Using pglogical to synchronize data across instances
All currently available RDS for PostgreSQL versions
support the pglogical
extension. The pglogical extension predates the functionally similar logical replication feature that
was introduced by PostgreSQL in version 10.
For more information, see Performing
logical replication for Amazon RDS for PostgreSQL.
The pglogical
extension supports logical replication between two or more
RDS for PostgreSQL DB instances. It also supports replication between different PostgreSQL
versions, and between databases running on RDS for PostgreSQL DB instances and Aurora PostgreSQL DB clusters. The pglogical
extension uses a publish-subscribe model to replicate changes to tables and other objects, such as sequences, from a
publisher to a subscriber. It relies on a replication slot to ensure that changes are synchronized from a publisher node
to a subscriber node, defined as follows.
The publisher node is the RDS for PostgreSQL DB instance that's the source of data to be replicated to other nodes. The publisher node defines the tables to be replicated in a publication set.
The subscriber node is the RDS for PostgreSQL DB instance that receives WAL updates from the publisher. The subscriber creates a subscription to connect to the publisher and get the decoded WAL data. When the subscriber creates the subscription, the replication slot is created on the publisher node.
Following, you can find information about setting up the pglogical
extension.
Topics
- Requirements and limitations for the pglogical extension
- Setting up the pglogical extension
- Setting up logical replication for RDS for PostgreSQL DB instance
- Reestablishing logical replication after a major upgrade
- Managing logical replication slots for RDS for PostgreSQL
- Parameter reference for the pglogical extension
Requirements and limitations for the pglogical extension
All currently available releases of RDS for PostgreSQL
support the pglogical
extension.
Both the publisher node and the subscriber node must be set up for logical replication.
The tables that you want to replicate from subscriber to publisher must have the same names and the same schema. These tables must also contain the same columns, and the columns must use the same data types. Both publisher and subscriber tables must have the same primary keys. We recommend that you use only the PRIMARY KEY as the unique constraint.
The tables on the subscriber node can have more permissive constraints than those on the publisher node for CHECK constraints and NOT NULL constraints.
The pglogical
extension provides features such as two-way replication that aren't
supported by the logical replication feature built into PostgreSQL (version 10 and higher).
For more information, see PostgreSQL
bi-directional replication using pglogical
Setting up the pglogical extension
To set up the pglogical
extension on your RDS for PostgreSQL DB instance
, you add pglogical
to the shared libraries
on the custom DB parameter group for your RDS for PostgreSQL DB instance.
You also
need to set the value of the rds.logical_replication
parameter to 1
, to turn on logical decoding. Finally, you
create the extension in the database. You can use the Amazon Web Services Management Console or the Amazon CLI for these tasks.
You must have permissions as the rds_superuser
role to perform these tasks.
The steps following assume that your RDS for PostgreSQL DB instance is associated with a custom DB parameter group. For information about creating a custom DB parameter group, see Working with parameter groups.
To set up the pglogical extension
Sign in to the Amazon Web Services Management Console and open the Amazon RDS console at https://console.amazonaws.cn/rds/
. -
In the navigation pane, choose your RDS for PostgreSQL DB instance.
-
Open the Configuration tab for your RDS for PostgreSQL DB instance. Among the Instance details, find the Parameter group link.
-
Choose the link to open the custom parameters associated with your RDS for PostgreSQL DB instance.
-
In the Parameters search field, type
shared_pre
to find theshared_preload_libraries
parameter. -
Choose Edit parameters to access the property values.
-
Add
pglogical
to the list in the Values field. Use a comma to separate items in the list of values. Find the
rds.logical_replication
parameter and set it to1
, to turn on logical replication.Reboot the RDS for PostgreSQL DB instance so that your changes take effect.
When the instance is available, you can use
psql
(or pgAdmin) to connect to the RDS for PostgreSQL DB instance.psql --host=
111122223333
.aws-region
.rds.amazonaws.com --port=5432 --username=postgres
--password --dbname=labdb
To verify that pglogical is initialized, run the following command.
