////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2017 ArangoDB 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 Simon Grätzer
////////////////////////////////////////////////////////////////////////////////

#include "RocksDBIncrementalSync.h"
#include "Basics/StaticStrings.h"
#include "Basics/StringUtils.h"
#include "Indexes/IndexIterator.h"
#include "Replication/DatabaseInitialSyncer.h"
#include "Replication/utilities.h"
#include "RocksDBEngine/RocksDBCollection.h"
#include "RocksDBIterators.h"
#include "SimpleHttpClient/SimpleHttpClient.h"
#include "SimpleHttpClient/SimpleHttpResult.h"
#include "StorageEngine/PhysicalCollection.h"
#include "Transaction/Helpers.h"
#include "Transaction/StandaloneContext.h"
#include "Utils/OperationOptions.h"
#include "VocBase/LocalDocumentId.h"
#include "VocBase/LogicalCollection.h"
#include "VocBase/ManagedDocumentResult.h"

#include <velocypack/Builder.h>
#include <velocypack/Iterator.h>
#include <velocypack/Slice.h>
#include <velocypack/velocypack-aliases.h>

namespace arangodb {
Result syncChunkRocksDB(
    DatabaseInitialSyncer& syncer, SingleCollectionTransaction* trx,
    InitialSyncerIncrementalSyncStats& stats,
    std::string const& keysId, uint64_t chunkId, std::string const& lowString,
    std::string const& highString,
    std::vector<std::pair<std::string, uint64_t>> const& markers) {

  // first thing we do is extend the batch lifetime
  if (!syncer._state.isChildSyncer) {
    syncer._batch.extend(syncer._state.connection, syncer._progress);
    syncer._state.barrier.extend(syncer._state.connection);
  }

  std::string const baseUrl = replutils::ReplicationUrl + "/keys";
  TRI_voc_tick_t const chunkSize = 5000;
  std::string const& collectionName = trx->documentCollection()->name();
  PhysicalCollection* physical = trx->documentCollection()->getPhysical();
  OperationOptions options;
  options.silent = true;
  options.ignoreRevs = true;
  options.isRestore = true;
  options.indexOperationMode = Index::OperationMode::internal;
  if (!syncer._state.leaderId.empty()) {
    options.isSynchronousReplicationFrom = syncer._state.leaderId;
  }

  LOG_TOPIC(TRACE, Logger::REPLICATION) << "syncing chunk. low: '" << lowString
                                        << "', high: '" << highString << "'";

  // no match
  // must transfer keys for non-matching range
  std::unique_ptr<httpclient::SimpleHttpResult> response;
  
  {
    std::string const url =
        baseUrl + "/" + keysId + "?type=keys&chunk=" + std::to_string(chunkId) +
        "&chunkSize=" + std::to_string(chunkSize) + "&low=" + lowString;

    syncer.setProgress(std::string("fetching keys chunk ") + std::to_string(chunkId) + " from " + url);

    // time how long the request takes
    double t = TRI_microtime();

    response.reset(syncer._state.connection.client->retryRequest(rest::RequestType::PUT, url, nullptr, 0,
                   replutils::createHeaders()));

    stats.waitedForKeys += TRI_microtime() - t;
    ++stats.numKeysRequests;
  }

  if (response == nullptr || !response->isComplete()) {
    return Result(TRI_ERROR_REPLICATION_NO_RESPONSE,
                  std::string("could not connect to master at ") +
                      syncer._state.master.endpoint + ": " +
                      syncer._state.connection.client->getErrorMessage());
  }

  TRI_ASSERT(response != nullptr);

  if (response->wasHttpError()) {
    return Result(TRI_ERROR_REPLICATION_MASTER_ERROR,
                  std::string("got invalid response from master at ") +
                      syncer._state.master.endpoint + ": HTTP " +
                      basics::StringUtils::itoa(response->getHttpReturnCode()) +
                      ": " + response->getHttpReturnMessage());
  }

  VPackBuilder builder;
  Result r = replutils::parseResponse(builder, response.get());
  response.reset(); // not needed anymore

  if (r.fail()) {
    return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                  std::string("got invalid response from master at ") +
                      syncer._state.master.endpoint + ": response is no array");
  }

