libkv

libkv provides a Go native library to store metadata.

The goal of libkv is to abstract common store operations for multiple distributed and/or local Key/Value store backends.

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 docker/leadership repository).

As of now, libkv offers support for Consul, Etcd, Zookeeper (Distributed store) and BoltDB (Local store).

Usage

libkv is meant to be used as an abstraction layer over existing distributed Key/Value stores. It is especially useful if you plan to support consul, etcd and zookeeper using the same codebase.

It is ideal if you plan for something written in Go that should support:

• A simple metadata storage, distributed or local
• A lightweight discovery service for your nodes
• A distributed lock mechanism

You can find examples of usage for libkv under in docs/examples.go. Optionally you can also take a look at the docker/swarm or docker/libnetwork repositories which are using docker/libkv for all the use cases listed above.

Supported versions

libkv supports:

• Consul versions >= 0.5.1 because it uses Sessions with Delete behavior for the use of TTLs (mimics zookeeper's Ephemeral node support), If you don't plan to use TTLs: you can use Consul version 0.4.0+.
• Etcd versions >= 2.0 because it uses the new coreos/etcd/client, this might change in the future as the support for APIv3 comes along and adds more capabilities.
• Zookeeper versions >= 3.4.5. Although this might work with previous version but this remains untested as of now.
• Boltdb, which shouldn't be subject to any version dependencies.

Interface

A storage backend in libkv should implement (fully or partially) this interface:

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(directory string, stopCh <-chan struct{}) (<-chan []*KVPair, error)
	NewLock(key string, options *LockOptions) (Locker, error)
	List(directory string) ([]*KVPair, error)
	DeleteTree(directory string) error
	AtomicPut(key string, value []byte, previous *KVPair, options *WriteOptions) (bool, *KVPair, error)
	AtomicDelete(key string, previous *KVPair) (bool, error)
	Close()
}

Compatibility matrix

Backend drivers in libkv are generally divided between local drivers and distributed drivers. Distributed backends offer enhanced capabilities like Watches and/or distributed Locks.

Local drivers are usually used in complement to the distributed drivers to store informations that only needs to be available locally.

Calls	Consul	Etcd	Zookeeper	BoltDB
Put	X	X	X	X
Get	X	X	X	X
Delete	X	X	X	X
Exists	X	X	X	X
Watch	X	X	X
WatchTree	X	X	X
NewLock (Lock/Unlock)	X	X	X
List	X	X	X	X
DeleteTree	X	X	X	X
AtomicPut	X	X	X	X
Close	X	X	X	X

Limitations

Distributed Key/Value stores often have different concepts for managing and formatting keys and their associated values. Even though libkv tries to abstract those stores aiming for some consistency, in some cases it can't be applied easily.

Please refer to the docs/compatibility.md to see what are the special cases for cross-backend compatibility.

Other than those special cases, you should expect the same experience for basic operations like Get/Put, etc.

Calls like WatchTree may return different events (or number of events) depending on the backend (for now, Etcd and Consul will likely return more events than Zookeeper that you should triage properly). Although you should be able to use it successfully to watch on events in an interchangeable way (see the docker/leadership repository or the pkg/discovery/kv package in docker/docker).

TLS

Only Consul and etcd have support for TLS and you should build and provide your own config.TLS object to feed the client. Support is planned for zookeeper.

Roadmap

• Make the API nicer to use (using options)
• Provide more options (consistency for example)
• Improve performance (remove extras Get/List operations)
• Better key formatting
• New backends?

Contributing

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

Copyright and license

Copyright © 2014-2016 Docker, Inc. All rights reserved, except as follows. Code is released under the Apache 2.0 license. The README.md file, and files in the "docs" folder are licensed under the Creative Commons Attribution 4.0 International License under the terms and conditions set forth in the file "LICENSE.docs". You may obtain a duplicate copy of the same license, titled CC-BY-SA-4.0, at http://creativecommons.org/licenses/by/4.0/.