SHOW shared_preload_libraries;
shared_preload_libraries -------------------------- rdsutils,pglogical (1 row)
Verify the setting that enables logical decoding, as follows.
SHOW wal_level;
wal_level ----------- logical (1 row)
Create the extension, as follows.
CREATE EXTENSION pglogical;
EXTENSION CREATED
Choose Save changes.
Open the Amazon RDS console at https://console.amazonaws.cn/rds/
. Choose your RDS for PostgreSQL DB instance from the Databases list to select it, and then choose Reboot from the Actions menu.
To setup the pglogical extension
To setup pglogical using the Amazon CLI, you call the modify-db-parameter-group operation to modify certain parameters in your custom parameter group as shown in the following procedure.
Use the following Amazon CLI command to add
pglogical
to theshared_preload_libraries
parameter.aws rds modify-db-parameter-group \ --db-parameter-group-name
custom-param-group-name
\ --parameters "ParameterName=shared_preload_libraries,ParameterValue=pglogical,ApplyMethod=pending-reboot" \ --regionaws-region
Use the following Amazon CLI command to set
rds.logical_replication
to1
to turn on the logical decoding capability for the RDS for PostgreSQL DB instance.aws rds modify-db-parameter-group \ --db-parameter-group-name
custom-param-group-name
\ --parameters "ParameterName=rds.logical_replication,ParameterValue=1,ApplyMethod=pending-reboot" \ --regionaws-region
-
Use the following Amazon CLI command to reboot the RDS for PostgreSQL DB instance so that the pglogical library is initialized.
aws rds reboot-db-instance \ --db-instance-identifier
your-instance
\ --regionaws-region
When the instance is available, use
psql
to connect to the RDS for PostgreSQL DB instance.psql --host=
111122223333
.aws-region
.rds.amazonaws.com --port=5432 --username=postgres
--password --dbname=labdb
Create the extension, as follows.
CREATE EXTENSION pglogical;
EXTENSION CREATED
Reboot the RDS for PostgreSQL DB instance using the following Amazon CLI command.
aws rds reboot-db-instance \ --db-instance-identifier
your-instance
\ --regionaws-region
Setting up logical replication for RDS for PostgreSQL DB instance
The following procedure shows you how to start logical replication between two RDS for PostgreSQL
DB instances. The steps assume that both the source (publisher) and the target (subscriber)
have the pglogical
extension set up as detailed
in Setting up the pglogical extension.
To create the publisher node and define the tables to replicate
These steps assume that your RDS for PostgreSQL DB instance has a database
that has one or more tables that you want to replicate to another node. You need to recreate the table
structure from the publisher on the subscriber, so first, get the table structure if necessary. You can do that
by using the psq1
metacommand \d
and then creating
the same table on the subscriber instance. The following procedure creates an example table
on the publisher (source) for demonstration purposes.tablename
Use
psql
to connect to the instance that has the table you want to use as a source for subscribers.psql --host=
source-instance
.aws-region
.rds.amazonaws.com --port=5432 --username=postgres
--password --dbname=labdb
If you don't have an existing table that you want to replicate, you can create a sample table as follows.
Create an example table using the following SQL statement.
CREATE TABLE docs_lab_table (a int PRIMARY KEY);
Populate the table with generated data by using the following SQL statement.
INSERT INTO docs_lab_table VALUES (generate_series(1,5000));
INSERT 0 5000
Verify that data exists in the table by using the following SQL statement.
SELECT count(*) FROM docs_lab_table;
Identify this RDS for PostgreSQL DB instance as the publisher node, as follows.