  VPackSlice const responseBody = builder.slice();
  if (!responseBody.isArray()) {
    return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                  std::string("got invalid response from master at ") +
                      syncer._state.master.endpoint + ": response is no array");
  }

  transaction::BuilderLeaser keyBuilder(trx);

  std::vector<size_t> toFetch;

  size_t const numKeys = static_cast<size_t>(responseBody.length());
  if (numKeys == 0) {
    return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                  std::string("got invalid response from master at ") +
                      syncer._state.master.endpoint +
                      ": response contains an empty chunk. Collection: " +
                      collectionName + " Chunk: " + std::to_string(chunkId));
  }
  TRI_ASSERT(numKeys > 0);

  ManagedDocumentResult mmdr;
  size_t i = 0;
  size_t nextStart = 0;

  for (VPackSlice pair : VPackArrayIterator(responseBody)) {
    if (!pair.isArray() || pair.length() != 2) {
      return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                    std::string("got invalid response from master at ") +
                        syncer._state.master.endpoint +
                        ": response key pair is no valid array");
    }

    // key
    VPackSlice const keySlice = pair.at(0);
    if (!keySlice.isString()) {
      return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                    std::string("got invalid response from master at ") +
                        syncer._state.master.endpoint +
                        ": response key is no string");
    }

    // rid
    if (markers.empty()) {
      // no local markers
      toFetch.emplace_back(i);
      i++;
      continue;
    }

    bool mustRefetch = false;

    // remove keys not present anymore
    while (nextStart < markers.size()) {
      std::string const& localKey = markers[nextStart].first;

      int res = keySlice.compareString(localKey);
      if (res > 0) {
        // we have a local key that is not present remotely
        keyBuilder->clear();
        keyBuilder->openObject();
        keyBuilder->add(StaticStrings::KeyString, VPackValue(localKey));
        keyBuilder->close();

        trx->remove(collectionName, keyBuilder->slice(), options);
        ++stats.numDocsRemoved;

        ++nextStart;
      } else if (res == 0) {
        // key match
        break;
      } else {
        // we have a remote key that is not present locally
        TRI_ASSERT(res < 0);
        mustRefetch = true;
        break;
      }
    }

    if (mustRefetch) {
      toFetch.emplace_back(i);
    } else {
      // see if key exists
      LocalDocumentId const documentId = physical->lookupKey(trx, keySlice);
      if (!documentId.isSet()) {
        // key not found locally
        toFetch.emplace_back(i);
      } else {
        TRI_voc_rid_t currentRevisionId = 0;
        if (physical->readDocument(trx, documentId, mmdr)) {
          currentRevisionId = transaction::helpers::extractRevFromDocument(
              VPackSlice(mmdr.vpack()));
        }
        if (!pair.at(1).isEqualString(TRI_RidToString(currentRevisionId))) {
          // key found, but revision id differs
          toFetch.emplace_back(i);
          ++nextStart;
        } else {
          // a match - nothing to do!
          ++nextStart;
        }
      }
    }

    i++;
  }

  // delete all keys at end of the range
  while (nextStart < markers.size()) {
    std::string const& localKey = markers[nextStart].first;

    if (localKey.compare(highString) > 0) {
      // we have a local key that is not present remotely
      keyBuilder->clear();
      keyBuilder->openObject(true);
      keyBuilder->add(StaticStrings::KeyString, VPackValue(localKey));
      keyBuilder->close();

      trx->remove(collectionName, keyBuilder->slice(), options);
      ++stats.numDocsRemoved;
    }
    ++nextStart;
  }

  if (toFetch.empty()) {
    // nothing to do
    return Result();
  }

  if (!syncer._state.isChildSyncer) {
    syncer._batch.extend(syncer._state.connection, syncer._progress);
    syncer._state.barrier.extend(syncer._state.connection);
  }

  LOG_TOPIC(TRACE, Logger::REPLICATION)
      << "will refetch " << toFetch.size() << " documents for this chunk";

  VPackBuilder keysBuilder;
  keysBuilder.openArray(false);
  for (auto const& it : toFetch) {
    keysBuilder.add(VPackValue(it));
  }
  keysBuilder.close();

  std::string const keyJsonString(keysBuilder.slice().toJson());

  size_t offsetInChunk = 0;

  while (true) {
    std::unique_ptr<httpclient::SimpleHttpResult> response;

