mirror of https://gitee.com/bigwinds/arangodb
232 lines
9.0 KiB
C++
232 lines
9.0 KiB
C++
////////////////////////////////////////////////////////////////////////////////
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/// DISCLAIMER
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///
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/// Copyright 2014-2018 ArangoDB GmbH, Cologne, Germany
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/// Copyright 2004-2014 triAGENS GmbH, Cologne, Germany
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///
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/// Licensed under the Apache License, Version 2.0 (the "License");
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/// you may not use this file except in compliance with the License.
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/// You may obtain a copy of the License at
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///
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/// http://www.apache.org/licenses/LICENSE-2.0
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///
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/// Unless required by applicable law or agreed to in writing, software
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/// distributed under the License is distributed on an "AS IS" BASIS,
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/// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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/// See the License for the specific language governing permissions and
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/// limitations under the License.
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///
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/// Copyright holder is ArangoDB GmbH, Cologne, Germany
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///
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/// @author Max Neunhoeffer
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/// @author Andreas Streichardt
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////////////////////////////////////////////////////////////////////////////////
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#ifndef ARANGOD_CLUSTER_FOLLOWER_INFO_H
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#define ARANGOD_CLUSTER_FOLLOWER_INFO_H 1
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#include "ClusterInfo.h"
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#include "Basics/Mutex.h"
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#include "Basics/ReadWriteLock.h"
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#include "Basics/Result.h"
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#include "Basics/WriteLocker.h"
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#include "StorageEngine/EngineSelectorFeature.h"
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#include "StorageEngine/StorageEngine.h"
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#include "VocBase/LogicalCollection.h"
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namespace arangodb {
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namespace velocypack {
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class Slice;
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}
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////////////////////////////////////////////////////////////////////////////////
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/// @brief a class to track followers that are in sync for a shard
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////////////////////////////////////////////////////////////////////////////////
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class FollowerInfo {
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// This is the list of real local followers
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std::shared_ptr<std::vector<ServerID> const> _followers;
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// This is the list of followers that have been insync BEFORE we
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// triggered a failover to this server.
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// The list is filled only temporarily, and will be deleted as
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// soon as we can guarantee at least so many followers locally.
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std::shared_ptr<std::vector<ServerID> const> _failoverCandidates;
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// The agencyMutex is used to synchronise access to the agency.
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// the _dataLock is used to sync the access to local data.
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// The _canWriteLock is used to protect flag if we do have enough followers
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// The locking ordering to avoid dead locks has to be as follows:
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// 1.) _agencyMutex
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// 2.) _canWriteLock
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// 3.) _dataLock
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mutable Mutex _agencyMutex;
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mutable arangodb::basics::ReadWriteLock _canWriteLock;
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mutable arangodb::basics::ReadWriteLock _dataLock;
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arangodb::LogicalCollection* _docColl;
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// if the latter is empty, then we are leading
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std::string _theLeader;
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bool _theLeaderTouched;
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// flag if we have enough insnc followers and can pass through writes
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bool _canWrite;
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public:
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explicit FollowerInfo(arangodb::LogicalCollection* d)
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: _followers(std::make_shared<std::vector<ServerID>>()),
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_failoverCandidates(std::make_shared<std::vector<ServerID>>()),
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_docColl(d),
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_theLeader(""),
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_theLeaderTouched(false),
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_canWrite(_docColl->replicationFactor() <= 1) {
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// On replicationfactor 1 we do not have any failover servers to maintain.
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// This should also disable satellite tracking.
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}
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////////////////////////////////////////////////////////////////////////////////
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/// @brief get information about current followers of a shard.
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////////////////////////////////////////////////////////////////////////////////
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std::shared_ptr<std::vector<ServerID> const> get() const {
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READ_LOCKER(readLocker, _dataLock);
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return _followers;
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}
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////////////////////////////////////////////////////////////////////////////////
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/// @brief get information about current followers of a shard.
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////////////////////////////////////////////////////////////////////////////////
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std::shared_ptr<std::vector<ServerID> const> getFailoverCandidates() const {
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READ_LOCKER(readLocker, _dataLock);
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return _failoverCandidates;
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}
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////////////////////////////////////////////////////////////////////////////////
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/// @brief Take over leadership for this shard.
