arangodb/arangod/Cluster/TakeoverShardLeadership.cpp

////////////////////////////////////////////////////////////////////////////////
/// DISCLAIMER
///
/// Copyright 2014-2018 ArangoDB GmbH, Cologne, Germany
/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
///
/// Licensed under the Apache License, Version 2.0 (the "License");
/// you may not use this file except in compliance with the License.
/// You may obtain a copy of the License at
///
///     http://www.apache.org/licenses/LICENSE-2.0
///
/// Unless required by applicable law or agreed to in writing, software
/// distributed under the License is distributed on an "AS IS" BASIS,
/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
/// See the License for the specific language governing permissions and
/// limitations under the License.
///
/// Copyright holder is ArangoDB GmbH, Cologne, Germany
///
/// @author Kaveh Vahedipour
/// @author Matthew Von-Maszewski
/// @author Max Neunhoeffer
////////////////////////////////////////////////////////////////////////////////

#include "TakeoverShardLeadership.h"
#include "MaintenanceFeature.h"

#include "ApplicationFeatures/ApplicationServer.h"
#include "Basics/VelocyPackHelper.h"
#include "Cluster/ClusterComm.h"
#include "Cluster/ClusterFeature.h"
#include "Cluster/ClusterInfo.h"
#include "Cluster/FollowerInfo.h"
#include "Logger/LogMacros.h"
#include "Logger/Logger.h"
#include "Logger/LoggerStream.h"
#include "Transaction/ClusterUtils.h"
#include "Transaction/Methods.h"
#include "Transaction/StandaloneContext.h"
#include "Utils/DatabaseGuard.h"
#include "Utils/SingleCollectionTransaction.h"
#include "VocBase/LogicalCollection.h"
#include "VocBase/Methods/Collections.h"
#include "VocBase/Methods/Databases.h"

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

using namespace arangodb;
using namespace arangodb::application_features;
using namespace arangodb::maintenance;
using namespace arangodb::methods;

TakeoverShardLeadership::TakeoverShardLeadership(MaintenanceFeature& feature,
                                             ActionDescription const& desc)
    : ActionBase(feature, desc) {
  std::stringstream error;

  _labels.emplace(FAST_TRACK);

  if (!desc.has(DATABASE)) {
    error << "database must be specified";
  }
  TRI_ASSERT(desc.has(DATABASE));

  if (!desc.has(COLLECTION)) {
    error << "collection must be specified. ";
  }
  TRI_ASSERT(desc.has(COLLECTION));

  if (!desc.has(SHARD)) {
    error << "shard must be specified";
  }
  TRI_ASSERT(desc.has(SHARD));

  if (!desc.has(THE_LEADER)) {
    error << "leader must be specified. ";
  }
  TRI_ASSERT(desc.has(THE_LEADER));

  if (!desc.has(LOCAL_LEADER)) {
    error << "local leader must be specified. ";
  }
  TRI_ASSERT(desc.has(LOCAL_LEADER));

  TRI_ASSERT(desc.has(OLD_CURRENT_COUNTER));

  if (!error.str().empty()) {
    LOG_TOPIC("2aa85", ERR, Logger::MAINTENANCE) << "TakeoverLeadership: " << error.str();
    _result.reset(TRI_ERROR_INTERNAL, error.str());
    setState(FAILED);
  }
}

TakeoverShardLeadership::~TakeoverShardLeadership() {}

static void sendLeaderChangeRequests(std::vector<ServerID> const& currentServers,
                              std::shared_ptr<std::vector<ServerID>>& realInsyncFollowers,
                              std::string const& databaseName, ShardID const& shardID,
                              std::string const& oldLeader) {

  auto cc = ClusterComm::instance();
  if (cc == nullptr) {
    // nullptr happens only during controlled shutdown
    return;
  }

  std::string const& sid = ServerState::instance()->getId();


  VPackBuilder bodyBuilder;
  {
    VPackObjectBuilder ob(&bodyBuilder);
    bodyBuilder.add("leaderId", VPackValue(sid));
    bodyBuilder.add("oldLeaderId", VPackValue(oldLeader));
    bodyBuilder.add("shard", VPackValue(shardID));
  }

  std::string const url = "/_db/" + databaseName + "/_api/replication/set-the-leader";

  std::vector<ClusterCommRequest> requests;
  auto body = std::make_shared<std::string>(bodyBuilder.toJson());
  for (auto const& srv : currentServers) {
    if (srv == sid) {
      continue; // ignore ourself
    }
    LOG_TOPIC("42516", DEBUG, Logger::MAINTENANCE)
      << "Sending " << bodyBuilder.toJson() << " to " << srv;
    requests.emplace_back("server:" + srv, RequestType::PUT, url, body);
  }

  cc->performRequests(requests, 3.0, Logger::COMMUNICATION, false);

