diff options
Diffstat (limited to 'doc/src/sgml/logicaldecoding.sgml')
| -rw-r--r-- | doc/src/sgml/logicaldecoding.sgml | 54 |
1 files changed, 50 insertions, 4 deletions
diff --git a/doc/src/sgml/logicaldecoding.sgml b/doc/src/sgml/logicaldecoding.sgml index dd9e83b08ea..5c5957e0d37 100644 --- a/doc/src/sgml/logicaldecoding.sgml +++ b/doc/src/sgml/logicaldecoding.sgml @@ -370,10 +370,10 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU <function>pg_create_logical_replication_slot</function></link>, or by using the <link linkend="sql-createsubscription-params-with-failover"> <literal>failover</literal></link> option of - <command>CREATE SUBSCRIPTION</command> during slot creation, and then calling - <link linkend="pg-sync-replication-slots"> - <function>pg_sync_replication_slots</function></link> - on the standby. By setting <link linkend="guc-sync-replication-slots"> + <command>CREATE SUBSCRIPTION</command> during slot creation. + Additionally, enabling <link linkend="guc-sync-replication-slots"> + <varname>sync_replication_slots</varname></link> on the standby + is required. By enabling <link linkend="guc-sync-replication-slots"> <varname>sync_replication_slots</varname></link> on the standby, the failover slots can be synchronized periodically in the slotsync worker. For the synchronization to work, it is mandatory to @@ -398,6 +398,52 @@ postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NU receiving the WAL up to the latest flushed position on the primary server. </para> + <note> + <para> + While enabling <link linkend="guc-sync-replication-slots"> + <varname>sync_replication_slots</varname></link> allows for automatic + periodic synchronization of failover slots, they can also be manually + synchronized using the <link linkend="pg-sync-replication-slots"> + <function>pg_sync_replication_slots</function></link> function on the standby. + However, this function is primarily intended for testing and debugging and + should be used with caution. Unlike automatic synchronization, it does not + include cyclic retries, making it more prone to synchronization failures, + particularly during initial sync scenarios where the required WAL files + or catalog rows for the slot may have already been removed or are at risk + of being removed on the standby. In contrast, automatic synchronization + via <varname>sync_replication_slots</varname> provides continuous slot + updates, enabling seamless failover and supporting high availability. + Therefore, it is the recommended method for synchronizing slots. + </para> + </note> + + <para> + When slot synchronization is configured as recommended, + and the initial synchronization is performed either automatically or + manually via pg_sync_replication_slot, the standby can persist the + synchronized slot only if the following condition is met: The logical + replication slot on the primary must retain WALs and system catalog + rows that are still available on the standby. This ensures data + integrity and allows logical replication to continue smoothly after + promotion. + If the required WALs or catalog rows have already been purged from the + standby, the slot will not be persisted to avoid data loss. In such + cases, the following log message may appear: +<programlisting> + LOG: could not synchronize replication slot "failover_slot" + DETAIL: Synchronization could lead to data loss as the remote slot needs WAL at LSN 0/3003F28 and catalog xmin 754, but the standby has LSN 0/3003F28 and catalog xmin 756 +</programlisting> + If the logical replication slot is actively used by a consumer, no + manual intervention is needed; the slot will advance automatically, + and synchronization will resume in the next cycle. However, if no + consumer is configured, it is advisable to manually advance the slot + on the primary using <link linkend="pg-logical-slot-get-changes"> + <function>pg_logical_slot_get_changes</function></link> or + <link linkend="pg-logical-slot-get-binary-changes"> + <function>pg_logical_slot_get_binary_changes</function></link>, + allowing synchronization to proceed. + </para> + <para> The ability to resume logical replication after failover depends upon the <link linkend="view-pg-replication-slots">pg_replication_slots</link>.<structfield>synced</structfield> |