Waiting for 9.2 – cascading streaming replication

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

Continue reading Waiting for 9.2 – cascading streaming replication

Write Ahead Log + Understanding postgresql.conf: checkpoint_segments, checkpoint_timeout, checkpoint_warning

While there are some docs on it, I decided to write about it, in perhaps more accessible language – not as a developer, but as PostgreSQL user.

Some parts (quite large parts) were described in one of my earlier posts, but I'll try to concentrate on WAL itself, and show a bit more in here.

Continue reading Write Ahead Log + Understanding postgresql.conf: checkpoint_segments, checkpoint_timeout, checkpoint_warning

Waiting for 9.1 – Unlogged tables

On 29th of December, Robert Haas committed interesting patch, which does:

Support unlogged tables.
 
The contents of an unlogged table aren't WAL-logged; thus, they are not
available on standby servers and are truncated whenever the database
system enters recovery.  Indexes on unlogged tables are also unlogged.
Unlogged GiST indexes are not currently supported.

(edited commit message, due to this mail.

Continue reading Waiting for 9.1 – Unlogged tables

Setting WAL Replication

There are several approaches on replication/failover – you might have heard of Slony, Londiste, pgPool and some other tools.

WAL Replication is different from all of them in one aspect – it doesn't let you query slave database (until 9.0, in which you actually can run read only queries on slave.

Since you can't run queries on slave, what is it good for? Well. It's good, and great in 1 very important aspect – all things that happen in database are replicated. Schema changes. Sequence modifications. Everything.

There is also drawback – you can't (as of now) replicate just one database. You replicate whole cluster (I don't like this word in this context – let's say: whole installation) of PostgreSQL. All databases that reside in given DATA directory.

So, the question is – how to set it up?

Continue reading Setting WAL Replication

Waiting for 9.0 – removal of 0000000001.history check

I tend to write about new features in new versions of PostgreSQL, but this patch actually fixes one of the things that annoy me a lot, so here it goes:

On 26th of January, Simon Riggs committed:

Log Message:
-----------
Fix longstanding gripe that we check for 0000000001.history at start of
archive recovery, even when we know it is never present.

If you've ever tried to roll your own restore_command script ( like pg_standby ) then you know that to the algorithm that's presented in docs You always had to add special case for file “0000000001.history" – which was never there, but somehow PostgreSQL always asked for it – despite the fact that it could never arrive.

Now, thanks to this small patch we will no longer need to add this “if" in any script. It's small, and it's not a new feature, but I am SO happy to see it.

Waiting for 9.0 – Streaming replication

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

Continue reading Waiting for 9.0 – Streaming replication

Waiting for 8.5 – Hot Standby

On 19th of December Simon Riggs committed a patch that will quite likely be the single most-talked-about change in PostgreSQL 8.5:

Log Message:
-----------
Allow read only connections during recovery, known as Hot Standby. 
 
Enabled by recovery_connections = on (default) and forcing archive recovery
using a recovery.conf. Recovery processing now emulates the original
transactions as they are replayed, providing full locking and MVCC behaviour
for read only queries. Recovery must enter consistent state before
connections are allowed, so there is a delay, typically short, before
connections succeed. Replay of recovering transactions can conflict and in
some cases deadlock with queries during recovery; these result in query
cancellation after max_standby_delay seconds have expired. Infrastructure
changes have minor effects on normal running, though introduce four new
types of WAL record. 
 
New test mode "make standbycheck" allows regression tests of
static command behaviour on a standby server while in recovery. Typical and
extreme dynamic behaviours have been checked via code inspection and manual
testing. Few port specific behaviours have been utilised, though primary
testing has been on Linux only so far. 
 
This commit is the basic patch. Additional changes will follow in this
release to enhance some aspects of behaviour, notably improved handling of
conflicts, deadlock detection and query cancellation. Changes to VACUUM FULL
are also required.
 
Simon Riggs, with significant and lengthy review by Heikki Linnakangas,
including streamlined redesign of snapshot creation and two-phase commit.
 
Important contributions from Florian Pflug, Mark Kirkwood, Merlin Moncure,
Greg Stark, Gianni Ciolli, Gabriele Bartolini, Hannu Krosing, Robert Haas,
Tatsuo Ishii, Hiroyuki Yamada plus support and feedback from many other
community members.

Continue reading Waiting for 8.5 – Hot Standby