etcd: Index | Examples | Files

package concurrency

import ""

Package concurrency implements concurrency operations on top of etcd such as distributed locks, barriers, and elections.



Package Files

doc.go election.go key.go mutex.go session.go stm.go


var (
    ErrElectionNotLeader = errors.New("election: not leader")
    ErrElectionNoLeader  = errors.New("election: no leader")

func NewLocker Uses

func NewLocker(s *Session, pfx string) sync.Locker

NewLocker creates a sync.Locker backed by an etcd mutex.

func NewSTM Uses

func NewSTM(c *v3.Client, apply func(STM) error, so ...stmOption) (*v3.TxnResponse, error)

NewSTM initiates a new STM instance, using serializable snapshot isolation by default.

func NewSTMReadCommitted Uses

func NewSTMReadCommitted(ctx context.Context, c *v3.Client, apply func(STM) error) (*v3.TxnResponse, error)

NewSTMReadCommitted is deprecated.

func NewSTMRepeatable Uses

func NewSTMRepeatable(ctx context.Context, c *v3.Client, apply func(STM) error) (*v3.TxnResponse, error)

NewSTMRepeatable is deprecated.

func NewSTMSerializable Uses

func NewSTMSerializable(ctx context.Context, c *v3.Client, apply func(STM) error) (*v3.TxnResponse, error)

NewSTMSerializable is deprecated.

func WithAbortContext Uses

func WithAbortContext(ctx context.Context) stmOption

WithAbortContext specifies the context for permanently aborting the transaction.

func WithIsolation Uses

func WithIsolation(lvl Isolation) stmOption

WithIsolation specifies the transaction isolation level.

func WithPrefetch Uses

func WithPrefetch(keys ...string) stmOption

WithPrefetch is a hint to prefetch a list of keys before trying to apply. If an STM transaction will unconditionally fetch a set of keys, prefetching those keys will save the round-trip cost from requesting each key one by one with Get().

type Election Uses

type Election struct {
    // contains filtered or unexported fields

func NewElection Uses

func NewElection(s *Session, pfx string) *Election

NewElection returns a new election on a given key prefix.

func ResumeElection Uses

func ResumeElection(s *Session, pfx string, leaderKey string, leaderRev int64) *Election

ResumeElection initializes an election with a known leader.

func (*Election) Campaign Uses

func (e *Election) Campaign(ctx context.Context, val string) error

Campaign puts a value as eligible for the election. It blocks until it is elected, an error occurs, or the context is cancelled.


cli, err := clientv3.New(clientv3.Config{Endpoints: endpoints})
if err != nil {
defer cli.Close()

// create two separate sessions for election competition
s1, err := concurrency.NewSession(cli)
if err != nil {
defer s1.Close()
e1 := concurrency.NewElection(s1, "/my-election/")

s2, err := concurrency.NewSession(cli)
if err != nil {
defer s2.Close()
e2 := concurrency.NewElection(s2, "/my-election/")

// create competing candidates, with e1 initially losing to e2
var wg sync.WaitGroup
electc := make(chan *concurrency.Election, 2)
go func() {
    defer wg.Done()
    // delay candidacy so e2 wins first
    time.Sleep(3 * time.Second)
    if err := e1.Campaign(context.Background(), "e1"); err != nil {
    electc <- e1
go func() {
    defer wg.Done()
    if err := e2.Campaign(context.Background(), "e2"); err != nil {
    electc <- e2

cctx, cancel := context.WithCancel(context.TODO())
defer cancel()

e := <-electc
fmt.Println("completed first election with", string((<-e.Observe(cctx)).Kvs[0].Value))

// resign so next candidate can be elected
if err := e.Resign(context.TODO()); err != nil {

e = <-electc
fmt.Println("completed second election with", string((<-e.Observe(cctx)).Kvs[0].Value))



completed first election with e2
completed second election with e1

func (*Election) Header Uses

func (e *Election) Header() *pb.ResponseHeader

Header is the response header from the last successful election proposal.

