Can coordinator know if part of consensus

[as51340]

Hi!

I am interested in whether server can quickly know whether it is part of the majority. Basically, want to know if there is a way to know if server knows the latest cluster state (so it can return it) even if it is not the leader.

If we can figure out that the server doesn't know the latest data, we could forward the request to the current leader

Tnx,
Andi

[greensky00]

If you were referring to followers knowing whether they are part of the quorum, unfortunately, that is not possible. Only the leader knows which members are part of the quorum at the given moment, by using adjust_commit_index API. You can refer to this document, you can selectively pin the quorum, or include all members in the quorum.

[as51340]

Not necessarily if followers are part of the quorum but I am interested more how clients can know whether they are sending request to the follower or to the leader.

Example:
3 coordinators: server1 is leader, server2 is follower and server3 is partitioned from server1 and 2. What if network between client and server3 isn't down and the request comes to server3? Server3 could serve the stale data because server1 and server2 could commit something in between while disconnected from server3.

Do you have such situation in Ebay or how do you work with it?

[greensky00]

You can do it in following way.

• Track the "last received response time" from each follower, using the provided callback function.
• You can also have a response time threshold, let's say that is X.
• By using the selective quorum that I mentioned above, the leader commits a log only after it is replicated to all members (i.e., quorum size == raft group size).
• However, if there is a member whose last response time exceeds 2 * X (server 3 in your example), the leader can exclude that member from the quorum.
• From that excluded member's (server 3) perspective, if it does not receive any message from the leader for more than X, it should step down and deny any requests from client.

In the way, clients can either receive the latest data or a request denial, so that they can try another member.

[greensky00]

Actually, the above things should be naturally supported by "full consensus mode", but this feature is currently experimental and not yet ready to use. We plan to enhance it soon.

[as51340]

great, tnx a lot!