README
¶
Leadership: Distributed Leader Election for Clustered Environments.
Leadership is a library for a cluster leader election on top of a distributed Key/Value store.
It's built using Swarm's pkg/store and is designed to work across multiple
storage backends.
You can use leadership with Consul, etcd and Zookeeper.
// Create a store using pkg/store.
client, err := store.NewStore("consul", []string{"127.0.0.1:8500"}, &store.Config{})
if err != nil {
panic(err)
}
underwood := leadership.NewCandidate(client, "service/swarm/leader", "underwood")
electedCh, _ := underwood.RunForElection()
for isElected := range electedCh {
// This loop will run every time there is a change in our leadership
// status.
if isElected {
// We won the election - we are now the leader.
// Let's do leader stuff, for example, sleep for a while.
log.Printf("I won the election! I'm now the leader")
time.Sleep(10 * time.Second)
// Tired of being a leader? You can resign anytime.
candidate.Resign()
} else {
// We lost the election but are still running for leadership.
// `elected == false` is the default state and is the first event
// we'll receive from the channel. After a successful election,
// this event can get triggered if someone else steals the
// leadership or if we resign.
log.Printf("Lost the election, let's try another time")
}
}
It is possible to follow an election in real-time and get notified whenever there is a change in leadership:
follower := leadership.NewFollower(client, "service/swarm/leader")
leaderCh, _ := follower.FollowElection()
for leader := <-leaderCh {
// Leader is a string containing the value passed to `NewCandidate`.
log.Printf("%s is now the leader", leader)
}
A typical use case for this is to be able to always send requests to the current leader.
Fault tolerance
Leadership returns an error channel for Candidates and Followers that you can use to be resilient to failures. For example, if the watch on the leader key fails because the store becomes unavailable, you can retry the process later.
func participate() {
// Create a store using pkg/store.
client, err := store.NewStore("consul", []string{"127.0.0.1:8500"}, &store.Config{})
if err != nil {
panic(err)
}
waitTime := 10 * time.Second
underwood := leadership.NewCandidate(client, "service/swarm/leader", "underwood")
go func() {
for {
run(underwood)
time.Sleep(waitTime)
// retry
}
}
}
func run(candidate *leadership.Candidate) {
electedCh, errCh := candidate.RunForElection()
for {
select {
case elected := <-electedCh:
if isElected {
// Do something
} else {
// Do something else
}
case err := <-errCh:
log.Error(err)
return
}
}
Documentation
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Index ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
This section is empty.
Types ¶
type Candidate ¶
type Candidate struct {
// contains filtered or unexported fields
}
Candidate runs the leader election algorithm asynchronously
func NewCandidate ¶
NewCandidate creates a new Candidate
func (*Candidate) Resign ¶
func (c *Candidate) Resign()
Resign forces the candidate to step-down and try again. If the candidate is not a leader, it doesn't have any effect. Candidate will retry immediately to acquire the leadership. If no-one else took it, then the Candidate will end up being a leader again.
func (*Candidate) RunForElection ¶
RunForElection starts the leader election algorithm. Updates in status are pushed through the ElectedCh channel.
ElectedCh is used to get a channel which delivers signals on acquiring or losing leadership. It sends true if we become the leader, and false if we lose it.
type Follower ¶
type Follower struct {
// contains filtered or unexported fields
}
Follower can follow an election in real-time and push notifications whenever there is a change in leadership.
func NewFollower ¶
NewFollower creates a new follower.
func (*Follower) FollowElection ¶
FollowElection starts monitoring the election.
Source Files