ApsaraDB RDS:Use the WAL log management feature

Background information

A replication slot is a mechanism in PostgreSQL. It retains the streams of changes that can be replayed in streaming replication. You can use replication slots to prevent data loss or duplicate replication in streaming replication and improve the efficiency and reliability of data replication.

If a large number of inactive replication slots exist and are not deleted at the earliest opportunity, WAL logs are continuously accumulated and consume a large number of storage resources. Accumulated WAL logs may exhaust disk space and cause an RDS instance to be locked. If an RDS instance is locked, the RDS instance cannot process write requests, which affects business.

ApsaraDB RDS for PostgreSQL allows you to delete inactive replication slots in the ApsaraDB RDS console or by calling API operations. This prevents WAL logs from occupying a large amount of disk space.

Usage notes

• You cannot delete active replication slots.

• If inactive replication slots are not deleted at the earliest opportunity, WAL logs are continuously accumulated and consume a large number of storage resources. If accumulated WAL logs exhaust disk space, an RDS instance becomes locked and cannot process write requests, which affects business.

Impacts

After an inactive replication slot is deleted, the following impacts occur:

• WAL logs that cannot be deleted due to the inactive replication slot on an RDS instance are automatically deleted.

• The deleted replication slot cannot be reused.

Procedure

1. Go to the Instances page. In the top navigation bar, select the region in which the RDS instance resides. Then, find the RDS instance and click the ID of the instance.

2. In the left-side navigation pane of the page that appears, click Logs.

3. On the WAL File Management tab, view information about all replication slots.

   The information about a replication slot maps the fields in the pg_replication_slots table in PostgreSQL. The following table describes the mappings.

   Parameter	Field in the pg_replication_slots table	Description
   Slot Name	slot_name	The name of the replication slot.
   Slot Type	slot_type	The type of the replication slot. Valid values: physical logical
   Plugin	plugin	By default, ApsaraDB RDS for PostgreSQL supports the following extensions: test_decoding pgoutput wal2json
   Temporary Slot	temporary	Specifies whether the replication slot is a temporary slot. A temporary slot is a special type of slot that exists only during the current session and is automatically deleted after the session ends. true: temporary slot false: permanent slot
   Database Name	database	The database in which the replication slot resides.
   Accumulated WAL Files	No field is mapped. The parameter is calculated by AliPG. Calculation method SELECT pg_size_pretty( pg_wal_lsn_diff( pg_current_wal_insert_lsn(), restart_lsn) ) AS wal_delay FROM pg_replication_slots;	The number of WAL logs that are accumulated for the replication slot. Unit: MB.
   Logical Subscription Latency	No field is mapped. The parameter is calculated by AliPG. Calculation method SELECT *, CAST(extract(epoch FROM replay_lag) AS BIGINT) AS sub_replay_lag FROM pg_stat_replication;	The latency of the logical subscription on the subscriber node that corresponds to the current replication slot. Unit: seconds.
   Whether Active	active	Specifies whether the replication slot is in use. Valid values: INACTIVE ACTIVE

4. Optional. Manage a replication slot for which INACTIVE is displayed in the Whether Active column. You must manage the inactive replication slot at the earliest opportunity because WAL logs are continuously accumulated for the replication slot.

   • If the replication slot is in use, change the state to ACTIVE.

   • If the replication slot is not in use, click Delete in the Actions column.

Related operations

• DescribeSlots: Queries information about all replication slots of an instance.

• DeleteSlot: Deletes replication slots from an instance.