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/**
* Copyright (C) 2020-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include <boost/none.hpp>
#include <boost/optional.hpp>
#include <memory>
#include <string>
#include "mongo/db/repl/is_master_response.h"
#include "mongo/db/repl/replication_coordinator.h"
#include "mongo/platform/atomic_word.h"
#include "mongo/platform/mutex.h"
#include "mongo/stdx/thread.h"
#include "mongo/util/time_support.h"
namespace mongo {
class Client;
namespace repl {
constexpr auto kTopologyVersionObserverName = "TopologyVersionObserver";
/**
* An utility to observe topology changes asynchronously and cache updates.
* `getCached()` is thread-safe (through `_mutex`), but its behavior is undefined during the
* execution of the constructor/destructor methods.
*
* The life-cycle for instances of this class (`_state`) is as follows:
* * Uninitialized: immediately after construction.
* Call `init()` to initialize the instance and start the observer thread.
* You may only call `init()` once -- otherwise, it will terminate the process.
* * Running: anytime after returning from `init()` and before calling `shutdown()`.
* Note that if the observer thread stops due to an error, it will set the state to Shutdown.
* * Shutdown: the object is ready for destruction.
* You must wait for `shutdown()` to return before deleting the object.
* Multiple, multithreaded calls to `shutdown()` is safe, and only one will proceed.
* After transitioning to shutdown, you can only call the destructor.
*
* constructor() -> init() -> getCached() ... getCached() -> shutdown() -> destructor()
*/
class TopologyVersionObserver final {
public:
static constexpr auto kDelayMS = Milliseconds(10);
TopologyVersionObserver() = default;
~TopologyVersionObserver() {
auto state = _state.load();
invariant(state == State::kShutdown || state == State::kUninitialized);
}
void init(ServiceContext* serviceContext,
ReplicationCoordinator* replCoordinator = nullptr) noexcept;
void shutdown() noexcept;
/**
* Returns a reference (shared pointer) to the cached version of `IsMasterResponse`.
* Note that the reference is initially set to `nullptr`.
* Also, the reference is set back to `nullptr` if the thread that updates `_cache` terminates
* due to an error (i.e., exception), or it receives an invalid response.
*/
std::shared_ptr<const IsMasterResponse> getCached() noexcept;
std::string toString() const;
/**
* Returns true if this TopologyVersionObserver background thread has stopped.
*
* Note that this funtion only returns true after _thread has started and ended, thus implies
* that getCached() will never return a valid IsMasterResponse again.
*/
bool isShutdown() const noexcept {
return _state.loadRelaxed() == State::kShutdown;
}
private:
enum class State {
kUninitialized,
kRunning,
kShutdown,
};
void _cacheIsMasterResponse(OperationContext*, boost::optional<TopologyVersion>);
void _workerThreadBody() noexcept;
/**
* Protects shared accesses to `_cache`, `_observerClient`, and serializes calls to `init()`
* and `shutdown()` methods.
*
* Accessing the cached `IsMasterResponse` follows a single-producer, multi-consumer model:
* consumers are readers of `_cache` and the producer is the observer thread. The assumption
* is that the contention on this lock is insignificant.
*/
mutable Mutex _mutex = MONGO_MAKE_LATCH(kTopologyVersionObserverName);
stdx::condition_variable _cv;
/**
* Tells the worker thread if it should continue to run
*
* This variable is set to true from false outside the worker thread
*/
AtomicWord<bool> _shouldShutdown;
// The reference to the latest cached version of `IsMasterResponse`
std::shared_ptr<const IsMasterResponse> _cache;
/**
* Represents the current state of the observer.
*
* This variable is only changed from the worker thread
*/
AtomicWord<State> _state;
// Holds a reference to the worker opCtx to allow `shutdown()` to stop the observer thread.
// This variable should only be set after the `_workerThreadBody()` checks _shouldShutdown.
// This means that we only ever have to interrupt the _workerOpCtx at most once because both its
// construction and its destruction synchronize with the cv in shutdown().
OperationContext* _workerOpCtx = nullptr;
boost::optional<stdx::thread> _thread;
ServiceContext* _serviceContext = nullptr;
ReplicationCoordinator* _replCoordinator = nullptr;
};
} // namespace repl
} // namespace mongo
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