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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 62. Generic WAL Records</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="index-cost-estimation.html" title="61.6. Index Cost Estimation Functions" /><link rel="next" href="btree.html" title="Chapter 63. B-Tree Indexes" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">Chapter 62. Generic WAL Records</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="index-cost-estimation.html" title="61.6. Index Cost Estimation Functions">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="internals.html" title="Part VII. Internals">Up</a></td><th width="60%" align="center">Part VII. Internals</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="btree.html" title="Chapter 63. B-Tree Indexes">Next</a></td></tr></table><hr></hr></div><div class="chapter" id="GENERIC-WAL"><div class="titlepage"><div><div><h2 class="title">Chapter 62. Generic WAL Records</h2></div></div></div><p>
Although all built-in WAL-logged modules have their own types of WAL
records, there is also a generic WAL record type, which describes changes
to pages in a generic way. This is useful for extensions that provide
custom access methods, because they cannot register their own WAL redo
routines.
</p><p>
The API for constructing generic WAL records is defined in
<code class="filename">access/generic_xlog.h</code> and implemented
in <code class="filename">access/transam/generic_xlog.c</code>.
</p><p>
To perform a WAL-logged data update using the generic WAL record
facility, follow these steps:
</p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p>
<code class="function">state = GenericXLogStart(relation)</code> — start
construction of a generic WAL record for the given relation.
</p></li><li class="listitem"><p>
<code class="function">page = GenericXLogRegisterBuffer(state, buffer, flags)</code>
— register a buffer to be modified within the current generic WAL
record. This function returns a pointer to a temporary copy of the
buffer's page, where modifications should be made. (Do not modify the
buffer's contents directly.) The third argument is a bit mask of flags
applicable to the operation. Currently the only such flag is
<code class="literal">GENERIC_XLOG_FULL_IMAGE</code>, which indicates that a full-page
image rather than a delta update should be included in the WAL record.
Typically this flag would be set if the page is new or has been
rewritten completely.
<code class="function">GenericXLogRegisterBuffer</code> can be repeated if the
WAL-logged action needs to modify multiple pages.
</p></li><li class="listitem"><p>
Apply modifications to the page images obtained in the previous step.
</p></li><li class="listitem"><p>
<code class="function">GenericXLogFinish(state)</code> — apply the changes to
the buffers and emit the generic WAL record.
</p></li></ol></div><p>
</p><p>
WAL record construction can be canceled between any of the above steps by
calling <code class="function">GenericXLogAbort(state)</code>. This will discard all
changes to the page image copies.
</p><p>
Please note the following points when using the generic WAL record
facility:
</p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p>
No direct modifications of buffers are allowed! All modifications must
be done in copies acquired from <code class="function">GenericXLogRegisterBuffer()</code>.
In other words, code that makes generic WAL records should never call
<code class="function">BufferGetPage()</code> for itself. However, it remains the
caller's responsibility to pin/unpin and lock/unlock the buffers at
appropriate times. Exclusive lock must be held on each target buffer
from before <code class="function">GenericXLogRegisterBuffer()</code> until after
<code class="function">GenericXLogFinish()</code>.
</p></li><li class="listitem"><p>
Registrations of buffers (step 2) and modifications of page images
(step 3) can be mixed freely, i.e., both steps may be repeated in any
sequence. Keep in mind that buffers should be registered in the same
order in which locks are to be obtained on them during replay.
</p></li><li class="listitem"><p>
The maximum number of buffers that can be registered for a generic WAL
record is <code class="literal">MAX_GENERIC_XLOG_PAGES</code>. An error will be thrown
if this limit is exceeded.
</p></li><li class="listitem"><p>
Generic WAL assumes that the pages to be modified have standard
layout, and in particular that there is no useful data between
<code class="structfield">pd_lower</code> and <code class="structfield">pd_upper</code>.
</p></li><li class="listitem"><p>
Since you are modifying copies of buffer
pages, <code class="function">GenericXLogStart()</code> does not start a critical
section. Thus, you can safely do memory allocation, error throwing,
etc. between <code class="function">GenericXLogStart()</code> and
<code class="function">GenericXLogFinish()</code>. The only actual critical section is
present inside <code class="function">GenericXLogFinish()</code>. There is no need to
worry about calling <code class="function">GenericXLogAbort()</code> during an error
exit, either.
</p></li><li class="listitem"><p>
<code class="function">GenericXLogFinish()</code> takes care of marking buffers dirty
and setting their LSNs. You do not need to do this explicitly.
</p></li><li class="listitem"><p>
For unlogged relations, everything works the same except that no
actual WAL record is emitted. Thus, you typically do not need to do
any explicit checks for unlogged relations.
</p></li><li class="listitem"><p>
The generic WAL redo function will acquire exclusive locks to buffers
in the same order as they were registered. After redoing all changes,
the locks will be released in the same order.
</p></li><li class="listitem"><p>
If <code class="literal">GENERIC_XLOG_FULL_IMAGE</code> is not specified for a
registered buffer, the generic WAL record contains a delta between
the old and the new page images. This delta is based on byte-by-byte
comparison. This is not very compact for the case of moving data
within a page, and might be improved in the future.
</p></li></ul></div><p>
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