func (*Election) Key Uses

func (e *Election) Key() string

Key returns the leader key if elected, empty string otherwise.

func (*Election) Leader Uses

func (e *Election) Leader(ctx context.Context) (*v3.GetResponse, error)

Leader returns the leader value for the current election.

func (*Election) Observe Uses

func (e *Election) Observe(ctx context.Context) <-chan v3.GetResponse

Observe returns a channel that reliably observes ordered leader proposals as GetResponse values on every current elected leader key. It will not necessarily fetch all historical leader updates, but will always post the most recent leader value.

The channel closes when the context is canceled or the underlying watcher is otherwise disrupted.

func (*Election) Proclaim Uses

func (e *Election) Proclaim(ctx context.Context, val string) error

Proclaim lets the leader announce a new value without another election.

func (*Election) Resign Uses

func (e *Election) Resign(ctx context.Context) (err error)

Resign lets a leader start a new election.

func (*Election) Rev Uses

func (e *Election) Rev() int64

Rev returns the leader key's creation revision, if elected.

type Isolation Uses

type Isolation int

Isolation is an enumeration of transactional isolation levels which describes how transactions should interfere and conflict.

const (
    // SerializableSnapshot provides serializable isolation and also checks
    // for write conflicts.
    SerializableSnapshot Isolation = iota
    // Serializable reads within the same transaction attempt return data
    // from the at the revision of the first read.
    // RepeatableReads reads within the same transaction attempt always
    // return the same data.
    // ReadCommitted reads keys from any committed revision.

type Mutex Uses

type Mutex struct {
    // contains filtered or unexported fields

Mutex implements the sync Locker interface with etcd

func NewMutex Uses

func NewMutex(s *Session, pfx string) *Mutex

func (*Mutex) Header Uses

func (m *Mutex) Header() *pb.ResponseHeader

Header is the response header received from etcd on acquiring the lock.

func (*Mutex) IsOwner Uses

func (m *Mutex) IsOwner() v3.Cmp

func (*Mutex) Key Uses

func (m *Mutex) Key() string

func (*Mutex) Lock Uses

func (m *Mutex) Lock(ctx context.Context) error

Lock locks the mutex with a cancelable context. If the context is canceled while trying to acquire the lock, the mutex tries to clean its stale lock entry.


cli, err := clientv3.New(clientv3.Config{Endpoints: endpoints})
if err != nil {
defer cli.Close()

// create two separate sessions for lock competition
s1, err := concurrency.NewSession(cli)
if err != nil {
defer s1.Close()
m1 := concurrency.NewMutex(s1, "/my-lock/")

s2, err := concurrency.NewSession(cli)
if err != nil {
defer s2.Close()
m2 := concurrency.NewMutex(s2, "/my-lock/")

// acquire lock for s1
if err := m1.Lock(context.TODO()); err != nil {
fmt.Println("acquired lock for s1")

m2Locked := make(chan struct{})
go func() {
    defer close(m2Locked)
    // wait until s1 is locks /my-lock/
    if err := m2.Lock(context.TODO()); err != nil {

if err := m1.Unlock(context.TODO()); err != nil {
fmt.Println("released lock for s1")

fmt.Println("acquired lock for s2")


acquired lock for s1
released lock for s1
acquired lock for s2

func (*Mutex) Unlock Uses

func (m *Mutex) Unlock(ctx context.Context) error

type STM Uses

type STM interface {
    // Get returns the value for a key and inserts the key in the txn's read set.
    // If Get fails, it aborts the transaction with an error, never returning.
    Get(key ...string) string
    // Put adds a value for a key to the write set.
    Put(key, val string, opts ...v3.OpOption)
    // Rev returns the revision of a key in the read set.
    Rev(key string) int64
    // Del deletes a key.
    Del(key string)
    // contains filtered or unexported methods

STM is an interface for software transactional memory.

