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readonly_member.md

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Read-Only Member (Learner)

Read-only member is a node who does not initiate or participate in leader election, and just receives new updates from leader. When we count the number of nodes in quorum, read-only member will not be included. For example, if there are 4 nodes where 3 nodes are normal members while the other one is a read-only member, the size of quorum is still 2.

Read-only member is useful when you want to replicate data to geo-distributed nodes in remote datacenter. If you set those nodes as read-only members, they are not counted in quorum so that you can still organize a quorum within the same datacenter, which keeps commit latency relatively low. Remote nodes will catch-up the leader eventually.

To make a member read-only, you need to set learner_ flag to true, when you generate srv_config:

srv_config normal_member(1, 0, "10.10.10.1:12345", "", false);
srv_config       learner(2, 0, "10.10.10.2:12345", "", true);

The major differences between read-only member and a member with priority = 0 are as follows:

  • Both read-only member and zero-priority member do not initiate leader election, and never be a leader.
  • Read-only member is not counted in quorum, while zero-priority member is counted in.
  • Read-only member does not receive vote request, while zero-priority member does. Zero-priority member can vote for others.