collections.find(cid);

    if (cid_itr != _metaState._collections.end()) {
      auto collections = _metaState._collections;

      _metaState._collections.erase(cid_itr);

      try {
        auto result = persistProperties(*this, _asyncSelf);

        if (!result.ok()) {
          _metaState._collections.swap(collections); // restore original collections

          return result;
        }
      } catch(...) {
        _metaState._collections.swap(collections); // restore original collections
        IR_LOG_EXCEPTION();

        return arangodb::Result(
          TRI_ERROR_INTERNAL,
          std::string("caught error during persistance of properties for arangosearch View '") + name() + "' while dropping collection ' " + std::to_string(cid) + "'"
        );
      }
    }

    wmutex.unlock(true); // downgrade to a read-lock
  }

  // ...........................................................................
  // if an errors occurs below than a drop retry would most likely happen
  // ...........................................................................

  try {
    if (_storePersisted) {
      _storePersisted._writer->documents().remove(std::move(filter));
    }
  } catch (arangodb::basics::Exception& e) {
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      e.code(),
      std::string("caught exception while removing from arangosearch view '") + name() + "', collection '" + std::to_string(cid) + "': " + e.what()
    );
  } catch (std::exception const& e) {
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("caught exception while removing from arangosearch view '") + name() + "', collection '" + std::to_string(cid) + "': " + e.what()
    );
  } catch (...) {
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("caught exception while removing from arangosearch view '") + name() + "', collection '" + std::to_string(cid) + "'"
    );
  }

  return arangodb::Result();
}

arangodb::Result IResearchView::dropImpl() {
  std::unordered_set<TRI_voc_cid_t> collections;
  std::unordered_set<TRI_voc_cid_t> stale;

  // drop all known links
  {
    ReadMutex mutex(_mutex);
    SCOPED_LOCK(mutex); // '_meta' can be asynchronously updated
    stale = _metaState._collections;
  }

  if (!stale.empty()) {
    // check link auth as per https://github.com/arangodb/backlog/issues/459
    if (arangodb::ExecContext::CURRENT) {
      for (auto& entry: stale) {
        auto collection = vocbase().lookupCollection(entry);

        if (collection
            && !arangodb::ExecContext::CURRENT->canUseCollection(vocbase().name(), collection->name(), arangodb::auth::Level::RO)) {
          return arangodb::Result(TRI_ERROR_FORBIDDEN);
        }
      }
    }

    arangodb::Result res;

    {
      if (!_updateLinksLock.try_lock()) {
        return arangodb::Result(
          TRI_ERROR_FAILED, // FIXME use specific error code
          std::string("failed to remove arangosearch view '") + name()
        );
      }

      ADOPT_SCOPED_LOCK_NAMED(_updateLinksLock, lock);

      res = IResearchLinkHelper::updateLinks(
        collections, vocbase(), *this, emptyObjectSlice(), stale
      );
    }

    if (!res.ok()) {
      return arangodb::Result(
        res.errorNumber(),
        std::string("failed to remove links while removing arangosearch view '") + name() + "': " + res.errorMessage()
      );
    }
  }

  _asyncTerminate.store(true); // mark long-running async jobs for terminatation
  updateProperties(_meta); // trigger reload of settings for async jobs
  _asyncSelf->reset(); // the view data-stores are being deallocated, view use is no longer valid (wait for all the view users to finish)
  _flushCallback.reset(); // unregister flush callback from flush thread

  WriteMutex mutex(_mutex); // members can be asynchronously updated
  SCOPED_LOCK(mutex);

  collections.insert(
    _metaState._collections.begin(), _metaState._collections.end()
  );

  auto collectionsCount = collections.size();

  for (auto& entry: collections) {
    auto collection = vocbase().lookupCollection(entry);

    if (!collection
        || !arangodb::iresearch::IResearchLink::find(*collection, *this)) {
      --collectionsCount;
    }
  }