ExampleSTM_apply shows how to use STM with a transactional transfer between balances.


cli, err := clientv3.New(clientv3.Config{Endpoints: endpoints})
if err != nil {
defer cli.Close()

// set up "accounts"
totalAccounts := 5
for i := 0; i < totalAccounts; i++ {
    k := fmt.Sprintf("accts/%d", i)
    if _, err = cli.Put(context.TODO(), k, "100"); err != nil {

exchange := func(stm concurrency.STM) error {
    from, to := rand.Intn(totalAccounts), rand.Intn(totalAccounts)
    if from == to {
        // nothing to do
        return nil
    // read values
    fromK, toK := fmt.Sprintf("accts/%d", from), fmt.Sprintf("accts/%d", to)
    fromV, toV := stm.Get(fromK), stm.Get(toK)
    fromInt, toInt := 0, 0
    fmt.Sscanf(fromV, "%d", &fromInt)
    fmt.Sscanf(toV, "%d", &toInt)

    // transfer amount
    xfer := fromInt / 2
    fromInt, toInt = fromInt-xfer, toInt+xfer

    // write back
    stm.Put(fromK, fmt.Sprintf("%d", fromInt))
    stm.Put(toK, fmt.Sprintf("%d", toInt))
    return nil

// concurrently exchange values between accounts
var wg sync.WaitGroup
for i := 0; i < 10; i++ {
    go func() {
        defer wg.Done()
        if _, serr := concurrency.NewSTM(cli, exchange); serr != nil {

// confirm account sum matches sum from beginning.
sum := 0
accts, err := cli.Get(context.TODO(), "accts/", clientv3.WithPrefix())
if err != nil {
for _, kv := range accts.Kvs {
    v := 0
    fmt.Sscanf(string(kv.Value), "%d", &v)
    sum += v

fmt.Println("account sum is", sum)


account sum is 500

type Session Uses

type Session struct {
    // contains filtered or unexported fields

Session represents a lease kept alive for the lifetime of a client. Fault-tolerant applications may use sessions to reason about liveness.

func NewSession Uses

func NewSession(client *v3.Client, opts ...SessionOption) (*Session, error)

NewSession gets the leased session for a client.

func (*Session) Client Uses

func (s *Session) Client() *v3.Client

Client is the etcd client that is attached to the session.

func (*Session) Close Uses

func (s *Session) Close() error

Close orphans the session and revokes the session lease.

func (*Session) Done Uses

func (s *Session) Done() <-chan struct{}

Done returns a channel that closes when the lease is orphaned, expires, or is otherwise no longer being refreshed.

func (*Session) Lease Uses

func (s *Session) Lease() v3.LeaseID

Lease is the lease ID for keys bound to the session.

func (*Session) Orphan Uses

func (s *Session) Orphan()

Orphan ends the refresh for the session lease. This is useful in case the state of the client connection is indeterminate (revoke would fail) or when transferring lease ownership.

type SessionOption Uses

type SessionOption func(*sessionOptions)

SessionOption configures Session.

func WithContext Uses

func WithContext(ctx context.Context) SessionOption

WithContext assigns a context to the session instead of defaulting to using the client context. This is useful for canceling NewSession and Close operations immediately without having to close the client. If the context is canceled before Close() completes, the session's lease will be abandoned and left to expire instead of being revoked.

func WithLease Uses

func WithLease(leaseID v3.LeaseID) SessionOption

WithLease specifies the existing leaseID to be used for the session. This is useful in process restart scenario, for example, to reclaim leadership from an election prior to restart.

func WithTTL Uses

func WithTTL(ttl int) SessionOption

WithTTL configures the session's TTL in seconds. If TTL is <= 0, the default 60 seconds TTL will be used.

Package concurrency imports 9 packages (graph) and is imported by 144 packages. Updated 2018-01-26. Refresh now. Tools for package owners.