Client Interaction

You can send application-defined commands to the Raft cluster.

sorock distinguishes the command into two types: read-write and read-only. The read-only command is called query and queries can be processed through the optimized path.

R/W commnand

R/W command is a normal application command that is inserted into the log to be applied later.

You can send a R/W command to the cluster with this API.

message WriteRequest {
  uint32 shard_id = 1;
  bytes message = 2;
  string request_id = 3;
}

request_id is to avoid doubly application. Let's think about this scenario:

1. The client sends a R/W command to add 1 to the value.
2. The leader server replicates the command to the followers but crashes before application (+response).
3. The client resends the command to a new leader after a timeout.
4. The result is adding 2 to the value whereas the expectation is 1.

To avoid this issue, request_id is added to identify the commands.

R/O command

The R/O command can bypass the log because it is safe to execute the query after the commit index at query time is applied. This is called read_index optimization.

You can send a R/O command to the cluster with the following API.

When read_locally is set to true, the request isn't proxied to the leader but processed locally.

message ReadRequest {
  uint32 shard_id = 1;
  bytes message = 2;
  bool read_locally = 3;
}