One of my clients is upgrading some servers. The procedure we have took some time to get to current state, and we found some potential problems, so decided to write more about it.
I had to deal with this question, or some version of it, quite a few times. So, decided to write a summary on what one could (or should) do, after data is in database, and application is running. Namely – setup some kind of replication and backups.
What to use, how, and why? This is what this post is all about.
On 13th of December, Heikki Linnakangas committed patch:
Allow a streaming replication standby to follow a timeline switch. Before this patch, streaming replication would refuse to start replicating if the timeline in the primary doesn't exactly match the standby. The situation where it doesn't match is when you have a master, and two standbys, and you promote one of the standbys to become new master. Promoting bumps up the timeline ID, and after that bump, the other standby would refuse to continue. There's significantly more timeline related logic in streaming replication now. First of all, when a standby connects to primary, it will ask the primary for any timeline history files that are missing from the standby. The missing files are sent using a new replication command TIMELINE_HISTORY, and stored in standby's pg_xlog directory. Using the timeline history files, the standby can follow the latest timeline present in the primary (recovery_target_timeline='latest'), just as it can follow new timelines appearing in an archive directory. START_REPLICATION now takes a TIMELINE parameter, to specify exactly which timeline to stream WAL from. This allows the standby to request the primary to send over WAL that precedes the promotion. The replication protocol is changed slightly (in a backwards-compatible way although there's little hope of streaming replication working across major versions anyway), to allow replication to stop when the end of timeline reached, putting the walsender back into accepting a replication command. Many thanks to Amit Kapila for testing and reviewing various versions of this patch.
On 19th of July, Simon Riggs committed patch:
Cascading replication feature for streaming log-based replication. Standby servers can now have WALSender processes, which can work with either WALReceiver or archive_commands to pass data. Fully updated docs, including new conceptual terms of sending server, upstream and downstream servers. WALSenders terminated when promote to master. Fujii Masao, review, rework and doc rewrite by Simon Riggs
On 6th of March, Simon Riggs committed patch:
Efficient transaction-controlled synchronous replication. If a standby is broadcasting reply messages and we have named one or more standbys in synchronous_standby_names then allow users who set synchronous_replication to wait for commit, which then provides strict data integrity guarantees. Design avoids sending and receiving transaction state information so minimises bookkeeping overheads. We synchronize with the highest priority standby that is connected and ready to synchronize. Other standbys can be defined to takeover in case of standby failure. This version has very strict behaviour; more relaxed options may be added at a later date. Simon Riggs and Fujii Masao, with reviews by Yeb Havinga, Jaime Casanova, Heikki Linnakangas and Robert Haas, plus the assistance of many other design reviewers.
As of yesterday OmniPITR got following changes/fixes:
- Fixed bug which caused immediate finish request be treated the same as smart finish request.
- Fixed problem with using omnipitr-backup-slave on PostgreSQL 9.0 slave, which is using streaming replication.
- Added option to omnipitr-restore, so that you can now use it for streaming-replication slaves
I'm very ashamed of the first thing (smart/immediate finish request), as it was simply my lack of test.
Second thing – the problem was that with streaming replication slave behaves a bit differently than normally, and so the backup procedure had to be modified.
As for third – restore command in streaming replication environment has to behave differently than normal restore command – i.e. it should finish, with error, as soon as it will be called for wal file that does not exist in wal archive. Which is direct opposite of what should be done normally.
Now, omnipitr-restore got switch (-sr) to make it work correctly in SR situation.
Links for svn/docs are listed on project page.
On 23rd of January, Magnus Hagander committed patch which adds:
Add pg_basebackup tool for streaming base backups This tool makes it possible to do the pg_start_backup/ copy files/pg_stop_backup step in a single command. There are still some steps to be done before this is a complete backup solution, such as the ability to stream the required WAL logs, but it's still usable, and could do with some buildfarm coverage. In passing, make the checkpoint request optionally fast instead of hardcoding it. Magnus Hagander, reviewed by Fujii Masao and Dimitri Fontaine
On 7th of January (I know, it was quite some time ago, I apologize for delay) Itagaki Takahiro committed patch:
New system view pg_stat_replication displays activity of wal sender processes. Itagaki Takahiro and Simon Riggs.
The BIG feature. The feature that made PostgreSQL leap from 8.4 to 9.0. Patch was written by Fujii Masao, and committed by Heikki Linnakangas on 15th of January 2010:
Log Message: ----------- Introduce Streaming Replication. This includes two new kinds of postmaster processes, walsenders and walreceiver. Walreceiver is responsible for connecting to the primary server and streaming WAL to disk, while walsender runs in the primary server and streams WAL from disk to the client. Documentation still needs work, but the basics are there. We will probably pull the replication section to a new chapter later on, as well as the sections describing file-based replication. But let's do that as a separate patch, so that it's easier to see what has been added/changed. This patch also adds a new section to the chapter about FE/BE protocol, documenting the protocol used by walsender/walreceivxer. Bump catalog version because of two new functions, pg_last_xlog_receive_location() and pg_last_xlog_replay_location(), for monitoring the progress of replication. Fujii Masao, with additional hacking by me