  // ArangoDB global consistency check, no known dangling links
  if (collectionsCount) {
    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("links still present while removing arangosearch view '") + std::to_string(id()) + "'"
    );
  }

  // ...........................................................................
  // if an exception occurs below than a drop retry would most likely happen
  // ...........................................................................
  try {
    if (_storePersisted) {
      _storePersisted._reader.reset(); // reset reader to release file handles
      _storePersisted._writer->close();
      _storePersisted._writer.reset();
      _storePersisted._directory->close();
      _storePersisted._directory.reset();
    }

    bool exists;

    // remove persisted data store directory if present
    if (_storePersisted._path.exists_directory(exists)
        && (!exists || _storePersisted._path.remove())) {
      return arangodb::Result(); // success
    }
  } catch (arangodb::basics::Exception& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while removing arangosearch view '" << name() << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      e.code(),
      std::string("caught exception while removing arangosearch view '") + name() + "': " + e.what()
    );
  } catch (std::exception const& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while removing arangosearch view '" << name() << "': " << e.what();
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("caught exception while removing arangosearch view '") + name() + "': " + e.what()
    );
  } catch (...) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while removing arangosearch view '" << name() << "'";
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      TRI_ERROR_INTERNAL,
      std::string("caught exception while removing arangosearch view '") + name() + "'"
    );
  }

  return arangodb::Result(
    TRI_ERROR_INTERNAL,
    std::string("failed to remove arangosearch view '") + name() + "'"
  );
}

bool IResearchView::emplace(TRI_voc_cid_t cid) {
  WriteMutex mutex(_mutex); // '_meta' can be asynchronously updated
  SCOPED_LOCK(mutex);
  arangodb::Result result;

  if (!_metaState._collections.emplace(cid).second) {
    return false;
  }

  try {
    result = persistProperties(*this, _asyncSelf);

    if (result.ok()) {
      return true;
    }
  } catch (arangodb::basics::Exception& e) {
    _metaState._collections.erase(cid); // undo meta modification
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception during persisting of logical view while emplacing collection ' " << cid
      << "' into arangosearch View '" << name() << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();
    throw;
  } catch (std::exception const& e) {
    _metaState._collections.erase(cid); // undo meta modification
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception during persisting of logical view while emplacing collection ' " << cid
      << "' into arangosearch View '" << name() << "': " << e.what();
    IR_LOG_EXCEPTION();
    throw;
  } catch (...) {
    _metaState._collections.erase(cid); // undo meta modification
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception during persisting of logical view while emplacing collection ' " << cid
      << "' into arangosearch View '" << name() << "'";
    IR_LOG_EXCEPTION();
    throw;
  }

  _metaState._collections.erase(cid); // undo meta modification
  LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
    << "failed to persist logical view while emplacing collection ' " << cid
    << "' into arangosearch View '" << name() << "': " << result.errorMessage();

  return false;
}

/*static*/ arangodb::ViewFactory const& IResearchView::factory() {
  static const ViewFactory factory;

  return factory;
}

arangodb::Result IResearchView::commit() {
  ReadMutex mutex(_mutex); // '_storePersisted' can be asynchronously updated
  SCOPED_LOCK(mutex);

  if (!_storePersisted) {
    return {}; // nothing more to do
  }

  try {
    LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
      << "starting persisted-sync sync for arangosearch view '" << name() << "'";

    _storePersisted._writer->commit(); // finishing flush transaction

    {
      SCOPED_LOCK(_readerLock);

      auto reader = _storePersisted._reader.reopen(); // update reader

      if (!reader) {
        // nothing more to do
        LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
          << "failed to update snapshot after commit, reuse the existing snapshot for arangosearch view '" << name() << "'";
        return {};
      }

      if (_storePersisted._reader != reader) {
         // update reader
         _storePersisted._reader = reader;

         // invalidate query cache if there were some data changes
         arangodb::aql::QueryCache::instance()->invalidate(
           &vocbase(), name()
         );
      }
    }

    LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
      << "finished persisted-sync sync for arangosearch view '" << name() << "'";

    return {};
  } catch (arangodb::basics::Exception const& e) {
    LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
      << "caught exception while committing store for arangosearch view '" << name() << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();
  } catch (std::exception const& e) {
    LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
      << "caught exception while committing store for arangosearch view '" << name() << "': " << e.what();
    IR_LOG_EXCEPTION();
  } catch (...) {
    LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
      << "caught exception while committing store for arangosearch view '" << name() << "'";
    IR_LOG_EXCEPTION();
  }

  return {TRI_ERROR_INTERNAL};
}

int IResearchView::insert(
    transaction::Methods& trx,
    TRI_voc_cid_t cid,
    arangodb::LocalDocumentId const& documentId,
    arangodb::velocypack::Slice const& doc,
    IResearchLinkMeta const& meta
) {
  TRI_ASSERT(_storePersisted);

  auto insertImpl = [this, cid, &meta, doc, &documentId](irs::index_writer::documents_context& ctx) ->int {
    FieldIterator body(doc, meta);

    if (!body.valid()) {
      return TRI_ERROR_NO_ERROR; // nothing to index
    }

    try {
      auto doc = ctx.insert();
      insertDocument(doc, body, cid, documentId.id());

      if (doc) {
        return TRI_ERROR_NO_ERROR;
      }

      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "failed inserting into arangosearch view '" << id()
        << "', collection '" << cid << "', revision '" << documentId.id() << "'";
    } catch (arangodb::basics::Exception const& e) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while inserting into arangosearch view '" << id()
        << "', collection '" << cid << "', revision '" << documentId.id() << "': " << e.code() << " " << e.what();
      IR_LOG_EXCEPTION();
    } catch (std::exception const& e) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while inserting into arangosearch view '" << id()
        << "', collection '" << cid << "', revision '" << documentId.id() << "': " << e.what();
      IR_LOG_EXCEPTION();
    } catch (...) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while inserting into arangosearch view '" << id()
        << "', collection '" << cid << "', revision '" << documentId.id() << "'";
      IR_LOG_EXCEPTION();
    }

    return TRI_ERROR_INTERNAL;
  };

  if (!trx.state()) {
    return TRI_ERROR_BAD_PARAMETER; // 'trx' and transaction state required
  }

  if (trx.isSingleOperationTransaction()) {
    auto ctx = _storePersisted._writer->documents();
    return insertImpl(ctx);
  }

  auto& state = *(trx.state());
  auto* ctx = ViewStateHelper::write(state, *this);

  if (!ctx) {
    auto ptr = irs::memory::make_unique<ViewStateWrite>(
      _asyncSelf->mutex(), *_storePersisted._writer
    ); // will aquire read-lock to prevent data-store deallocation

    if (!_asyncSelf->get()) {
      return TRI_ERROR_INTERNAL; // the current view is no longer valid (checked after ReadLock aquisition)
    }

    ctx = ptr.get();

    if (!ViewStateHelper::write(state, *this, std::move(ptr))
        || !trx.addStatusChangeCallback(&_trxWriteCallback)) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "failed to store state into a TransactionState for insert into arangosearch view '" << name() << "'"
        << "', tid '" << state.id() << "', collection '" << cid << "', revision '" << documentId.id() << "'";

      return TRI_ERROR_INTERNAL;
    }
  }

  TRI_ASSERT(ctx);

  if (_inRecovery) {
    ctx->remove(cid, documentId.id());
  }

  return insertImpl(*ctx);
}

int IResearchView::insert(
    transaction::Methods& trx,
    TRI_voc_cid_t cid,
    std::vector<std::pair<arangodb::LocalDocumentId, arangodb::velocypack::Slice>> const& batch,
    IResearchLinkMeta const& meta
) {
  TRI_ASSERT(_storePersisted);

  auto insertImpl = [this, cid, &batch, &meta](irs::index_writer::documents_context& ctx) -> int {
    auto begin = batch.begin();
    auto const end = batch.end();

