canonical / Raft

Fully asynchronous C implementation of the Raft consensus protocol.

The library has modular design: its core part implements only the core Raft algorithm logic, in a fully platform independent way. On top of that, a pluggable interface defines the I/O implementation for networking (send/receive RPC messages) and disk persistence (store log entries and snapshots).

A stock implementation of the I/O interface is provided when building the library with default options. It is based on libuv and should fit the vast majority of use cases. The only catch is that it currently requires Linux, since it uses the Linux AIO API for disk I/O. Patches are welcome to add support for more platforms.

See raft.h for full documentation.

License

This raft C library is released under a slightly modified version of LGPLv3, that includes a copyright exception letting users to statically link the library code in their project and release the final work under their own terms. See the full license text.

Features

This implementation includes all the basic features described in the Raft dissertation:

• Leader election
• Log replication
• Log compaction
• Membership changes

It also includes a few optional enhancements:

• Optimistic pipelining to reduce log replication latency
• Writing to leader's disk in parallel
• Automatic stepping down when the leader loses quorum
• Leadership transfer extension
• Pre-vote protocol

Building

autoreconf -i
./configure --enable-example
make

Example

The best way to understand how to use the library is probably reading the code of the example server included in the source code.

You can also see the example server in action by running:

./example/cluster

which spawns a little cluster of 3 servers, runs a sample workload, and randomly stops and restarts a server from time to time.

Quick guide

It is recommended that you read raft.h for documentation details, but here's a quick high-level guide of what you'll need to do (error handling is omitted for brevity).

Create an instance of the stock raft_io interface implementation (or implement your own one if the one that comes with the library really does not fit):

const char *dir = "/your/raft/data";
struct uv_loop_s loop;
struct raft_uv_transport transport;
struct raft_io io;
uv_loop_init(&loop);
raft_uv_tcp_init(&transport, &loop);
raft_uv_init(&io, &loop, dir, &transport);

Define your application Raft FSM, implementing the raft_fsm interface:

struct raft_fsm
{
  void *data;
  int (*apply)(struct raft_fsm *fsm, const struct raft_buffer *buf, void **result);
  int (*snapshot)(struct raft_fsm *fsm, struct raft_buffer *bufs[], unsigned *n_bufs);
  int (*restore)(struct raft_fsm *fsm, struct raft_buffer *buf);
}

Pick a unique ID and address for each server and initialize the raft object:

unsigned id = 1;
const char *address = "192.168.1.1:9999";
struct raft raft;
raft_init(&raft, &io, &fsm, id, address);

If it's the first time you start the cluster, create a configuration object containing each server that should be present in the cluster (typically just one, since you can grow your cluster at a later point using raft_add and raft_promote) and bootstrap:

struct raft_configuration configuration;
raft_configuration_init(&configuration);
raft_configuration_add(&configuration, 1, "192.168.1.1:9999", true);
raft_bootstrap(&raft, &configuration);

Start the raft server:

raft_start(&raft);
uv_run(&loop, UV_RUN_DEFAULT);

Asynchronously submit requests to apply new commands to your application FSM:

static void apply_callback(struct raft_apply *req, int status, void *result) {
  /* ... */
}

struct raft_apply req;
struct raft_buffer buf;
buf.len = ...; /* The length of your FSM entry data */
buf.base = ...; /* Your FSM entry data */
raft_apply(&raft, &req, &buf, 1, apply_callback);

To add more servers to the cluster use the raft_add() and raft_promote APIs.

Notable users

• dqlite

Credits

Of course the biggest thanks goes to Diego Ongaro :) (the original author of the Raft dissertation)

A lot of ideas and inspiration was taken from other Raft implementations such as:

• CoreOS' Go implementation for etcd
• Hashicorp's Go raft
• Willem's C implementation
• LogCabin's C++ implementation