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Add a README, a License and expand documentation with examples
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‎LICENSE‎

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‎README.md‎

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![Go](https://github.com/vimeo/leaderelection/workflows/Go/badge.svg)
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[![GoDoc](https://godoc.org/github.com/vimeo/leaderelection?status.svg)](https://godoc.org/github.com/vimeo/leaderelection)
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# LeaderElection
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LeaderElection is a library for electing a leader with a pluggable backend (a
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[`RaceDecider`]).
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## [`RaceDecider`]
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The [`RaceDecider`] implementation needs to provide a conditional-write or
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transactional-write mechanism to ensure that only one write succeeds per
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election-cycle, but otherwise has very minimal requirements.
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Currently, this implementation only has two implementations:
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- [Google Cloud Storage][`gcs`]: intended for easy production use while running in GCP, using a GCS object for locking.
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- [Memory][`memory`]: intended for tests.
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## Electing a leader
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The [`Config`] struct contains callbacks and tunables for the leader election
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"campaign".
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Each process that would like to acquire leadership must register callbacks for
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all three of `OnElected`, `OnOusting` and `LeaderChanged`, as well as specify
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unique `LeaderID` and `HostPort`s (the latter two are used for communication, so
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some use-cases may be able to ignore them)
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The `TermLength` is the length of time that a leader acquires leadership for,
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and the length of any extension. This duration should be long enough to get
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useful work done, but short enough that you won't have a problem if the leader
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dies and no one takes over the remainder of the lease term. The `Config.Acquire`
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method takes care of extending the lease twice per term to reduce the likelihood
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of spuriously losing the lock.
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`MaxClockSkew` specifies the corrections added to and subtracted from sleeps
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and leases to account for a lack of perfect synchronization among the clocks of
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all candidates.
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`ConnectionParams` is a generic byte-payload side-channel. The `legrpc` package
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uses it for the GRPC [`ServiceConfig`], but other use-cases may stash other
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serialized data there (may be `nil`).
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`Clock` is an instance of a `clocks.Clock`, which should be `nil` outside of
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tests, in which case it uses `clocks.DefaultClock()`.
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### There's more to Leadership than getting elected
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The `OnElected` callback takes two arguments which indicate the state of the
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leadership lock with different degrees of certainty.
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The `ctx` argument is a context derived from the one passed to `Acquire` that
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will be canceled upon losing the leadership role. This is an explicit
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cancellation by the goroutine handling lease renewals and acquisition, and as
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such is subject to normal thread-scheduling delay caveats (particularly relevant
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when operating with heavy CPU-contention).
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To address the thread-scheduling-delay issues plaguing use of `ctx`, the
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second argument to `OnElected` is a `*TimeView` containing the current
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expiration time. This pointer tracks an atomic value which is updated every time
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the lease is extended. Before taking any action that requires holding
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leadership, one should always check that the time returned by `t.Get()` is in
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the future. `TimeView` has a `ValueInFuture()` convenience-method to facilitate
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such checks for quick operations against the correct clock.
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## Picking a `RaceDecider`
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The [`gcs`] [`RaceDecider`] is the only currently usable implementation. It
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requires a Google Cloud Storage client, a bucket and an object.
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In tests, one can use the [`memory`] [`RaceDecider`], as that implementation
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is trivially fast and lacks any external dependencies, but doesn't work outside
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a single process.
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## Watching an Election
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Leader election is useful on its own only for a subset of use cases. Many times
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it is necessary for other processes to observe a leader election to send
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requests to the correct process. As such, one can define a [`WatchConfig`] and
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call [`Watch()`] on it. The callback will be called sequentially for every
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lease-extension and acquisition, thus allowing an observer to track the
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expiration of the current leader's leadership term.
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[`RaceDecider`]: https://pkg.go.dev/github.com/vimeo/leaderelection?tab=doc#RaceDecider
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[`WatchConfig`]: https://pkg.go.dev/github.com/vimeo/leaderelection?tab=doc#WatchConfig
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[`Watch()`]: https://pkg.go.dev/github.com/vimeo/leaderelection?tab=doc#WatchConfig.Watch
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[`Config`]: https://pkg.go.dev/github.com/vimeo/leaderelection?tab=doc#Config
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[`ServiceConfig`]: https://github.com/grpc/grpc/blob/master/doc/service_config.md
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[`gcs`]: https://pkg.go.dev/github.com/vimeo/leaderelection/gcs
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[`memory`]: https://pkg.go.dev/github.com/vimeo/leaderelection/memory

