Please see `ticketd <https://github.com/willemt/ticketd>`_ for a real life use of the Raft library.
It's best to have 2 separate threads. One for handling peer traffic, and a another for handling client traffic.
Initializing the Raft server
============================
We instantiate a new Raft server using `raft_new`.
.. code-block:: c
void* raft = raft_new();
We provide our callbacks to the raft server using `raft_set_callbacks`
We MUST implement the following callbacks: `send_requestvote`, `send_appendentries`, `applylog`, `persist_vote`, `persist_term`, `log_offer`, and `log_pop`.
We tell the Raft server what the cluster configuration is by using the `raft_add_node` function. For example, if we have 5 servers [*] in our cluster, we call `raft_add_node` 5 [*] times.
.. [*] 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.
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 [*]
.. [*] 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.
.. code-block:: c
msg_entry_response_t response;
e = raft_recv_entry(raft, node_idx, &entry, &response);
You should popuate 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` 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.*
.. code-block:: c
msg_entry_response_t response;
e = raft_recv_entry(sv->raft, sv->node_idx, &entry, &response);
*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.*
.. code-block:: c
e = raft_recv_appendentries_response(sv->raft, conn->node_idx, &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.
We use the `raft_get_current_leader` function to check who is the current leader.
*Example of ticketd sending a 301 HTTP redirect response:*
e = uv_write(&conn->write, conn->stream, bufs, 1, __peer_write_cb);
if (-1 == e)
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_idx`. This struct is more complicated to serialize because the `m->entries` array might be populated.
*Example from ticketd showing how the callback is implemented:*