C implementation of the Raft Consensus protocol, BSD licensed
C C++ Makefile Shell

README.rst

https://travis-ci.org/willemt/raft.png https://coveralls.io/repos/willemt/raft/badge.png

C implementation of the Raft consensus protocol, BSD licensed.

See raft.h for full documentation.

See ticketd for real life use of this library.

Networking is out of scope for this project. The implementor will need to do all the plumbing.

There are no dependencies, however https://github.com/willemt/linked-List-queue is required for testing.

Building

make tests

Quality Assurance

We use the following methods to ensure that the library is safe:

Single file amalgamation

The source has been amalgamated into a single raft.h header file. Use clib to download the source into your project's deps folder, ie:

brew install clib
clib install willemt/raft_amalgamation

The file is stored in the deps folder like below:

deps/raft/raft.h

How to integrate with this library

See ticketd for an example of how to integrate with this library.

If you don't have access to coroutines it's easiest to use two separate threads - one for handling Raft peer traffic, and another for handling client traffic.

Be aware that this library is not thread safe. You will need to ensure that the library's functions are called exclusively.

Initializing the Raft server

Instantiate a new Raft server using raft_new.

void* raft = raft_new();

We tell the Raft server what the cluster configuration is by using the raft_add_node function. For example, if we have 5 servers [1] in our cluster, we call raft_add_node 5 [2] times.

raft_add_node(raft, connection_user_data, node_id, peer_is_self);

Where:

  • connection_user_data is a pointer to user data.
  • peer_is_self is boolean indicating that this is the current server's server index.
  • node_id is the unique integer ID of the node. Peers use this to identify themselves. This SHOULD be a random integer.
[1]AKA "Raft peer"
[2]We have to also include the Raft server itself in the raft_add_node calls. When we call raft_add_node for the Raft server, we set peer_is_self to 1.

Calling raft_periodic() periodically

We need to call raft_periodic at periodic intervals.

raft_periodic(raft, 1000);

Example using a libuv timer:

static void __periodic(uv_timer_t* handle)
{
    raft_periodic(sv->raft, PERIOD_MSEC);
}

uv_timer_t *periodic_req;
periodic_req = malloc(sizeof(uv_timer_t));
periodic_req->data = sv;
uv_timer_init(&peer_loop, periodic_req);
uv_timer_start(periodic_req, __periodic, 0, 1000);

Receiving the entry (ie. client sends entry to Raft cluster)

Our Raft application receives log entries from the client.

When this happens we need to:

  • Redirect the client to the Raft cluster leader (if necessary)
  • Append the entry to our log
  • Block until the log entry has been committed [3]
[3]When the log entry has been replicated across a majority of servers in the Raft cluster

Append the entry to our log

We call raft_recv_entry when we want to append the entry to the log.

msg_entry_response_t response;
e = raft_recv_entry(raft,  &entry, &response);

You should populate the entry struct with the log entry the client has sent. After the call completes the response parameter is populated and can be used by the raft_msg_entry_response_committed function to check if the log entry has been committed or not.

Blocking until the log entry has been committed

When the server receives a log entry from the client, it has to block until the entry is committed. This is necessary as our Raft server has to replicate the log entry with the other peers of the Raft cluster.

The raft_recv_entry function does not block! This means you will need to implement the blocking functionality yourself.

Example below is from the ticketd client thread. This shows that we need to block on client requests. ticketd does the blocking by waiting on a conditional, which is signalled by the peer thread. The separate thread is responsible for handling traffic between Raft peers.

msg_entry_response_t response;

e = raft_recv_entry(sv->raft, &entry, &response);
if (0 != e)
    return h2oh_respond_with_error(req, 500, "BAD");

/* block until the entry is committed */
int done = 0;
do {
    uv_cond_wait(&sv->appendentries_received, &sv->raft_lock);
    e = raft_msg_entry_response_committed(sv->raft, &r);
    switch (e)
    {
        case 0:
            /* not committed yet */
            break;
        case 1:
            done = 1;
            uv_mutex_unlock(&sv->raft_lock);
            break;
        case -1:
            uv_mutex_unlock(&sv->raft_lock);
            return h2oh_respond_with_error(req, 400, "TRY AGAIN");
    }
} while (!done);

Example from ticketd of the peer thread. When an appendentries response is received from a Raft peer, we signal to the client thread that an entry might be committed.

e = raft_recv_appendentries_response(sv->raft, conn->node, &m.aer);
uv_cond_signal(&sv->appendentries_received);

Redirecting the client to the leader

When we receive an entry log from the client it's possible we might not be a leader.