‎campaign.go‎

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@@ -126,7 +126,7 @@ const jitterTermMaxFraction = 0.1
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func (c *campaign) manageWin(ctx context.Context, winningEntry *entry.RaceEntry, wg *sync.WaitGroup) (*entry.RaceEntry, error) {
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// we won!
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tv := TimeView{}
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tv := TimeView{clock: c.clock}
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tv.Set(winningEntry.TermExpiry)
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// run the elected callback in a background goroutine and cancel the
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// context on ousting

‎campaign_test.go‎

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import (
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"context"
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"fmt"
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"math/rand"
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"os"
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"strconv"
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cancel()
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}
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func ExampleConfig_Acquire() {
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ctx, cancel := context.WithCancel(context.Background())
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defer cancel()
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d := memory.NewDecider()
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now := time.Date(2020, 5, 6, 0, 0, 0, 0, time.UTC)
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fc := clocks.NewFakeClock(now)
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tvLeaderIndicator := (*TimeView)(nil)
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electedCh := make(chan struct{})
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c := Config{
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Decider: d,
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HostPort: "127.0.0.1:8080",
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LeaderID: "yabadabadoo",
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OnElected: func(ctx context.Context, tv *TimeView) {
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fmt.Printf("I won! Leader term expiration: %s; held: %t\n",
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tv.Get(), tv.ValueInFuture())
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tvLeaderIndicator = tv
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close(electedCh)
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},
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OnOusting: func(ctx context.Context) {
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// make sure we've already set `tvLeaderIndictor` before touching it
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<-electedCh
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fmt.Printf("I lost! Holding Lock: %t; expires: %s\n",
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// Note that ValueInFuture will return true
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// here because the clock is still within the
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// term.
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tvLeaderIndicator.ValueInFuture(),
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tvLeaderIndicator.Get())
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},
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LeaderChanged: func(ctx context.Context, entry entry.RaceEntry) {
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fmt.Printf("%q won\n", entry.LeaderID)
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},
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TermLength: time.Minute * 30,
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MaxClockSkew: time.Second * 10,
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Clock: fc,
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}
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acquireCh := make(chan error, 1)
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go func() {
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acquireCh <- c.Acquire(ctx)
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}()
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fc.AwaitSleepers(1)
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<-electedCh
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fc.Advance(c.TermLength / 2)
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// Wait for the lease renewal to happen before cancelling (so the
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// output is predictable)
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fc.AwaitSleepers(1)
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cancel()
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// Acquire blocks until all callbacks return (there's an internal WaitGroup)
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<-acquireCh
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// advance past the end of the current term (after an extension)
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fc.Advance(c.TermLength + time.Minute)
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fmt.Printf("Still Leading: %t; expiry: %s; current time: %s\n",
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tvLeaderIndicator.ValueInFuture(),
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tvLeaderIndicator.Get(), fc.Now())
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// Unordered output:
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// I won! Leader term expiration: 2020-05-06 00:30:00 +0000 UTC; held: true
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// I lost! Holding Lock: true; expires: 2020-05-06 00:45:00 +0000 UTC
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// "" won
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// Still Leading: false; expiry: 2020-05-06 00:45:00 +0000 UTC; current time: 2020-05-06 00:46:00 +0000 UTC
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}

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