README.md

etcd-leaderelection

A go leader election module that can be used to track leader changes.

Install

go get -u github.com/ahmagdy/leaderelection 

Use case:

This package is intended to be used when multiple instances are running of service, and only a single instance can be the leader, or a single instance can do a specific job.

Example:

It's mostly intended for services built on top of a framework that supports app lifecycle. e.g. Uber fx.

leaderElection := leaderelection.New(etcdClient, zapLogger, instanceName, watcher)

fx.Invoke(func(lc fx.Lifecycle) {
    lc.Append(fx.Hook{
        OnStart: func(context.Context) error { return leaderElection.Start(ctx) },
        OnStop: func(context.Context) error { return leaderElection.Stop(ctx) },
    })
})

Without lifecycle support:

func run(ctx context.Context, logger *zap.Logger, instanceName string) error {
	etcdClient, err := newETCDClient(etcdSeedHostPorts)
	if err != nil {
		return nil
	}

	watcher := &WatcherService{logger: logger}

	leaderElection, err := leaderelection.New(etcdClient, logger, instanceName, watcher)
	if err != nil {
		return err
	}
	
	if err := leaderElection.Start(ctx); err != nil {
		return fmt.Errorf("failed to start leader election: %w", err)
	}

	defer func() {
		if err := leaderElection.Stop(ctx); err != nil {
			logger.Error("failed to stop leader election", zap.Error(err))
		}
	}()
	
}

Watcher interface to act on leadership changes

I tried to keep the interface minimal without fancy options.

Feel free to contribute or open a ticket if you feel like it's missing an event or can be extended.

var _ leaderelection.EventsWatcher = (*WatcherService)(nil)

type WatcherService struct {
	logger *zap.Logger
}

func (w *WatcherService) OnGainedLeadership() {
	w.logger.Info("I am the new leader")
}

func (w *WatcherService) OnLostLeadership() {
	w.logger.Info("I am no longer the leader")
}

Please check the /example package for more go examples.

License:

Apache License 2.0