    {
      std::string const url =
          baseUrl + "/" + keysId + "?type=docs&chunk=" + std::to_string(chunkId) +
          "&chunkSize=" + std::to_string(chunkSize) + "&low=" + lowString +
          "&offset=" + std::to_string(offsetInChunk);

      syncer.setProgress(std::string("fetching documents chunk ") + std::to_string(chunkId) + " (" +
                 std::to_string(toFetch.size()) + " keys) for collection '" +
                 collectionName + "' from " + url);

      double t = TRI_microtime();

      response.reset(syncer._state.connection.client->retryRequest(
            rest::RequestType::PUT, url, keyJsonString.c_str(),
            keyJsonString.size(), replutils::createHeaders()));

      stats.waitedForDocs += TRI_microtime() - t;
      stats.numDocsRequested += toFetch.size();
      ++stats.numDocsRequests;
    }

    if (response == nullptr || !response->isComplete()) {
      return Result(TRI_ERROR_REPLICATION_NO_RESPONSE,
                    std::string("could not connect to master at ") +
                        syncer._state.master.endpoint + ": " +
                        syncer._state.connection.client->getErrorMessage());
    }

    TRI_ASSERT(response != nullptr);

    if (response->wasHttpError()) {
      return Result(
          TRI_ERROR_REPLICATION_MASTER_ERROR,
          std::string("got invalid response from master at ") +
              syncer._state.master.endpoint + ": HTTP " +
              basics::StringUtils::itoa(response->getHttpReturnCode()) + ": " +
              response->getHttpReturnMessage());
    }

    VPackBuilder builder;
    Result r = replutils::parseResponse(builder, response.get());

    if (r.fail()) {
      return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                    std::string("got invalid response from master at ") +
                        syncer._state.master.endpoint +
                        ": response is no array");
    }

    VPackSlice const slice = builder.slice();
    if (!slice.isArray()) {
      return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                    std::string("got invalid response from master at ") +
                        syncer._state.master.endpoint +
                        ": response is no array");
    }

    size_t foundLength = slice.length();

    for (auto const& it : VPackArrayIterator(slice)) {
      if (it.isNull()) {
        continue;
      }

      if (!it.isObject()) {
        return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                      std::string("got invalid response from master at ") +
                          syncer._state.master.endpoint +
                          ": document is no object");
      }

      VPackSlice const keySlice = it.get(StaticStrings::KeyString);

      if (!keySlice.isString()) {
        return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                      std::string("got invalid response from master at ") +
                          syncer._state.master.endpoint +
                          ": document key is invalid");
      }

      VPackSlice const revSlice = it.get(StaticStrings::RevString);

      if (!revSlice.isString()) {
        return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                      std::string("got invalid response from master at ") +
                          syncer._state.master.endpoint +
                          ": document revision is invalid");
      }

      LocalDocumentId const documentId = physical->lookupKey(trx, keySlice);

      auto removeConflict =
          [&](std::string const& conflictingKey) -> OperationResult {
        VPackBuilder conflict;
        conflict.add(VPackValue(conflictingKey));
        LocalDocumentId conflictId = physical->lookupKey(trx, conflict.slice());
        if (conflictId.isSet()) {
          ManagedDocumentResult mmdr;
          bool success = physical->readDocument(trx, conflictId, mmdr);
          if (success) {
            VPackSlice conflictingKey(mmdr.vpack());
            return trx->remove(collectionName, conflictingKey, options);
          }
        }
        return OperationResult(TRI_ERROR_ARANGO_DOCUMENT_NOT_FOUND);
      };

      if (!documentId.isSet()) {
        // INSERT
        OperationResult opRes = trx->insert(collectionName, it, options);
        if (opRes.fail()) {
          if (opRes.is(TRI_ERROR_ARANGO_UNIQUE_CONSTRAINT_VIOLATED) &&
              opRes.errorMessage() > keySlice.copyString()) {
            // remove conflict and retry
            auto inner = removeConflict(opRes.errorMessage());
            if (inner.fail()) {
              return opRes.result;
            }
            opRes = trx->insert(collectionName, it, options);
            if (opRes.fail()) {
              return opRes.result;
            }
          } else {
            return opRes.result;
          }
        }
      } else {
        // REPLACE
        OperationResult opRes = trx->replace(collectionName, it, options);
        if (opRes.fail()) {
          if (opRes.is(TRI_ERROR_ARANGO_UNIQUE_CONSTRAINT_VIOLATED) &&
              opRes.errorMessage() > keySlice.copyString()) {
            // remove conflict and retry
            auto inner = removeConflict(opRes.errorMessage());
            if (inner.fail()) {
              return opRes.result;
            }
            opRes = trx->replace(collectionName, it, options);
            if (opRes.fail()) {
              return opRes.result;
            }
          } else {
            return opRes.result;
          }
        }
      }
      ++stats.numDocsInserted;
    }

