Performing logical replication for Amazon RDS for PostgreSQL - Amazon Relational Database Service
Understanding logical replication and logical decodingWorking with logical replication slots

Performing logical replication for Amazon RDS for PostgreSQL

Starting with version 10.4, RDS for PostgreSQL supports the publication and subscription SQL syntax that was introduced in PostgreSQL 10. To learn more, see Logical replication in the PostgreSQL documentation.

Note

In addition to the native PostgreSQL logical replication feature introduced in PostgreSQL 10, RDS for PostgreSQL also supports the pglogical extension. For more information, see Using pglogical to synchronize data across instances.

Following, you can find information about setting up logical replication for an RDS for PostgreSQL DB instance.

Understanding logical replication and logical decoding

RDS for PostgreSQL supports the streaming of write-ahead log (WAL) changes using PostgreSQL's logical replication slots. It also supports using logical decoding. You can set up logical replication slots on your instance and stream database changes through these slots to a client such as pg_recvlogical. You create logical replication slots at the database level, and they support replication connections to a single database.

The most common clients for PostgreSQL logical replication are AWS Database Migration Service or a custom-managed host on an Amazon EC2 instance. The logical replication slot has no information about the receiver of the stream. Also, there's no requirement that the target be a replica database. If you set up a logical replication slot and don't read from the slot, data can be written and quickly fill up your DB instance's storage.

You turn on PostgreSQL logical replication and logical decoding for Amazon RDS with a parameter, a replication connection type, and a security role. The client for logical decoding can be any client that can establish a replication connection to a database on a PostgreSQL DB instance.

To turn on logical decoding for an RDS for PostgreSQL DB instance

  1. Make sure that the user account that you're using has these roles:

    • The rds_superuser role so you can turn on logical replication

    • The rds_replication role to grant permissions to manage logical slots and to stream data using logical slots

  2. Set the rds.logical_replication static parameter to 1. As part of applying this parameter, also set the parameters wal_level, max_wal_senders, max_replication_slots, and max_connections. These parameter changes can increase WAL generation, so set the rds.logical_replication parameter only when you are using logical slots.

  3. Reboot the DB instance for the static rds.logical_replication parameter to take effect.

  4. Create a logical replication slot as explained in the next section. This process requires that you specify a decoding plugin. Currently, RDS for PostgreSQL supports the test_decoding and wal2json output plugins that ship with PostgreSQL.

For more information on PostgreSQL logical decoding, see the PostgreSQL documentation.

Working with logical replication slots

You can use SQL commands to work with logical slots. For example, the following command creates a logical slot named test_slot using the default PostgreSQL output plugin test_decoding.

SELECT * FROM pg_create_logical_replication_slot('test_slot', 'test_decoding');
slot_name    | xlog_position
-----------------+---------------
regression_slot | 0/16B1970
(1 row)

To list logical slots, use the following command.

SELECT * FROM pg_replication_slots;

To drop a logical slot, use the following command.

SELECT pg_drop_replication_slot('test_slot');
pg_drop_replication_slot
-----------------------
(1 row)

For more examples on working with logical replication slots, see Logical decoding examples in the PostgreSQL documentation.

After you create the logical replication slot, you can start streaming. The following example shows how logical decoding is controlled over the streaming replication protocol. This example uses the program pg_recvlogical, which is included in the PostgreSQL distribution. Doing this requires that client authentication is set up to allow replication connections.

pg_recvlogical -d postgres --slot test_slot -U postgres
    --host -instance-name.111122223333.aws-region.rds.amazonaws.com 
    -f -  --start

To see the contents of the pg_replication_origin_status view, query the pg_show_replication_origin_status function.

SELECT * FROM pg_show_replication_origin_status();
local_id | external_id | remote_lsn | local_lsn
----------+-------------+------------+-----------
(0 rows)