  // This code intentionally ignores all errors
  realInsyncFollowers = std::make_shared<std::vector<ServerID>>();
  for (auto const& req : requests) {
    ClusterCommResult const& result = req.result;
    if (result.status == CL_COMM_RECEIVED && result.errorCode == TRI_ERROR_NO_ERROR) {
      if (result.result && result.result->getHttpReturnCode() == 200) {
        realInsyncFollowers->push_back(result.serverID);
      }
    }
  }
}

static void handleLeadership(LogicalCollection& collection,
                      std::string const& localLeader,
                      std::string const& plannedLeader,
                      std::string const& databaseName,
                      uint64_t oldCounter, MaintenanceFeature& feature) {
  auto& followers = collection.followers();

  if (plannedLeader.empty()) {   // Planned to lead
    if (!localLeader.empty()) {  // We were not leader, assume leadership
      // This will block the thread until we fetched a new current version
      // in maintenance main thread.
      feature.waitForLargerCurrentCounter(oldCounter);
      auto currentInfo = ClusterInfo::instance()->getCollectionCurrent(
          databaseName, std::to_string(collection.planId()));
      if (currentInfo == nullptr) {
        // Collection has been dropped we cannot continue here.
        return;
      }
      TRI_ASSERT(currentInfo != nullptr);
      std::vector<ServerID> currentServers = currentInfo->servers(collection.name());
      std::shared_ptr<std::vector<ServerID>> realInsyncFollowers;

      if (currentServers.size() > 0) {
        std::string& oldLeader = currentServers.at(0);
        // Check if the old leader has resigned and stopped all write
        // (if so, we can assume that all servers are still in sync)
        if (oldLeader.at(0) == '_') {
          // remove the underscore from the list as it is useless anyway
          oldLeader = oldLeader.substr(1);

          // Update all follower and tell them that we are the leader now
          sendLeaderChangeRequests(currentServers, realInsyncFollowers, databaseName, collection.name(), oldLeader);
        }
      }

      std::vector<ServerID> failoverCandidates = currentInfo->failoverCandidates(collection.name());
      followers->takeOverLeadership(failoverCandidates, realInsyncFollowers);
      transaction::cluster::abortFollowerTransactionsOnShard(collection.id());
    }
  } else {  // Planned to follow
    if (localLeader.empty()) {
      // Note that the following does not delete the follower list
      // and that this is crucial, because in the planned leader
      // resign case, updateCurrentForCollections will report the
      // resignation together with the old in-sync list to the
      // agency. If this list would be empty, then the supervision
      // would be very angry with us!
      followers->setTheLeader(plannedLeader);
      transaction::cluster::abortLeaderTransactionsOnShard(collection.id());
    }
    // Note that if we have been a follower to some leader
    // we do not immediately adjust the leader here, even if
    // the planned leader differs from what we have set locally.
    // The setting must only be adjusted once we have
    // synchronized with the new leader and negotiated
    // a leader/follower relationship!
  }
}

bool TakeoverShardLeadership::first() {
  std::string const& database = _description.get(DATABASE);
  std::string const& collection = _description.get(COLLECTION);
  std::string const& shard = _description.get(SHARD);
  std::string const& plannedLeader = _description.get(THE_LEADER);
  std::string const& localLeader = _description.get(LOCAL_LEADER);
  std::string const& oldCounterString = _description.get(OLD_CURRENT_COUNTER);
  uint64_t oldCounter = basics::StringUtils::uint64(oldCounterString);

  try {
    DatabaseGuard guard(database);
    auto& vocbase = guard.database();
    Result found = methods::Collections::lookup(
        vocbase, shard, [&](std::shared_ptr<LogicalCollection> const& coll) -> void {
          TRI_ASSERT(coll);
          LOG_TOPIC("5632a", DEBUG, Logger::MAINTENANCE)
              << "trying to become leader of shard '" << database << "/" << shard;
          // We adjust local leadership, note that the planned
          // resignation case is not handled here, since then
          // ourselves does not appear in shards[shard] but only
          // "_" + ourselves.
          handleLeadership(*coll, localLeader, plannedLeader,
                           vocbase.name(), oldCounter, feature());
        });

    if (found.fail()) {
      std::stringstream error;
      error << "TakeoverShardLeadership: failed to lookup local collection " << shard << "in database " + database;
      LOG_TOPIC("65342", ERR, Logger::MAINTENANCE) << error.str();
      _result = actionError(TRI_ERROR_ARANGO_DATA_SOURCE_NOT_FOUND, error.str());
    }
  } catch (std::exception const& e) {
    std::stringstream error;

    error << "action " << _description << " failed with exception " << e.what();
    LOG_TOPIC("79443", WARN, Logger::MAINTENANCE)
        << "TakeoverShardLeadership: " << error.str();
    _result.reset(TRI_ERROR_INTERNAL, error.str());
  }

  if (_result.fail()) {
    _feature.storeShardError(database, collection, shard,
                             _description.get(SERVER_ID), _result);
  }

  notify();
  return false;
}