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/// Also inject information of a insync followers that we knew about
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/// before a failover to this server has happened
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/// The second parameter may be nullptr. It is an additional list
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/// of declared to be insync followers. If it is nullptr the follower
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/// list is initialised empty.
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////////////////////////////////////////////////////////////////////////////////
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void takeOverLeadership(std::vector<ServerID> const& previousInsyncFollowers,
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std::shared_ptr<std::vector<ServerID>> realInsyncFollowers);
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//////////////////////////////////////////////////////////////////////////////
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/// @brief add a follower to a shard, this is only done by the server side
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/// of the "get-in-sync" capabilities. This reports to the agency under
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/// `/Current` but in asynchronous "fire-and-forget" way. The method
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/// fails silently, if the follower information has since been dropped
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/// (see `dropFollowerInfo` below).
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//////////////////////////////////////////////////////////////////////////////
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Result add(ServerID const& s);
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//////////////////////////////////////////////////////////////////////////////
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/// @brief remove a follower from a shard, this is only done by the
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/// server if a synchronous replication request fails. This reports to
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/// the agency under `/Current` but in an asynchronous "fire-and-forget"
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/// way.
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//////////////////////////////////////////////////////////////////////////////
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Result remove(ServerID const& s);
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//////////////////////////////////////////////////////////////////////////////
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/// @brief clear follower list, no changes in agency necesary
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//////////////////////////////////////////////////////////////////////////////
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void clear();
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//////////////////////////////////////////////////////////////////////////////
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/// @brief check whether the given server is a follower
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//////////////////////////////////////////////////////////////////////////////
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bool contains(ServerID const& s) const;
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//////////////////////////////////////////////////////////////////////////////
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/// @brief set leadership
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//////////////////////////////////////////////////////////////////////////////
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void setTheLeader(std::string const& who) {
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// Empty leader => we are now new leader.
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// This needs to be handled with takeOverLeadership
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TRI_ASSERT(!who.empty());
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WRITE_LOCKER(writeLocker, _dataLock);
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_theLeader = who;
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_theLeaderTouched = true;
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief get the leader
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//////////////////////////////////////////////////////////////////////////////
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std::string getLeader() const {
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READ_LOCKER(readLocker, _dataLock);
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return _theLeader;
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief see if leader was explicitly set
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//////////////////////////////////////////////////////////////////////////////
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bool getLeaderTouched() const {
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READ_LOCKER(readLocker, _dataLock);
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return _theLeaderTouched;
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}
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bool allowedToWrite() {
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{
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auto engine = arangodb::EngineSelectorFeature::ENGINE;
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TRI_ASSERT(engine != nullptr);
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if (engine->inRecovery()) {
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return true;
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}
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READ_LOCKER(readLocker, _canWriteLock);
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if (_canWrite) {
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// Someone has decided we can write, fastPath!
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#ifdef ARANGODB_ENABLE_MAINTAINER_MODE
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// Invariant, we can only WRITE if we do not have other failover candidates
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READ_LOCKER(readLockerData, _dataLock);
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TRI_ASSERT(_followers->size() == _failoverCandidates->size());
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// Our follower list only contains followers, numFollowers + leader
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// needs to be at least minReplicationFactor.
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TRI_ASSERT(_followers->size() + 1 >= _docColl->minReplicationFactor());
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#endif
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return _canWrite;
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}
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READ_LOCKER(readLockerData, _dataLock);
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TRI_ASSERT(_docColl != nullptr);
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if (_followers->size() + 1 < _docColl->minReplicationFactor()) {
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// We know that we still do not have enough followers
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return false;
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}
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}
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return updateFailoverCandidates();
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}
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//////////////////////////////////////////////////////////////////////////////
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/// @brief Inject the information about followers into the builder.
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/// Builder needs to be an open object and is not allowed to contain
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/// the keys "servers" and "failoverCandidates".
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//////////////////////////////////////////////////////////////////////////////
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void injectFollowerInfo(arangodb::velocypack::Builder& builder) const {
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READ_LOCKER(readLockerData, _dataLock);
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injectFollowerInfoInternal(builder);
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}
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private:
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void injectFollowerInfoInternal(arangodb::velocypack::Builder& builder) const;
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bool updateFailoverCandidates();
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Result persistInAgency(bool isRemove) const;
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arangodb::velocypack::Builder newShardEntry(arangodb::velocypack::Slice oldValue) const;
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};
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} // end namespace arangodb
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#endif
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