    if (foundLength >= toFetch.size()) {
      break;
    }

    // try again in next round
    offsetInChunk = foundLength;
  }

  return Result();
}

Result handleSyncKeysRocksDB(DatabaseInitialSyncer& syncer,
                             arangodb::LogicalCollection* col,
                             std::string const& keysId) {
  double const startTime = TRI_microtime();

  syncer.setProgress(std::string("collecting local keys for collection '") + col->name() + "'");

  if (syncer.isAborted()) {
    return Result(TRI_ERROR_REPLICATION_APPLIER_STOPPED);
  }

  if (!syncer._state.isChildSyncer) {
    syncer._batch.extend(syncer._state.connection, syncer._progress);
    syncer._state.barrier.extend(syncer._state.connection);
  }
  
  TRI_voc_tick_t const chunkSize = 5000;
  std::string const baseUrl = replutils::ReplicationUrl + "/keys";
  
  InitialSyncerIncrementalSyncStats stats;

  std::unique_ptr<httpclient::SimpleHttpResult> response;

  {
    std::string const url =
        baseUrl + "/" + keysId + "?chunkSize=" + std::to_string(chunkSize);

    syncer.setProgress(std::string("fetching remote keys chunks for collection '") + col->name() + "' from " + url);
    
    auto const headers = replutils::createHeaders();
    
    double t = TRI_microtime();
    response.reset(syncer._state.connection.client->retryRequest(rest::RequestType::GET, url, nullptr, 0, headers));

    stats.waitedForInitial += TRI_microtime() - t;
  }

  if (response == nullptr || !response->isComplete()) {
    return Result(TRI_ERROR_REPLICATION_NO_RESPONSE,
                  std::string("could not connect to master at ") +
                      syncer._state.master.endpoint + ": " +
                      syncer._state.connection.client->getErrorMessage());
  }

  TRI_ASSERT(response != nullptr);

  if (response->wasHttpError()) {
    return Result(TRI_ERROR_REPLICATION_MASTER_ERROR,
                  std::string("got invalid response from master at ") +
                      syncer._state.master.endpoint + ": HTTP " +
                      basics::StringUtils::itoa(response->getHttpReturnCode()) +
                      ": " + response->getHttpReturnMessage());
  }

  VPackBuilder builder;
  Result r = replutils::parseResponse(builder, response.get());

  if (r.fail()) {
    return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                  std::string("got invalid response from master at ") +
                      syncer._state.master.endpoint + ": response is no array");
  }

  VPackSlice const chunkSlice = builder.slice();

  if (!chunkSlice.isArray()) {
    return Result(TRI_ERROR_REPLICATION_INVALID_RESPONSE,
                  std::string("got invalid response from master at ") +
                      syncer._state.master.endpoint + ": response is no array");
  }

  ManagedDocumentResult mmdr;
  OperationOptions options;
  options.silent = true;
  options.ignoreRevs = true;
  options.isRestore = true;
  if (!syncer._state.leaderId.empty()) {
    options.isSynchronousReplicationFrom = syncer._state.leaderId;
  }

  VPackBuilder keyBuilder;
  size_t const numChunks = static_cast<size_t>(chunkSlice.length());

  // remove all keys that are below first remote key or beyond last remote key
  if (numChunks > 0) {
    // first chunk
    SingleCollectionTransaction trx(
      transaction::StandaloneContext::Create(syncer.vocbase()),
      col,
      AccessMode::Type::EXCLUSIVE
    );

    trx.addHint(
        transaction::Hints::Hint::RECOVERY);  // to turn off waitForSync!
    trx.addHint(transaction::Hints::Hint::NO_TRACKING);
    trx.addHint(transaction::Hints::Hint::NO_INDEXING);

    Result res = trx.begin();

    if (!res.ok()) {
      return Result(
          res.errorNumber(),
          std::string("unable to start transaction: ") + res.errorMessage());
    }

    VPackSlice chunk = chunkSlice.at(0);

    TRI_ASSERT(chunk.isObject());
    auto lowSlice = chunk.get("low");
    TRI_ASSERT(lowSlice.isString());

    // last high
    chunk = chunkSlice.at(numChunks - 1);
    TRI_ASSERT(chunk.isObject());

    auto highSlice = chunk.get("high");
    TRI_ASSERT(highSlice.isString());

    StringRef lowRef(lowSlice);
    StringRef highRef(highSlice);

