store

libkv

libkv provides a Go native library to store metadata.

The goal of libkv is to abstract common store operations for multiple Key/Value backends and offer the same experience no matter which one of the backend you want to use.

For example, you can use it to store your metadata or for service discovery to register machines and endpoints inside your cluster.

You can also easily implement a generic Leader Election on top of it (see the swarm/leadership package).

As of now, libkv offers support for Consul, Etcd and Zookeeper.

Example of usage

Create a new store and use Put/Get

package main

import (
	"fmt"
	"time"

	log "github.com/Sirupsen/logrus"
	"github.com/docker/swarm/store"
)

func main() {
	client := "localhost:8500"

	// Initialize a new store with consul
	kv, err = store.NewStore(
		store.CONSUL, // or "consul"
		[]string{client},
		&store.Config{
			ConnectionTimeout: 10*time.Second,
		},
	)
	if err != nil {
		log.Error("Cannot create store consul")
	}

	key := "foo"
	err = kv.Put(key, []byte("bar"), nil)
	if err != nil {
		log.Error("Error trying to put value at key `", key, "`")
	}

	pair, err := kv.Get(key)
	if err != nil {
		log.Error("Error trying accessing value at key `", key, "`")
	}

	log.Info("value: ", string(pair.Value))
}

Details

You should expect the same experience for basic operations like Get/Put, etc.

However calls like WatchTree are limited to the common denominator and you should only expect events when nodes are added or deleted (although Etcd and Consul will likely return more events that you should triage).

Create a new storage backend

A new storage backend should include those calls:

type Store interface {
	Put(key string, value []byte, options *WriteOptions) error
	Get(key string) (*KVPair, error)
	Delete(key string) error
	Exists(key string) (bool, error)
	Watch(key string, stopCh <-chan struct{}) (<-chan *KVPair, error)
	WatchTree(prefix string, stopCh <-chan struct{}) (<-chan []*KVPair, error)
	NewLock(key string, options *LockOptions) (Locker, error)
	List(prefix string) ([]*KVPair, error)
	DeleteTree(prefix string) error
	AtomicPut(key string, value []byte, previous *KVPair, options *WriteOptions) (bool, *KVPair, error)
	AtomicDelete(key string, previous *KVPair) (bool, error)
	Close()
}

In the case of Swarm and to be eligible as a discovery backend only, a K/V store implementation should at least offer Get, Put, WatchTree and List.

Put should support usage of ttl to be able to remove entries in case of a node failure.

You can get inspiration from existing backends to create a new one. This interface could be subject to changes to improve the experience of using the library and contributing to a new backend.

##Future

A few points on the ROADMAP:

• Make the API nicer to use
• Improve performance (remove extras Get operations)
• Provide with more options (consistency for example)
• Add more exhaustive tests
• New backends?

##Contributing

Want to hack on libkv? Docker's contributions guidelines apply.

##Copyright and license

Code and documentation copyright 2015 Docker, inc. Code released under the Apache 2.0 license. Docs released under Creative commons.