    try {
      {
        for (FieldIterator body; begin != end; ++begin) {
          body.reset(begin->second, meta);

          if (!body.valid()) {
            // find first valid document
            continue;
          }

          auto doc = ctx.insert();
          insertDocument(doc, body, cid, begin->first.id());

          if (!doc) {
            LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
              << "failed inserting batch into arangosearch view '" << id() << "', collection '" << cid;
            return TRI_ERROR_INTERNAL;
          }
        }
      }
    } catch (arangodb::basics::Exception const& e) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while inserting batch into arangosearch view '" << id() << "', collection '" << cid << "': " << e.code() << " " << e.what();
      IR_LOG_EXCEPTION();
    } catch (std::exception const& e) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while inserting batch into arangosearch view '" << id() << "', collection '" << cid << "': " << e.what();
      IR_LOG_EXCEPTION();
    } catch (...) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "caught exception while inserting batch into arangosearch view '" << id() << "', collection '" << cid << "'";
      IR_LOG_EXCEPTION();
    }

    return TRI_ERROR_NO_ERROR;
  };

  if (!trx.state()) {
    return TRI_ERROR_BAD_PARAMETER; // 'trx' and transaction state required
  }

  auto& state = *(trx.state());
  auto* ctx = ViewStateHelper::write(state, *this);

  if (!ctx) {
    auto ptr = irs::memory::make_unique<ViewStateWrite>(
      _asyncSelf->mutex(), *_storePersisted._writer
    ); // will aquire read-lock to prevent data-store deallocation

    if (!_asyncSelf->get()) {
      return TRI_ERROR_INTERNAL; // the current view is no longer valid (checked after ReadLock aquisition)
    }

    ctx = ptr.get();

    if (!ViewStateHelper::write(state, *this, std::move(ptr))
        || !trx.addStatusChangeCallback(&_trxWriteCallback)) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "failed to store state into a TransactionState for insert into arangosearch view '" << name() << "'"
        << "', tid '" << state.id() << "', collection '" << cid << "'";

      return TRI_ERROR_INTERNAL;
    }
  }

  TRI_ASSERT(ctx);

  if (_inRecovery) {
    for (auto const& doc : batch) {
      ctx->remove(cid, doc.first.id());
    }
  }

  return insertImpl(*ctx);
}

size_t IResearchView::memory() const {
  ReadMutex mutex(_mutex); // view members can be asynchronously updated
  SCOPED_LOCK(mutex);
  size_t size = sizeof(IResearchView);

  {
    auto meta = std::atomic_load(&_meta);
    SCOPED_LOCK(meta->read()); // '_meta' can be asynchronously updated

    size += meta->memory();
  }

  size += _metaState.memory();

  if (_storePersisted) {
    // FIXME TODO this is incorrect since '_storePersisted' is on disk and not in memory
    size += directoryMemory(*(_storePersisted._directory), id());
    size += _storePersisted._path.native().size() * sizeof(irs::utf8_path::native_char_t);
  }

  return size;
}

void IResearchView::open() {
  auto* engine = arangodb::EngineSelectorFeature::ENGINE;

  if (engine) {
    _inRecovery = engine->inRecovery();
  } else {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "failure to get storage engine while opening arangosearch view: " << name();
    // assume not inRecovery()
  }

  WriteMutex mutex(_mutex); // '_meta' can be asynchronously updated
  SCOPED_LOCK(mutex);

  if (_storePersisted) {
    return; // view already open
  }

  try {
    auto format = irs::formats::get(IRESEARCH_STORE_FORMAT);

    if (format) {
      if (!_storePersisted._path.mkdir()) {
#if defined(_WIN32)
        errno = TRI_MapSystemError(GetLastError());
#endif

        throw std::runtime_error(
          std::string("failed to create data directory for arangosearch view '") + name()
            + "' at: '" + _storePersisted._path.utf8()
            + "' error: '" + std::to_string(errno) + "'"
        );
      }