    LogicalCollection* coll = trx.documentCollection();
    auto iterator = createPrimaryIndexIterator(&trx, coll);

    VPackBuilder builder;
    iterator.next(
        [&](rocksdb::Slice const& rocksKey, rocksdb::Slice const& rocksValue) {
          StringRef docKey(RocksDBKey::primaryKey(rocksKey));
          if (docKey.compare(lowRef) < 0 || docKey.compare(highRef) > 0) {
            builder.clear();
            builder.add(velocypack::ValuePair(docKey.data(), docKey.size(),
                                              velocypack::ValueType::String));
            trx.remove(col->name(), builder.slice(), options);
            ++stats.numDocsRemoved;
          }
        },
        std::numeric_limits<std::uint64_t>::max());

    res = trx.commit();

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

  {
    if (syncer.isAborted()) {
      return Result(TRI_ERROR_REPLICATION_APPLIER_STOPPED);
    }

    SingleCollectionTransaction trx(
      transaction::StandaloneContext::Create(syncer.vocbase()),
      col,
      AccessMode::Type::EXCLUSIVE
    );

    trx.addHint(
        transaction::Hints::Hint::RECOVERY);  // to turn off waitForSync!
    trx.addHint(transaction::Hints::Hint::NO_TRACKING);
    trx.addHint(transaction::Hints::Hint::NO_INDEXING);

    Result res = trx.begin();

    if (!res.ok()) {
      return Result(
          res.errorNumber(),
          std::string("unable to start transaction: ") + res.errorMessage());
    }

    // We do not take responsibility for the index.
    // The LogicalCollection is protected by trx.
    // Neither it nor it's indexes can be invalidated

    size_t currentChunkId = 0;

    std::string lowKey;
    std::string highKey;
    std::string hashString;
    uint64_t localHash = 0x012345678;
    // chunk keys + revisionId
    std::vector<std::pair<std::string, uint64_t>> markers;
    bool foundLowKey = false;

    auto resetChunk = [&]() -> void {
      if (!syncer._state.isChildSyncer) {
        syncer._batch.extend(syncer._state.connection, syncer._progress);
        syncer._state.barrier.extend(syncer._state.connection);
      }

      syncer.setProgress(std::string("processing keys chunk ") + std::to_string(currentChunkId) +
                 " for collection '" + col->name() + "'");

      // read remote chunk
      TRI_ASSERT(chunkSlice.isArray());
      TRI_ASSERT(chunkSlice.length() >
                 0);  // chunkSlice.at will throw otherwise
      VPackSlice chunk = chunkSlice.at(currentChunkId);
      if (!chunk.isObject()) {
        THROW_ARANGO_EXCEPTION_MESSAGE(
            TRI_ERROR_REPLICATION_INVALID_RESPONSE,
            std::string("got invalid response from master at ") +
                syncer._state.master.endpoint + ": chunk is no object");
      }

      VPackSlice const lowSlice = chunk.get("low");
      VPackSlice const highSlice = chunk.get("high");
      VPackSlice const hashSlice = chunk.get("hash");
      if (!lowSlice.isString() || !highSlice.isString() ||
          !hashSlice.isString()) {
        THROW_ARANGO_EXCEPTION_MESSAGE(
            TRI_ERROR_REPLICATION_INVALID_RESPONSE,
            std::string("got invalid response from master at ") +
                syncer._state.master.endpoint +
                ": chunks in response have an invalid format");
      }

      // now reset chunk information
      markers.clear();
      lowKey = lowSlice.copyString();
      highKey = highSlice.copyString();
      hashString = hashSlice.copyString();
      localHash = 0x012345678;
      foundLowKey = false;
    };
    // set to first chunk
    resetChunk();

    // tempBuilder is reused inside compareChunk
    VPackBuilder tempBuilder;

    std::function<void(std::string, std::uint64_t)> compareChunk =
        [&trx, &col, &options, &foundLowKey, &markers, &localHash, &hashString,
         &syncer, &currentChunkId, &numChunks, &keysId, &resetChunk,
         &compareChunk, &lowKey, &highKey, 
         &tempBuilder, &stats](std::string const& docKey, std::uint64_t docRev) {
          bool rangeUnequal = false;
          bool nextChunk = false;

          tempBuilder.clear(); 
          tempBuilder.add(VPackValue(docKey));
          VPackSlice docKeySlice(tempBuilder.slice());
          int cmp1 = docKey.compare(lowKey);
          int cmp2 = docKey.compare(highKey);

