Before PostgreSQL got streaming replication, back in version 9.0, people kept asking when we’re going to get replication. That was a common conversation-starter when standing at a conference booth. I don’t hear that anymore, but this dialogue still happens every now and then:
– I have streaming replication set up, with a master and standby. How do I perform failover?
– That’s easy, just kill the old master node, and run “pg_ctl promote” on the standby.
– Cool. And how do I fail back to the old master?
– Umm, well, you have to take a new base backup from the new master, and re-build the node from scratch..
– Huh, what?!?
pg_rewind is a better answer to that. One way to think of it is that it’s like rsync on steroids. Like rsync, it copies files that differ between the source and target. The trick is in how it determines which files have changed. Rsync compares timestamps, file sizes and checksums, but pg_rewind understands the PostgreSQL file formats, and reads the WAL to get that information instead.
I started hacking on pg_rewind about a year ago, while working for VMware. I got it working, but it was a bit of a pain to maintain. Michael Paquier helped to keep it up-to-date, whenever upstream changes in PostgreSQL broke it. A big pain was that it has to scan the WAL, and understand all different WAL record types – miss even one and you might end up with a corrupt database. I made big changes to the way WAL-logging works in 9.5, to make that easier. All WAL record types now contain enough information to know what block it applies to, in a common format. That slashed the amount of code required in pg_rewind, and made it a lot easier to maintain.
I have just committed pg_rewind into the PostgreSQL git repository, and it will be included in the upcoming 9.5 version. I always intended pg_rewind to be included in PostgreSQL itself; I started it as a standalone project to be able to develop it faster, outside the PostgreSQL release cycle, so I’m glad it finally made it into the main distribution now. Please give it a lot of testing!
PS. I gave a presentation on pg_rewind in Nordic PGDay 2015. It was a great conference, and I think people enjoyed the presentation. Have a look at the slides for an overview on how pg_rewind works. Also take a look at the page in the user manual.