Simon Willison’s Weblog

Blogmarks tagged replication, postgresql

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Scaling Postgres with Read Replicas & Using WAL to Counter Stale Reads (via) The problem with sending writes to the primary and balancing reads across replicas is dealing with replica lag—what if you write to the primary and then read from a replica that hasn’t had the new state applied to it yet? Brandur Leach dives deep into an elegant solution using PostgreSQL’s LSN (log sequence numbers) accesesed using pg_last_wal_replay_lsn(). An observer process continuously polls the replicas for their most recently applied LSN and stores them in a table. A column in the Users table then records the min_lsn valid for that user, updating it to the pg_current_wal_lsn() of the primary whenever that user makes a write. Combining the two allows the application to randomly select a replica that is up-to-date for the purposes of a specific user any time it needs to make a read. # 18th November 2017, 6:42 pm

PostgreSQL 10 Released. Highlights include major improvements to parallelized queries, quorum commit for synchronous replication (sounds reminiscent of Cassandra) and logical replication, which allows modifications to specific tables to be replicated to different clusters. They’re also changing their versioning scheme to Major.Minor, so the next minor release will be 10.1 and the next major release will be 11. # 5th October 2017, 3:31 pm

PostgreSQL 9.0 Beta 1 Now Available. With asynchronous streaming replication. # 5th May 2010, 2:36 pm

PostgreSQL 8.5alpha3 now available. “Hot Standby, allowing read-only connections during recovery, provides a built-in master-slave replication solution.” Woohoo! # 23rd December 2009, 9:57 am

PostgreSQL 8.5 alpha 2 is out. “P.S. If you’re wondering about Hot Standby and Synchronous Replication, they’re still under heavy development and still (at this point) expected to be in 8.5.”—Hot Standby is PostgreSQL-speak for MySQL-style master/slave replication for scaling your reads. # 28th October 2009, 9:02 am

Londiste Tutorial. Master/slave replication for PostgreSQL, developed and used by Skype. # 9th September 2009, 11:06 pm

PostgreSQL Development Priorities. The top two for 8.4 are “Simple built-in replication” and “Upgrade-in-place”, Josh Berkus is seeking feedback on priorities for future work on 8.5. # 28th May 2009, 8:08 pm

What happened to Hot Standby? Hot Standby (the ability to have read-only replication slaves) has been dropped from PostgreSQL 8.4 and is now scheduled for 8.5. “Making hard decisions to postpone features which aren’t quite ready is how PostgreSQL makes sure that our DBMS is ”bulletproof“ and that we release close to on-time every year”. # 8th March 2009, 9:28 am