          if (cmp1 < 0) {
            // smaller values than lowKey mean they don't exist remotely
            trx.remove(col->name(), docKeySlice, options);
            ++stats.numDocsRemoved;
            return;
          }
           
          if (cmp1 >= 0 && cmp2 <= 0) {
            // we only need to hash we are in the range
            if (cmp1 == 0) {
              foundLowKey = true;
            }

            markers.emplace_back(docKey, docRev);  // revision as uint64
            // don't bother hashing if we have't found lower key
            if (foundLowKey) {
              localHash ^= docKeySlice.hashString();
  
              tempBuilder.clear();
              // use a temporary char buffer for building to rid string
              char ridBuffer[21];
              tempBuilder.add(TRI_RidToValuePair(docRev, &ridBuffer[0]));
              localHash ^= tempBuilder.slice().hashString();  // revision as string

              if (cmp2 == 0) {  // found highKey
                rangeUnequal = std::to_string(localHash) != hashString;
                nextChunk = true;
              }
            } else if (cmp2 == 0) {  // found high key, but not low key
              rangeUnequal = true;
              nextChunk = true;
            }
          } else if (cmp2 > 0) {  // higher than highKey
            // current range was unequal and we did not find the
            // high key. Load range and skip to next
            rangeUnequal = true;
            nextChunk = true;
          }

          TRI_ASSERT(!rangeUnequal || nextChunk);  // A => B
          if (nextChunk) {  // we are out of range, see next chunk
            if (rangeUnequal && currentChunkId < numChunks) {
              Result res =
                  syncChunkRocksDB(syncer, &trx, stats, keysId, currentChunkId, lowKey,
                                   highKey, markers);
              if (!res.ok()) {
                THROW_ARANGO_EXCEPTION(res);
              }
            }
            currentChunkId++;
            if (currentChunkId < numChunks) {
              resetChunk();
              // key is higher than upper bound, recheck the current document
              if (cmp2 > 0) {
                compareChunk(docKey, docRev);
              }
            }
          }
        };  // compare chunk - end

    LogicalCollection* coll = trx.documentCollection();
    auto iterator = createPrimaryIndexIterator(&trx, coll);
    iterator.next(
        [&](rocksdb::Slice const& rocksKey, rocksdb::Slice const& rocksValue) {
          std::string docKey = RocksDBKey::primaryKey(rocksKey).toString();
          TRI_voc_rid_t docRev;
          if (!RocksDBValue::revisionId(rocksValue, docRev)) {
            // for collections that do not have the revisionId in the value
            auto documentId = RocksDBValue::documentId(
                rocksValue);  // we want probably to do this instead
            if (col->readDocument(&trx, documentId, mmdr) == false) {
              TRI_ASSERT(false);
              return;
            }
            VPackSlice doc(mmdr.vpack());
            docRev = TRI_ExtractRevisionId(doc);
          }
          compareChunk(docKey, docRev);
        },
        std::numeric_limits<std::uint64_t>::max());  // no limit on documents

    // we might have missed chunks, if the keys don't exist at all locally
    while (currentChunkId < numChunks) {
      Result res = syncChunkRocksDB(syncer, &trx, stats, keysId, currentChunkId,
                                    lowKey, highKey, markers);
      if (!res.ok()) {
        THROW_ARANGO_EXCEPTION(res);
      }
      currentChunkId++;
      if (currentChunkId < numChunks) {
        resetChunk();
      }
    }

    res = trx.commit();
    if (!res.ok()) {
      return res;
    }
  }

  syncer.setProgress(
      std::string("incremental sync statistics for collection '") + col->name() + "': " + 
      "keys requests: " + std::to_string(stats.numKeysRequests) + ", " +
      "docs requests: " + std::to_string(stats.numDocsRequests) + ", " +
      "number of documents requested: " + std::to_string(stats.numDocsRequested) + ", " +
      "number of documents inserted: " + std::to_string(stats.numDocsInserted) + ", " +
      "number of documents removed: " + std::to_string(stats.numDocsRemoved) + ", " +
      "waited for initial: " + std::to_string(stats.waitedForInitial) + " s, " +
      "waited for keys: " + std::to_string(stats.waitedForKeys) + " s, " +
      "waited for docs: " + std::to_string(stats.waitedForDocs) + " s, " + 
      "total time: " + std::to_string(TRI_microtime() - startTime) + " s");

  return Result();
}
}  // namespace arangodb