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-rw-r--r--doc/src/sgml/high-availability.sgml54
-rw-r--r--doc/src/sgml/ref/allfiles.sgml1
-rw-r--r--doc/src/sgml/ref/wait_for.sgml234
-rw-r--r--doc/src/sgml/reference.sgml1
4 files changed, 290 insertions, 0 deletions
diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
index b47d8b4106e..742deb037b7 100644
--- a/doc/src/sgml/high-availability.sgml
+++ b/doc/src/sgml/high-availability.sgml
@@ -1376,6 +1376,60 @@ synchronous_standby_names = 'ANY 2 (s1, s2, s3)'
</sect3>
</sect2>
+ <sect2 id="read-your-writes-consistency">
+ <title>Read-Your-Writes Consistency</title>
+
+ <para>
+ In asynchronous replication, there is always a short window where changes
+ on the primary may not yet be visible on the standby due to replication
+ lag. This can lead to inconsistencies when an application writes data on
+ the primary and then immediately issues a read query on the standby.
+ However, it is possible to address this without switching to synchronous
+ replication.
+ </para>
+
+ <para>
+ To address this, PostgreSQL offers a mechanism for read-your-writes
+ consistency. The key idea is to ensure that a client sees its own writes
+ by synchronizing the WAL replay on the standby with the known point of
+ change on the primary.
+ </para>
+
+ <para>
+ This is achieved by the following steps. After performing write
+ operations, the application retrieves the current WAL location using a
+ function call like this.
+
+ <programlisting>
+postgres=# SELECT pg_current_wal_insert_lsn();
+pg_current_wal_insert_lsn
+--------------------
+0/306EE20
+(1 row)
+ </programlisting>
+ </para>
+
+ <para>
+ The <acronym>LSN</acronym> obtained from the primary is then communicated
+ to the standby server. This can be managed at the application level or
+ via the connection pooler. On the standby, the application issues the
+ <xref linkend="sql-wait-for"/> command to block further processing until
+ the standby's WAL replay process reaches (or exceeds) the specified
+ <acronym>LSN</acronym>.
+
+ <programlisting>
+postgres=# WAIT FOR LSN '0/306EE20';
+ status
+--------
+ success
+(1 row)
+ </programlisting>
+ Once the command returns a status of success, it guarantees that all
+ changes up to the provided <acronym>LSN</acronym> have been applied,
+ ensuring that subsequent read queries will reflect the latest updates.
+ </para>
+ </sect2>
+
<sect2 id="continuous-archiving-in-standby">
<title>Continuous Archiving in Standby</title>
diff --git a/doc/src/sgml/ref/allfiles.sgml b/doc/src/sgml/ref/allfiles.sgml
index f5be638867a..e167406c744 100644
--- a/doc/src/sgml/ref/allfiles.sgml
+++ b/doc/src/sgml/ref/allfiles.sgml
@@ -188,6 +188,7 @@ Complete list of usable sgml source files in this directory.
<!ENTITY update SYSTEM "update.sgml">
<!ENTITY vacuum SYSTEM "vacuum.sgml">
<!ENTITY values SYSTEM "values.sgml">
+<!ENTITY waitFor SYSTEM "wait_for.sgml">
<!-- applications and utilities -->
<!ENTITY clusterdb SYSTEM "clusterdb.sgml">
diff --git a/doc/src/sgml/ref/wait_for.sgml b/doc/src/sgml/ref/wait_for.sgml
new file mode 100644
index 00000000000..3b8e842d1de
--- /dev/null
+++ b/doc/src/sgml/ref/wait_for.sgml
@@ -0,0 +1,234 @@
+<!--
+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>
diff --git a/doc/src/sgml/reference.sgml b/doc/src/sgml/reference.sgml
index ff85ace83fc..2cf02c37b17 100644
--- a/doc/src/sgml/reference.sgml
+++ b/doc/src/sgml/reference.sgml
@@ -216,6 +216,7 @@
&update;
&vacuum;
&values;
+ &waitFor;
</reference>
generated by cgit v1.2.3 (git 2.46.0) at 2025-11-07 20:58:26 +0000