      _storePersisted._directory =
        irs::directory::make<irs::mmap_directory>(_storePersisted._path.utf8());

      if (_storePersisted._directory) {
        // do not lock index, ArangoDB has it's own lock
        irs::index_writer::options options;

        TRI_ASSERT(_meta);
        options.lock_repository = false;
        options.segment_count_max = _meta->_writebufferActive;
        options.segment_memory_max = _meta->_writebufferSizeMax;
        options.segment_pool_size = _meta->_writebufferIdle;

        // create writer before reader to ensure data directory is present
        _storePersisted._writer = irs::index_writer::make(
          *(_storePersisted._directory), format, irs::OM_CREATE | irs::OM_APPEND, options
        );

        if (_storePersisted._writer) {
          _storePersisted._writer->commit(); // initialize 'store'
          _storePersisted._reader = irs::directory_reader::open(
            *(_storePersisted._directory)
          );

          if (_storePersisted._reader) {
            registerFlushCallback();
            updateProperties(_meta);

            return; // success
          }

          _storePersisted._writer.reset(); // unlock the directory
        }
      }
    }
  } catch (arangodb::basics::Exception const& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while opening arangosearch view '" << id() << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();
    throw;
  } catch (std::exception const& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while opening arangosearch view '" << id() << "': " << e.what();
    IR_LOG_EXCEPTION();
    throw;
  } catch (...) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while opening arangosearch view '" << id() << "'";
    IR_LOG_EXCEPTION();
    throw;
  }

  LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
    << "failed to open arangosearch view '" << name() << "' at: " << _storePersisted._path.utf8();

  throw std::runtime_error(
    std::string("failed to open arangosearch view '") + name() + "' at: " + _storePersisted._path.utf8()
  );
}

int IResearchView::remove(
    transaction::Methods& trx,
    TRI_voc_cid_t cid,
    arangodb::LocalDocumentId const& documentId
) {
  TRI_ASSERT(_storePersisted);

  if (!trx.state()) {
    return TRI_ERROR_BAD_PARAMETER; // 'trx' and transaction state required
  }

  auto& state = *(trx.state());
  auto* ctx = ViewStateHelper::write(state, *this);

  if (!ctx) {
    auto ptr = irs::memory::make_unique<ViewStateWrite>(
      _asyncSelf->mutex(), *_storePersisted._writer
    ); // will aquire read-lock to prevent data-store deallocation

    if (!_asyncSelf->get()) {
      return TRI_ERROR_INTERNAL; // the current view is no longer valid (checked after ReadLock aquisition)
    }

    ctx = ptr.get();

    if (!ViewStateHelper::write(state, *this, std::move(ptr))
        || !trx.addStatusChangeCallback(&_trxWriteCallback)) {
      LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "failed to store state into a TransactionState for insert into arangosearch view '" << name() << "'"
        << "', tid '" << state.id() << "', collection '" << cid << "', revision '" << documentId.id() << "'";

      return TRI_ERROR_INTERNAL;
    }
  }

  TRI_ASSERT(ctx);

  // ...........................................................................
  // if an exception occurs below than the transaction is droped including all
  // all of its fid stores, no impact to iResearch View data integrity
  // ...........................................................................
  try {
    ctx->remove(cid, documentId.id());

    return TRI_ERROR_NO_ERROR;
  } catch (arangodb::basics::Exception const& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while removing from arangosearch view '" << id()
      << "', tid '" << state.id() << "', collection '" << cid << "', revision '" << documentId.id() << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();
  } catch (std::exception const& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while removing from arangosearch view '" << id()
      << "', tid '" << state.id() << "', collection '" << cid << "', revision '" << documentId.id() << "': " << e.what();
    IR_LOG_EXCEPTION();
  } catch (...) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while removing from arangosearch view '" << id()
      << "', tid '" << state.id() << "', collection '" << cid << "', revision '" << documentId.id() << "'";
    IR_LOG_EXCEPTION();
  }

