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Comment by sealjam | Hacker News Reader

I'm by no means an expert on any of this stuff, but:

> They only require one other replica (as opposed to a quorum) to be reachable from the master for the master to continue acknowledging writes (from your comment)

> Orchestrator considers a number of variables during this process and is built on top of Raft for consensus... (from the article)

Doesn't quite make sense to me. Doesn't raft require the master to wait for a quorum before committing writes? I understood it as a pretty important aspect.

> A candidate must contact a majority of the cluster in order to be elected, which means that every committed entry must be present in at least one of those servers (from the raft paper [1])

I understood that the bold is only true if commits are acknowledged by the quorum

Edit: I'm not implying your post is incorrect, just trying to understand how the two fit together

[1]: https://raft.github.io/raft.pdf

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> Doesn't quite make sense to me. Doesn't raft require the master to wait for a quorum before committing writes? I understood it as a pretty important aspect.

Different kinds of masters. I've forced myself to always include the application if terminology gets reused across applications in a discussion. As far as I understand this>

- The mysql-master only commits an sql-level-transaction if a mysql-replica acknowledges replication of the transaction. This avoids data loss during a failover.

- The orchestrator-nodes elect an orchestrator-master via raft mechanics.

- After this, the orchestrator-master starts pondering about the mysql-master / mysql-replica situation and potentially promotes the most current mysql-replica to mysql-master.

- This promotion requires writes on an orchestrator/raft level, which in turn requires the orchestrator-master to get an acknowledgement from the orchestrator-quorum.

This was my first thought, you beat me to it. It seems like they contradict each other. Is it Raft with quorum or is it a single replica node that is caught up with master? You can't have both (unless you only have three nodes).

> unless you only have three nodes

Interestingly enough this seems to be how managed sql solutions do it. You just have one primary and one failover replica in another zone. So a quorum write is just a synchronous write to both (2 out of 2). You don't have the split-brain problem because the original primary can't make progress if it can't contact the replica.

https://cloud.google.com/sql/docs/mysql/high-availability

https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/Conce...

Github's problem is that they were trying to be too smart and allow any of their replicas to be candidate masters without increasing their quorum size. In theory this is higher availability than the managed sql solutions (they can be available even if their entire coast gets nuked) but they do it at the cost of consistency in the more common failure scenarios.

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