If we aren't currently the leader of the raft cluster, we MUST send a redirect error message to the client. This is so that the client can connect directly to the leader in future connections. This enables future requests to be faster (ie. no redirects are required after the first redirect until the leader changes).

We use the raft_get_current_leader function to check who is the current leader.

Example of ticketd sending a 301 HTTP redirect response:

/* redirect to leader if needed */
raft_node_t* leader = raft_get_current_leader_node(sv->raft);
if (!leader)
{
    return h2oh_respond_with_error(req, 503, "Leader unavailable");
}
else if (raft_node_get_id(leader) != sv->node_id)
{
    /* send redirect */
    peer_connection_t* conn = raft_node_get_udata(leader);
    char leader_url[LEADER_URL_LEN];
    static h2o_generator_t generator = { NULL, NULL };
    static h2o_iovec_t body = { .base = "", .len = 0 };
    req->res.status = 301;
    req->res.reason = "Moved Permanently";
    h2o_start_response(req, &generator);
    snprintf(leader_url, LEADER_URL_LEN, "http://%s:%d/",
             inet_ntoa(conn->addr.sin_addr), conn->http_port);
    h2o_add_header(&req->pool,
                   &req->res.headers,
                   H2O_TOKEN_LOCATION,
                   leader_url,
                   strlen(leader_url));
    h2o_send(req, &body, 1, 1);
    return 0;
}

Function callbacks

You provide your callbacks to the Raft server using raft_set_callbacks.

The following callbacks MUST be implemented: send_requestvote, send_appendentries, applylog, persist_vote, persist_term, log_offer, and log_pop.

Example of function callbacks being set:

raft_cbs_t raft_callbacks = {
    .send_requestvote            = __send_requestvote,
    .send_appendentries          = __send_appendentries,
    .applylog                    = __applylog,
    .persist_vote                = __persist_vote,
    .persist_term                = __persist_term,
    .log_offer                   = __raft_logentry_offer,
    .log_poll                    = __raft_logentry_poll,
    .log_pop                     = __raft_logentry_pop,
    .log                         = __raft_log,
};

char* user_data = "test";

raft_set_callbacks(raft, &raft_callbacks, user_data);

send_requestvote()

For this callback we have to serialize a msg_requestvote_t struct, and then send it to the peer identified by node.

Example from ticketd showing how the callback is implemented:

static int __send_requestvote(
    raft_server_t* raft,
    void *udata,
    raft_node_t* node,
    msg_requestvote_t* m
    )
{
    peer_connection_t* conn = raft_node_get_udata(node);

    uv_buf_t bufs[1];
    char buf[RAFT_BUFLEN];
    msg_t msg = {
        .type              = MSG_REQUESTVOTE,
        .rv                = *m
    };
    __peer_msg_serialize(tpl_map("S(I$(IIII))", &msg), bufs, buf);
    int e = uv_try_write(conn->stream, bufs, 1);
    if (e < 0)
        uv_fatal(e);
    return 0;
}

send_appendentries()

For this callback we have to serialize a msg_appendentries_t struct, and then send it to the peer identified by node. This struct is more complicated to serialize because the m->entries array might be populated.

Example from ticketd showing how the callback is implemented:

static int __send_appendentries(
    raft_server_t* raft,
    void *user_data,
    raft_node_t* node,
    msg_appendentries_t* m
    )
{
    uv_buf_t bufs[3];

    peer_connection_t* conn = raft_node_get_udata(node);

    char buf[RAFT_BUFLEN], *ptr = buf;
    msg_t msg = {
        .type              = MSG_APPENDENTRIES,
        .ae                = {
            .term          = m->term,
            .prev_log_idx  = m->prev_log_idx,
            .prev_log_term = m->prev_log_term,
            .leader_commit = m->leader_commit,
            .n_entries     = m->n_entries
        }
    };
    ptr += __peer_msg_serialize(tpl_map("S(I$(IIIII))", &msg), bufs, ptr);

