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Doc: improve discussion of race conditions involved in LISTEN.

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The user docs didn't really explain how to use LISTEN safely,
so clarify that.  Also clean up some fuzzy-headed explanations
in comments.  No code changes.

Discussion: https://postgr.es/m/3ac7f397-4d5f-be8e-f354-440020675694@gmail.com
This commit is contained in:
Tom Lane
2019-11-24 18:03:39 -05:00
parent 6b802cfc7f
commit d3aa114ac4
2 changed files with 65 additions and 36 deletions
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31
doc/src/sgml/ref/listen.sgml
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@ -63,13 +63,6 @@ LISTEN <replaceable class="parameter">channel</replaceable>
<command>LISTEN</command> or <command>UNLISTEN</command> directly. See the
documentation for the interface you are using for more details.
</para>
<para>
<xref linkend="sql-notify"/>
contains a more extensive
discussion of the use of <command>LISTEN</command> and
<command>NOTIFY</command>.
</para>
</refsect1>
<refsect1>
@ -96,10 +89,34 @@ LISTEN <replaceable class="parameter">channel</replaceable>
within a transaction that later rolls back, the set of notification
channels being listened to is unchanged.
</para>
<para>
A transaction that has executed <command>LISTEN</command> cannot be
prepared for two-phase commit.
</para>
<para>
There is a race condition when first setting up a listening session:
if concurrently-committing transactions are sending notify events,
exactly which of those will the newly listening session receive?
The answer is that the session will receive all events committed after
an instant during the transaction's commit step. But that is slightly
later than any database state that the transaction could have observed
in queries. This leads to the following rule for
using <command>LISTEN</command>: first execute (and commit!) that
command, then in a new transaction inspect the database state as needed
by the application logic, then rely on notifications to find out about
subsequent changes to the database state. The first few received
notifications might refer to updates already observed in the initial
database inspection, but this is usually harmless.
</para>
<para>
<xref linkend="sql-notify"/>
contains a more extensive
discussion of the use of <command>LISTEN</command> and
<command>NOTIFY</command>.
</para>
</refsect1>
<refsect1>