  return TRI_ERROR_INTERNAL;
}

PrimaryKeyIndexReader* IResearchView::snapshot(
    transaction::Methods& trx,
    IResearchView::Snapshot mode /*= IResearchView::Snapshot::Find*/
) const {
  auto* state = trx.state();

  if (!state) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "failed to get transaction state while creating arangosearch view snapshot";

    return nullptr;
  }

  auto* cookie = ViewStateHelper::read(*state, *this);

  switch (mode) {
    case Snapshot::Find:
      return cookie ? &cookie->_snapshot : nullptr;
    case Snapshot::FindOrCreate:
      if (cookie) {
        return &cookie->_snapshot;
      }
      break;
    case Snapshot::SyncAndReplace: {
      // ingore existing cookie, recreate snapshot
      auto const res = const_cast<IResearchView*>(this)->commit();

      if (!res.ok()) {
        LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
          << "failed to sync while creating snapshot for arangosearch view '" << name()
          << "', previous snapshot will be used instead, error: '" << res.errorMessage() << "'";
      }
    } break;
  }

  std::unique_ptr<ViewStateRead> cookiePtr;
  CompoundReader* reader = nullptr;

  if (!cookie) {
    // will acquire read-lock to prevent data-store deallocation
    cookiePtr = irs::memory::make_unique<ViewStateRead>(_asyncSelf->mutex());
    reader = &cookiePtr->_snapshot;
  } else {
    reader = &cookie->_snapshot;
    reader->clear();
  }

  TRI_ASSERT(reader);

  if (!_asyncSelf->get()) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "current view no longer valid while creating arangosearch view "
        << "snapshot";
    return nullptr; // the current view is no longer valid (checked after ReadLock aquisition)
  }

  try {
    ReadMutex mutex(_mutex); // _storePersisted can be asynchronously updated
    SCOPED_LOCK(mutex);

    if (_storePersisted) {
      reader->add(_storePersisted._reader);
    }
  } catch (arangodb::basics::Exception& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while collecting readers for snapshot of arangosearch view '" << name()
      << "', tid '" << state->id() << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();

    return nullptr;
  } catch (std::exception const& e) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while collecting readers for snapshot of arangosearch view '" << name()
      << "', tid '" << state->id() << "': " << e.what();
    IR_LOG_EXCEPTION();

    return nullptr;
  } catch (...) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
      << "caught exception while collecting readers for snapshot of arangosearch view '" << name()
      << "', tid '" << state->id() << "'";
    IR_LOG_EXCEPTION();

    return nullptr;
  }

  if (cookiePtr && !ViewStateHelper::read(*state, *this, std::move(cookiePtr))) {
    LOG_TOPIC(WARN, arangodb::iresearch::TOPIC)
        << "failed to store state into a TransactionState for snapshot of arangosearch view '" << name()
        << "', tid '" << state->id() << "'";

    return nullptr;
  }

  return reader;
}

IResearchView::AsyncSelf::ptr IResearchView::self() const {
  return _asyncSelf;
}

arangodb::Result IResearchView::updateProperties(
    arangodb::velocypack::Slice const& slice,
    bool partialUpdate
) {
  try {
    auto links = slice.hasKey(StaticStrings::LinksField)
               ? slice.get(StaticStrings::LinksField)
               : arangodb::velocypack::Slice::emptyObjectSlice();
    auto res = _inRecovery
             ? arangodb::Result() // do not validate if in recovery
             : IResearchLinkHelper::validateLinks(vocbase(), links);

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

    WriteMutex mutex(_mutex); // '_metaState' can be asynchronously read
    SCOPED_LOCK_NAMED(mutex, mtx);

