Flexible Quorums

OmniPaxos also offers support for flexible quorums. Normally a node needs to consult a majority of nodes to become a leader and then, as a leader, consult a majority of nodes to append to the log. Flexible quorums allow for a trade off between the number of nodes that need to be consulted in each of these two scenarios.

OmniPaxos can be configured to use a flexible quorum as follows:

use omnipaxos::{ClusterConfig, OmniPaxosConfig, ServerConfig, util::FlexibleQuroum};

let flex_quorum = FlexibleQuorum {
    read_quorum_size: 5,
    write_quorum_size: 3,

let cluster_config = ClusterConfig {
    configuration_id: 1,
    nodes: vec![1, 2, 3, 4, 5, 6, 7],
    flexible_quorum: Some(flex_quorum),
let server_config = ServerConfig {
    pid: 1,
let config = OmniPaxosConfig {

In OmniPaxos, becoming a leader involves reading from the log and so read_quorum_size is the number of nodes to consult in order to become a leader. The write_quorum_size is the number of nodes to consult in order to append to the log.

In order to guarantee safety, the read_quorum_size and write_quorum_size must overlap (i.e. read_quorum_size + write_quorum_size > # of nodes in cluster). In practice this means that reducing the write_quorum_size requires increasing the read_quorum_size. However, appending to the log is a much more common occurrence than electing a leader, so reducing the write_quorum_size can be beneficial.