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Re: [HACKERS] Re: [COMMITTERS] pgsql: Make standby server continuously retry restoring the next WAL

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>, Aidan Van Dyk <aidan(at)highrise(dot)ca>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, PostgreSQL Docs <pgsql-docs(at)postgresql(dot)org>
Subject: Re: [HACKERS] Re: [COMMITTERS] pgsql: Make standby server continuously retry restoring the next WAL
Date: 2010-03-25 10:15:15
Message-ID: 4BAB37B3.109@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-docspgsql-hackers
(cc'ing docs list)

Simon Riggs wrote:
> The lack of docs begins to show a lack of coherent high-level design
> here.

Yeah, I think you're right. It's becoming hard to keep track of how it's
supposed to behave.

> By now, I've forgotten what this thread was even about. The major
> design decision in this that keeps showing up is "remove pg_standby, at
> all costs" but no reason has ever been given for that. I do believe
> there is a "better way", but we won't find it by trial and error, even
> if we had time to do so.

This has nothing to do with pg_standby.

> Please work on some clear docs for the failure modes in this system.
> That way we can all read them and understand them, or point out further
> issues. Moving straight to code is not a solution to this, since what we
> need now is to all agree on the way forwards. If we ignore this, then
> there is considerable risk that streaming rep will have a fatal
> operational flaw.
> 
> Please just document/diagram how it works now, highlighting the problems
> that still remain to be solved. We're all behind you and I'm helping
> wherever I can.

Ok, here's my attempt at the docs. Read it as a replacement for the
"High Availability, Load Balancing, and Replication" chapter, but of
course many of the sections will be unchanged, as indicated below.

-------------
Chapter 25. High Availability, Load Balancing, and Replication

25.1 Comparison of different solutions

<no changes>


25.2 Log-Shipping Standby servers

<overview from current "File-based Log Shipping" section. With small
changes so that it applies to the built-in standby mode as well as
pg_standby like solutions>

A standby server can also be used for read-only queries. This is called
Hot Standby mode, see chapter XXX

25.2.1 Planning

Set up two servers with identical hardware ...

<two first paragraphs of current File-based log-shipping / Planning section>

25.2.3 Standby mode

In standby mode, the server continously applies WAL received from the
master server. The standby server can receive WAL from a WAL archive
(see restore_command) or directly from the master over a TCP connection
(streaming replication). The standby server will also attempt to restore
any WAL found in the standby's pg_xlog. That typically happens after a
server restart, to replay again any WAL streamed from the master before
the restart, but you can also manually copy files to pg_xlog at any time
to have them replayed.

At startup, the standby begins by restoring all WAL available in the
archive location, calling restore_command. Once it reaches the end of
WAL available there and restore_command fails, it tries to restore any
WAL available in the pg_xlog directory (possibly stored there by
streaming replication before restart). If that fails, and streaming
replication has been configured, the standby tries to connect to the
master server and stream WAL from it. If that fails or streaming
replication is not configured, or if the connection is disconnected
later on, the standby goes back to step 1 and tries to restoring the
file from the archive again. This loop of retries from the archive,
pg_xlog, and via streaming replication goes on until the server is
stopped or failover is triggered by a trigger file.

A corrupt or half-finished WAL file in the archive, or streamed from the
master, causes a PANIC and immediate shutdown of the standby server. A
corrupt WAL file is always a serious event which requires administrator
action. If you want to recover a WAL file known to be corrupt as far as
it can be, you can copy the file manually into pg_xlog.

Standby mode is exited and the server switches to normal operation, when
a trigger file is found (trigger_file). Before failover, it will restore
any WAL available in the archive or in pg_xlog, but won't try to connect
to the master or wait for files to become available in the archive.


25.2.4 Preparing Master for Standby servers

Set up continous archiving to a WAL archive on the master, as described
in the chapter "Continous Archiving and Point-In-Time_recovery". The
archive location should be accessible from the standby even when the
master is down, ie. it should reside on the standby server itself or
another trusted server, not on the master server.

If you want to use streaming replication, set up authentication to allow
streaming replication connections. Set max_wal_senders.

Take a base backup as described in chapter Continous Archiving and
Point-In-Time_recovery / Making a Base Backup.

25.2.4.1 Authentication for streaming replication

Ensure that listen_addresses allows connections from the standby server.

<current Streaming Replication / Authentication section, describing
pg_hba.conf>


25.2.5 Setting up the standby server

1. Take a base backup, and copy it to the standby

2. Create a restore_command to restore files from the WAL archive.

3. Set standby_mode=on

4. If you want to use streaming replicaton, set primary_conninfo


You can use restartpoint_command to prune the archive of files no longer
needed by the standby.

You can have any number of standby servers as long as you set
max_wal_senders high enough in the master to allow them to be connected
simultaneously.

25.2.6 Archive recovery based log shipping

An alternative to the built-in standby mode desribed in the previous
sections is to use a restore_command that polls the archive location.
This was the only option available in versions 8.4 and below. In this
setup, set standby_mode=off, because you are implementing the polling
required for a standby server yourself. See contrib/pg_standby for a
reference implementation of this.

Note that the in this mode, the server will apply WAL one file at a
time, so if you use the standby server for queries (see Hot Standby),
there is a bigger delay between an action in the master and when the
action becomes visible in the standby, corresponding the time it takes
to fill up the WAL file. archive_timeout can be used to make that delay
shorter. Also note that you can't combine streaming replication with
this method.

<all the sections from current File-based log shipping that don't apply
to built-in standby mode go here>

25.3 Hot Standby

<no changes>

25.4 Incrementally Updated backups

<no changes>

-------------


-- 
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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