    // check link auth as per https://github.com/arangodb/backlog/issues/459
    if (arangodb::ExecContext::CURRENT) {
      // check existing links
      for (auto& entry: _metaState._collections) {
        auto collection = vocbase().lookupCollection(entry);

        if (collection
            && !arangodb::ExecContext::CURRENT->canUseCollection(vocbase().name(), collection->name(), arangodb::auth::Level::RO)) {
          return arangodb::Result(
            TRI_ERROR_FORBIDDEN,
            std::string("while updating arangosearch definition, error: collection '") + collection->name() + "' not authorised for read access"
          );
        }
      }
    }

    std::string error;
    IResearchViewMeta meta;

    {
      auto viewMeta = std::atomic_load(&_meta);
      SCOPED_LOCK(viewMeta->write());
      IResearchViewMeta* metaPtr = viewMeta.get();
      auto& initialMeta = partialUpdate ? *metaPtr : IResearchViewMeta::DEFAULT();

      if (!meta.init(slice, error, initialMeta)) {
        return arangodb::Result(
          TRI_ERROR_BAD_PARAMETER,
          error.empty()
          ? (std::string("failed to update arangosearch view '") + name() + "' from definition: " + slice.toString())
          : (std::string("failed to update arangosearch view '") + name() + "' from definition, error in attribute '" + error + "': " + slice.toString())
        );
      }

      // reset non-updatable values to match current meta
      meta._locale = viewMeta->_locale;
      meta._writebufferActive = viewMeta->_writebufferActive;
      meta._writebufferIdle = viewMeta->_writebufferIdle;
      meta._writebufferSizeMax = viewMeta->_writebufferSizeMax;

      if (arangodb::ServerState::instance()->isDBServer()) {
        viewMeta = std::make_shared<AsyncMeta>(); // create an instance not shared with cluster-view
      }

      static_cast<IResearchViewMeta&>(*viewMeta) = std::move(meta);
      updateProperties(viewMeta); // trigger reload of settings for async jobs
    }

    mutex.unlock(true); // downgrade to a read-lock

    if (links.isEmptyObject() && (partialUpdate || _inRecovery.load())) { // ignore missing links coming from WAL (inRecovery)
      return res;
    }

    // ...........................................................................
    // update links if requested (on a best-effort basis)
    // indexing of collections is done in different threads so no locks can be held and rollback is not possible
    // as a result it's also possible for links to be simultaneously modified via a different callflow (e.g. from collections)
    // ...........................................................................

    std::unordered_set<TRI_voc_cid_t> collections;

    if (partialUpdate) {
      mtx.unlock(); // release lock

      SCOPED_LOCK(_updateLinksLock);

      return IResearchLinkHelper::updateLinks(
        collections, vocbase(), *this, links
      );
    }

    auto stale = _metaState._collections;

    mtx.unlock(); // release lock

    SCOPED_LOCK(_updateLinksLock);

    return IResearchLinkHelper::updateLinks(
      collections, vocbase(), *this, links, stale
    );  } catch (arangodb::basics::Exception& e) {
    LOG_TOPIC(WARN, iresearch::TOPIC)
      << "caught exception while updating properties for arangosearch view '" << name() << "': " << e.code() << " " << e.what();
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      e.code(),
      std::string("error updating properties for arangosearch view '") + name() + "'"
    );
  } catch (std::exception const& e) {
    LOG_TOPIC(WARN, iresearch::TOPIC)
      << "caught exception while updating properties for arangosearch view '" << name() << "': " << e.what();
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      TRI_ERROR_BAD_PARAMETER,
      std::string("error updating properties for arangosearch view '") + name() + "'"
    );
  } catch (...) {
    LOG_TOPIC(WARN, iresearch::TOPIC)
      << "caught exception while updating properties for arangosearch view '" << name() << "'";
    IR_LOG_EXCEPTION();

    return arangodb::Result(
      TRI_ERROR_BAD_PARAMETER,
      std::string("error updating properties for arangosearch view '") + name() + "'"
    );
  }
}

arangodb::Result IResearchView::updateProperties(
  std::shared_ptr<AsyncMeta> const& meta
) {
  if (!meta) {
    return arangodb::Result(TRI_ERROR_BAD_PARAMETER);
  }

