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Comment by jfbaro | Hacker News Reader

It's not a limitation that affects most of users, but lack of official multi master support makes it less suitable for "always on" databases. But to be honest multi master is something hard to get right in ANY database. PostgreSQL is great for many use cases and good enough for many more!

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Lately I've been getting a lot of notifications for BDR (short for Bi-Directional Replication) from EDB; the term 'Always On' also appears in those notifications, apparently to woo the Oracle audiences. You might want to take a look into that.

Disclosures/info: I used to work for EDB. BDR was developed by 2nd Quadrant, which was recently acquired by EDB.

The basic HA support is there, and is very well described in the docs. What Postgres doesn't provide is the mechanism to pick when to automatically promote a replica to primary, but that's because this is a system-level concern that's entirely outside the database featureset itself.

https://www.postgres-xl.org/

Why would you need multi master to be "always on"?

If for always on or High Availability setups you want two DBs running that are in constant communication, with every write being committed on both machines synchronously - so if one of them goes down the other can take its place immediately with no downtime.

Postgres has no built in support yet, but tools like STOLON exist.

Postgres doesn't support synchronous replication yet? It's had it since 9.1.

I think you're referring to something else.

PostgreSQL has master-slave replication (one read/write master, and multiple read-only standbys), the other users are talking about multi-master replication (all the databases are read/write).

I'm aware. What I'm asking is why multi master is required. You can have a read only slave that is promoted to the master. It's that little bit of downtime during failover in question? Doesn't pgpool make that possible without downtime?

If you use multi master and DNS failover you still have partial downtime for every session routed to the unhealthy master. That doesn't seem to solve the problem, just a little better.

Yup. The synchronous replication described in the docs will allow the configuration of a hot standby, but I don’t think it allows writing to the standby, which is what true master-master does.

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