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140 lines
3.6 KiB
C++
140 lines
3.6 KiB
C++
#pragma once
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#include "ZooKeeper.h"
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#include "KeeperException.h"
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#include <functional>
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#include <memory>
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#include <common/logger_useful.h>
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#include <Common/CurrentMetrics.h>
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#include <Core/BackgroundSchedulePool.h>
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namespace ProfileEvents
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{
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extern const Event LeaderElectionAcquiredLeadership;
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}
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namespace CurrentMetrics
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{
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extern const Metric LeaderElection;
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}
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namespace zkutil
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{
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/** Implements leader election algorithm described here: http://zookeeper.apache.org/doc/r3.4.5/recipes.html#sc_leaderElection
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*/
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class LeaderElection
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{
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public:
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using LeadershipHandler = std::function<void()>;
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/** handler is called when this instance become leader.
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*
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* identifier - if not empty, must uniquely (within same path) identify participant of leader election.
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* It means that different participants of leader election have different identifiers
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* and existence of more than one ephemeral node with same identifier indicates an error.
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*/
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LeaderElection(DB::BackgroundSchedulePool & pool_, const std::string & path_, ZooKeeper & zookeeper_, LeadershipHandler handler_, const std::string & identifier_ = "")
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: pool(pool_), path(path_), zookeeper(zookeeper_), handler(handler_), identifier(identifier_)
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, log_name("LeaderElection (" + path + ")")
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, log(&Logger::get(log_name))
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{
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task = pool.createTask(log_name, [this] { threadFunction(); });
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createNode();
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}
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void shutdown()
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{
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if (shutdown_called)
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return;
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shutdown_called = true;
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task->deactivate();
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}
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~LeaderElection()
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{
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releaseNode();
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}
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private:
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DB::BackgroundSchedulePool & pool;
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DB::BackgroundSchedulePool::TaskHolder task;
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std::string path;
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ZooKeeper & zookeeper;
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LeadershipHandler handler;
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std::string identifier;
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std::string log_name;
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Logger * log;
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EphemeralNodeHolderPtr node;
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std::string node_name;
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std::atomic<bool> shutdown_called {false};
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CurrentMetrics::Increment metric_increment{CurrentMetrics::LeaderElection};
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void createNode()
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{
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shutdown_called = false;
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node = EphemeralNodeHolder::createSequential(path + "/leader_election-", zookeeper, identifier);
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std::string node_path = node->getPath();
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node_name = node_path.substr(node_path.find_last_of('/') + 1);
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task->activateAndSchedule();
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}
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void releaseNode()
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{
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shutdown();
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node = nullptr;
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}
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void threadFunction()
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{
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bool success = false;
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try
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{
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Strings children = zookeeper.getChildren(path);
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std::sort(children.begin(), children.end());
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auto it = std::lower_bound(children.begin(), children.end(), node_name);
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if (it == children.end() || *it != node_name)
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throw Poco::Exception("Assertion failed in LeaderElection");
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if (it == children.begin())
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{
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ProfileEvents::increment(ProfileEvents::LeaderElectionAcquiredLeadership);
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handler();
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return;
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}
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if (!zookeeper.existsWatch(path + "/" + *(it - 1), nullptr, task->getWatchCallback()))
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task->schedule();
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success = true;
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}
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catch (const KeeperException & e)
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{
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DB::tryLogCurrentException(log);
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if (e.code == Coordination::ZSESSIONEXPIRED)
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return;
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}
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catch (...)
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{
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DB::tryLogCurrentException(log);
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}
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if (!success)
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task->scheduleAfter(10 * 1000);
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}
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};
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using LeaderElectionPtr = std::shared_ptr<LeaderElection>;
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}
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