  std::atomic_store(&_meta, meta);

  if (_asyncFeature) {
    _asyncFeature->asyncNotify();
  }

  return arangodb::Result();
}

void IResearchView::registerFlushCallback() {
  auto* flush = getFlushFeature();

  if (!flush) {
    // feature not registered
    return;
  }

  auto viewSelf = self();

  flush->registerCallback(this, [viewSelf]() {
    static struct NoopFlushTransaction: arangodb::FlushTransaction {
      NoopFlushTransaction(): FlushTransaction("ArangoSearchNoop") {}
      virtual arangodb::Result commit() override {
        return arangodb::Result(TRI_ERROR_ARANGO_DATA_SOURCE_NOT_FOUND);
      }
    } noopFlushTransaction;
    SCOPED_LOCK_NAMED(viewSelf->mutex(), lock);

    if (!*viewSelf) {
      return arangodb::FlushFeature::FlushTransactionPtr(
        &noopFlushTransaction, [](arangodb::FlushTransaction*)->void {}
      );
    }

    auto trx = arangodb::FlushFeature::FlushTransactionPtr(
      viewSelf->get(),
      [](arangodb::FlushTransaction* trx)->void {
        ADOPT_SCOPED_LOCK_NAMED(static_cast<IResearchView*>(trx)->self()->mutex(), lock);
      }
    );

    lock.release(); // unlocked in distructor above

    return trx;
  });

  // noexcept
  _flushCallback.reset(this); // mark for future unregistration
}

bool IResearchView::visitCollections(
    LogicalView::CollectionVisitor const& visitor
) const {
  ReadMutex mutex(_mutex);
  SCOPED_LOCK(mutex);

  for (auto& cid: _metaState._collections) {
    if (!visitor(cid)) {
      return false;
    }
  }

  return true;
}

void IResearchView::FlushCallbackUnregisterer::operator()(IResearchView* view) const noexcept {
  arangodb::FlushFeature* flush = nullptr;

  if (!view || !(flush = getFlushFeature())) {
    return;
  }

  try {
    flush->unregisterCallback(view);
  } catch (...) {
    // suppress all errors
  }
}

void IResearchView::verifyKnownCollections() {
  // do not need to lock '_metaState' since only caller is a single-threaded recovery callback
  auto cids = _metaState._collections;

  {
    struct DummyTransaction : transaction::Methods {
      explicit DummyTransaction(std::shared_ptr<transaction::Context> const& ctx)
        : transaction::Methods(ctx) {
      }
    };

    transaction::StandaloneContext context(vocbase());
    std::shared_ptr<transaction::Context> dummy;  // intentionally empty
    DummyTransaction trx(std::shared_ptr<transaction::Context>(dummy, &context)); // use aliasing constructor

    if (!appendKnownCollections(cids, *snapshot(trx, Snapshot::FindOrCreate))) {
      LOG_TOPIC(ERR, arangodb::iresearch::TOPIC)
        << "failed to collect collection IDs for arangosearch view '" << id() << "'";

      return;
    }
  }

  for (auto cid : cids) {
    auto collection = vocbase().lookupCollection(cid);

    if (!collection) {
      // collection no longer exists, drop it and move on
      LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
        << "collection '" << cid
        << "' no longer exists! removing from arangosearch view '"
        << id() << "'";
      drop(cid);
    } else {
      // see if the link still exists, otherwise drop and move on
      auto link = IResearchLink::find(*collection, *this);
      if (!link) {
        LOG_TOPIC(TRACE, arangodb::iresearch::TOPIC)
          << "collection '" << cid
          << "' no longer linked! removing from arangosearch view '"
          << id() << "'";
        drop(cid);
      }
    }
  }
}

} // iresearch
} // arangodb

// -----------------------------------------------------------------------------
// --SECTION--                                                       END-OF-FILE
// -----------------------------------------------------------------------------