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110 lines
2.2 KiB
C++
110 lines
2.2 KiB
C++
#pragma once
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#include <zkutil/ZooKeeper.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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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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LeaderElection(const std::string & path_, ZooKeeper & zookeeper_, LeadershipHandler handler_, const std::string & identifier_ = "")
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: path(path_), zookeeper(zookeeper_), handler(handler_), identifier(identifier_)
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{
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createNode();
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}
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void yield()
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{
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releaseNode();
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createNode();
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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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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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EphemeralNodeHolderPtr node;
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std::string node_name;
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std::thread thread;
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std::atomic<bool> shutdown {false};
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zkutil::EventPtr event = std::make_shared<Poco::Event>();
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void createNode()
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{
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shutdown = 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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thread = std::thread(&LeaderElection::threadFunction, this);
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}
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void releaseNode()
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{
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shutdown = true;
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event->set();
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if (thread.joinable())
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thread.join();
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node = nullptr;
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}
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void threadFunction()
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{
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while (!shutdown)
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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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handler();
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return;
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}
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if (zookeeper.exists(path + "/" + *(it - 1), nullptr, event))
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event->wait();
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success = true;
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}
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catch (...)
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{
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DB::tryLogCurrentException("LeaderElection");
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}
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if (!success)
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event->tryWait(10 * 1000);
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}
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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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