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<!--
doc/src/sgml/ref/wait_for.sgml
PostgreSQL documentation
-->
<refentry id="sql-wait-for">
<indexterm zone="sql-wait-for">
<primary>WAIT FOR</primary>
</indexterm>
<refmeta>
<refentrytitle>WAIT FOR</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>WAIT FOR</refname>
<refpurpose>wait for target <acronym>LSN</acronym> to be replayed, optionally with a timeout</refpurpose>
</refnamediv>
<refsynopsisdiv>
<synopsis>
WAIT FOR LSN '<replaceable class="parameter">lsn</replaceable>' [ WITH ( <replaceable class="parameter">option</replaceable> [, ...] ) ]
<phrase>where <replaceable class="parameter">option</replaceable> can be:</phrase>
TIMEOUT '<replaceable class="parameter">timeout</replaceable>'
NO_THROW
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
Waits until recovery replays <parameter>lsn</parameter>.
If no <parameter>timeout</parameter> is specified or it is set to
zero, this command waits indefinitely for the
<parameter>lsn</parameter>.
On timeout, or if the server is promoted before
<parameter>lsn</parameter> is reached, an error is emitted,
unless <literal>NO_THROW</literal> is specified in the WITH clause.
If <parameter>NO_THROW</parameter> is specified, then the command
doesn't throw errors.
</para>
<para>
The possible return values are <literal>success</literal>,
<literal>timeout</literal>, and <literal>not in recovery</literal>.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><replaceable class="parameter">lsn</replaceable></term>
<listitem>
<para>
Specifies the target <acronym>LSN</acronym> to wait for.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>WITH ( <replaceable class="parameter">option</replaceable> [, ...] )</literal></term>
<listitem>
<para>
This clause specifies optional parameters for the wait operation.
The following parameters are supported:
<variablelist>
<varlistentry>
<term><literal>TIMEOUT</literal> '<replaceable class="parameter">timeout</replaceable>'</term>
<listitem>
<para>
When specified and <parameter>timeout</parameter> is greater than zero,
the command waits until <parameter>lsn</parameter> is reached or
the specified <parameter>timeout</parameter> has elapsed.
</para>
<para>
The <parameter>timeout</parameter> might be given as integer number of
milliseconds. Also it might be given as string literal with
integer number of milliseconds or a number with unit
(see <xref linkend="config-setting-names-values"/>).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>NO_THROW</literal></term>
<listitem>
<para>
Specify to not throw an error in the case of timeout or
running on the primary. In this case the result status can be get from
the return value.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Outputs</title>
<variablelist>
<varlistentry>
<term><literal>success</literal></term>
<listitem>
<para>
This return value denotes that we have successfully reached
the target <parameter>lsn</parameter>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>timeout</literal></term>
<listitem>
<para>
This return value denotes that the timeout happened before reaching
the target <parameter>lsn</parameter>.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>not in recovery</literal></term>
<listitem>
<para>
This return value denotes that the database server is not in a recovery
state. This might mean either the database server was not in recovery
at the moment of receiving the command, or it was promoted before
reaching the target <parameter>lsn</parameter>.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
<command>WAIT FOR</command> command waits till
<parameter>lsn</parameter> to be replayed on standby.
That is, after this command execution, the value returned by
<function>pg_last_wal_replay_lsn</function> should be greater or equal
to the <parameter>lsn</parameter> value. This is useful to achieve
read-your-writes-consistency, while using async replica for reads and
primary for writes. In that case, the <acronym>lsn</acronym> of the last
modification should be stored on the client application side or the
connection pooler side.
</para>
<para>
<command>WAIT FOR</command> command should be called on standby.
If a user runs <command>WAIT FOR</command> on primary, it
will error out unless <parameter>NO_THROW</parameter> is specified in the WITH clause.
However, if <command>WAIT FOR</command> is
called on primary promoted from standby and <literal>lsn</literal>
was already replayed, then the <command>WAIT FOR</command> command just
exits immediately.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
You can use <command>WAIT FOR</command> command to wait for
the <type>pg_lsn</type> value. For example, an application could update
the <literal>movie</literal> table and get the <acronym>lsn</acronym> after
changes just made. This example uses <function>pg_current_wal_insert_lsn</function>
on primary server to get the <acronym>lsn</acronym> given that
<varname>synchronous_commit</varname> could be set to
<literal>off</literal>.
<programlisting>
postgres=# UPDATE movie SET genre = 'Dramatic' WHERE genre = 'Drama';
UPDATE 100
postgres=# SELECT pg_current_wal_insert_lsn();
pg_current_wal_insert_lsn
--------------------
0/306EE20
(1 row)
</programlisting>
Then an application could run <command>WAIT FOR</command>
with the <parameter>lsn</parameter> obtained from primary. After that the
changes made on primary should be guaranteed to be visible on replica.
<programlisting>
postgres=# WAIT FOR LSN '0/306EE20';
status
--------
success
(1 row)
postgres=# SELECT * FROM movie WHERE genre = 'Drama';
genre
-------
(0 rows)
</programlisting>
</para>
<para>
If the target LSN is not reached before the timeout, the error is thrown.
<programlisting>
postgres=# WAIT FOR LSN '0/306EE20' WITH (TIMEOUT '0.1s');
ERROR: timed out while waiting for target LSN 0/306EE20 to be replayed; current replay LSN 0/306EA60
</programlisting>
</para>
<para>
The same example uses <command>WAIT FOR</command> with
<parameter>NO_THROW</parameter> option.
<programlisting>
postgres=# WAIT FOR LSN '0/306EE20' WITH (TIMEOUT '100ms', NO_THROW);
status
--------
timeout
(1 row)
</programlisting>
</para>
</refsect1>
</refentry>
|
