A C++14 client library for leader election using etcd.
This library is work in progress, it correctly handles a leader election against a single etcd server, I am currently working on supporting failovers in the etcd cluster.
This library is built using cmake(1). On Linux and other Unix variants the usual commands to build it are:
cmake .
make
make test
make install
Leader election is a building block for large distributed systems. The context is some component of the distributed system that needs to be replicated for increased availability, but that cannot have more than one instance active at a time because the component acts as a coordinator to distribute some non-trivially parallelizable workload.
The common solution is to perform a leader election between the instances of the component in question. The instance that "wins" the election plays the leader role and remains active. The other instances passively wait until the leader terminates, gracefully or not.
Gee-H solves this coordination problem using the
etcd lock service.
The examples/observe_election.cpp example shows how to watch an election.
After some initialization boilerplate the main class is created using:
auto channel = grpc::CreateChannel(...);
auto queue = std::make_shared<gh::active_completion_queue>();
gh::watch_election (queue.cq(), channel, election_name);the class monitors the given election (election_name in this case).
The application can create callbacks to receive updates about the election:
auto subscriber = [](std::string const& key, std::string const& value) {
if (key.empty()) {
std::cout << "no current leader" << std::endl;
return;
}
std::cout << "current leader is " << key << ", with value=" << value << std::endl;
};
auto token = election_observer->subscribe(std::move(subscriber));The examples/join_election.cpp example shows how to join an election.
After some initialization boilerplate the main class is created using:
auto channel = grpc::CreateChannel(...);
auto queue = std::make_shared<gh::active_completion_queue>();
gh::leader_election election(queue, channel, "my-election", "my-value", std::chrono::seconds(5));The application can then block until the current candidate wins the election:
auto fut = election.campaign();
bool elected = fut.get();Gee-H was a radio navigation system developed during
World War II. It has nothing to do with leader election, or C++14, or the etcd server.
I just like short names for namespaces: gh:: in this case.
gee-h is distributed under the Apache License, Version 2.0.
The full licensing terms can be found in the LICENSE file.
Copyright 2017 Carlos O'Ryan
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.