    /* appendentries with payload */
    if (0 < m->n_entries)
    {
        tpl_bin tb = {
            .sz   = m->entries[0].data.len,
            .addr = m->entries[0].data.buf
        };

        /* list of entries */
        tpl_node *tn = tpl_map("IIIB",
            &m->entries[0].id,
            &m->entries[0].term,
            &m->entries[0].type,
            &tb);
        size_t sz;
        tpl_pack(tn, 0);
        tpl_dump(tn, TPL_GETSIZE, &sz);
        e = tpl_dump(tn, TPL_MEM | TPL_PREALLOCD, ptr, RAFT_BUFLEN);
        assert(0 == e);
        bufs[1].len = sz;
        bufs[1].base = ptr;
        e = uv_try_write(conn->stream, bufs, 2);
        if (e < 0)
            uv_fatal(e);

        tpl_free(tn);
    }
    else
    {
        /* keep alive appendentries only */
        e = uv_try_write(conn->stream, bufs, 1);
        if (e < 0)
            uv_fatal(e);
    }

    return 0;
}

applylog()

This callback is all what is needed to interface the FSM with the Raft library. Depending on your application, you might want to save the commit_idx to disk inside this callback.

persist_vote() & persist_term()

These callbacks simply save data to disk, so that when the Raft server is rebooted it starts from a valid state. This is necessary to ensure safety.

log_offer()

For this callback the user needs to add a log entry. The log MUST be synced to disk before this callback can return.

log_poll()

For this callback the user needs to remove the eldest log entry [4]. The log MUST be synced to disk before this callback can return.

This callback only needs to be implemented to support log compaction.

log_pop()

For this callback the user needs to remove the youngest log entry [5]. The log MUST be synced to disk before this callback can return.

[4]The log entry at the front of the log
[5]The log entry at the back of the log

Receving traffic from peers

To receive Append Entries, Append Entries response, Request Vote, and Request Vote response messages, you need to deserialize the bytes into the message's corresponding struct.

The table below shows the structs that you need to deserialize-to or deserialize-from:

Message Type Struct Function
Append Entries msg_appendentries_t raft_recv_appendentries
Append Entries response msg_appendentries_response_t raft_recv_appendentries_response
Request Vote msg_requestvote_t raft_recv_requestvote
Request Vote response msg_requestvote_response_t raft_recv_requestvote_response

Example of how we receive an Append Entries message, and reply to it:

msg_appendentries_t ae;
msg_appendentries_response_t response;
char buf_in[1024], buf_out[1024];
size_t len_in, len_out;

read(socket, buf_in, &len_in);

deserialize_appendentries(buf_in, len_in, &ae);

e = raft_recv_requestvote(sv->raft, conn->node, &ae, &response);

serialize_appendentries_response(&response, buf_out, &len_out);

write(socket, buf_out, &len_out);

Membership changes

Membership changes are managed on the Raft log. You need two log entries to add a server to the cluster. While to remove you only need one log entry. There are two log entries for adding a server because we need to ensure that the new server's log is up to date before it can take part in voting.

It's highly recommended that when a node is added to the cluster that its node ID is random. This is especially important if the server was once connected to the cluster.

Adding a node

  1. Append the configuration change using raft_recv_entry. Make sure the entry has the type set to RAFT_LOGTYPE_ADD_NONVOTING_NODE
  2. Inside the log_offer callback, when a log with type RAFT_LOGTYPE_ADD_NONVOTING_NODE is detected, we add a non-voting node by calling raft_add_non_voting_node.
  3. Once node_has_sufficient_logs callback fires, append a configuration finalization log entry using raft_recv_entry. Make sure the entry has a type set to RAFT_LOGTYPE_ADD_NODE
  4. Inside the log_offer callback, when you receive a log with type RAFT_LOGTYPE_ADD_NODE, we then set the node to voting by using raft_add_node

Removing a node

  1. Append the configuration change using raft_recv_entry. Make sure the entry has the type set to RAFT_LOGTYPE_REMOVE_NODE
  2. Inside the log_offer callback, when a log with type RAFT_LOGTYPE_REMOVE_NODE is detected, we remove the node by calling raft_remove_node
  3. Once the RAFT_LOGTYPE_REMOVE_NODE configuration change log is applied in the applylog callback we shutdown the server if it is to be removed.

Todo

  • Log compaction