SELECT pglogical.create_node( node_name := '
docs_lab_provider
', dsn := 'host=source-instance
.aws-region
.rds.amazonaws.com port=5432 dbname=labdb
');create_node ------------- 3410995529 (1 row)
Add the table that you want to replicate to the default replication set. For more information about replication sets, see Replication sets
in the pglogical documentation. SELECT pglogical.replication_set_add_table('default', '
docs_lab_table
', 'true', NULL, NULL);replication_set_add_table --------------------------- t (1 row)
The publisher node setup is complete. You can now set up the subscriber node to receive the updates from the publisher.
To set up the subscriber node and create a subscription to receive updates
These steps assume that the
RDS for PostgreSQL DB instance has been set up with the
pglogical
extension. For more information, see Setting up the pglogical extension.
Use
psql
to connect to the instance that you want to receive updates from the publisher.psql --host=
target-instance
.aws-region
.rds.amazonaws.com --port=5432 --username=postgres
--password --dbname=labdb
-
On the subscriber RDS for PostgreSQL DB instance,create the same table that exists on the publisher. For this example, the table is
docs_lab_table
. You can create the table as follows.CREATE TABLE docs_lab_table (a int PRIMARY KEY);
Verify that this table is empty.
SELECT count(*) FROM docs_lab_table;
count ------- 0 (1 row)
Identify this RDS for PostgreSQL DB instance as the subscriber node, as follows.
SELECT pglogical.create_node( node_name := '
docs_lab_target
', dsn := 'host=target-instance
.aws-region
.rds.amazonaws.com port=5432 sslmode=require dbname=labdb
user=postgres
password=********
');create_node ------------- 2182738256 (1 row)
Create the subscription.
SELECT pglogical.create_subscription( subscription_name := 'docs_lab_subscription', provider_dsn := 'host=
source-instance
.aws-region
.rds.amazonaws.com port=5432 sslmode=require dbname=labdb
user=postgres
password=*******
', replication_sets := ARRAY['default'], synchronize_data := true, forward_origins := '{}' );create_subscription --------------------- 1038357190 (1 row)
When you complete this step, the data from the table on the publisher is created in the table on the subscriber. You can verify that this has occurred by using the following SQL query.
SELECT count(*) FROM docs_lab_table;
count ------- 5000 (1 row)
From this point forward, changes made to the table on the publisher are replicated to the table on the subscriber.
Reestablishing logical replication after a major upgrade
Before you can perform a major version upgrade of an RDS for PostgreSQL DB instance that's set up as a publisher node for logical replication, you must drop all replication slots, even those that aren't active. We recommend that you temporarily divert database transactions from the publisher node, drop the replication slots, upgrade the RDS for PostgreSQL DB instance, and then re-establish and restart replication.
The replication slots are hosted on the publisher node only. The RDS for PostgreSQL subscriber node in a logical replication scenario has no slots to drops, but it can't be upgraded to a major version while it's designated as a subscriber node with a subscription to the publisher. Before upgrading the RDS for PostgreSQL subscriber node, drop the subscription and the node. For more information, see Managing logical replication slots for RDS for PostgreSQL.
Determining that logical replication has been disrupted
You can determine that the replication process has been disrupted by querying either the publisher node or the subscriber node, as follows.
To check the publisher node
Use
psql
to connect to the publisher node, and then query thepg_replication_slots
function. Note the value in the active column. Normally, this will returnt
(true), showing that replication is active. If the query returnsf
(false), it's an indication that replication to the subscriber has stopped.SELECT slot_name,plugin,slot_type,active FROM pg_replication_slots;
slot_name | plugin | slot_type | active -------------------------------------------+------------------+-----------+-------- pgl_labdb_docs_labcb4fa94_docs_lab3de412c | pglogical_output | logical | f (1 row)
To check the subscriber node
On the subscriber node, you can check the status of replication in three different ways.
Look through the PostgreSQL logs on the subscriber node to find failure messages. The log identifies failure with messages that include exit code 1, as shown following.
2022-07-06 16:17:03 UTC::@:[7361]:LOG: background worker "pglogical apply 16404:2880255011" (PID 14610) exited with exit code 1 2022-07-06 16:19:44 UTC::@:[7361]:LOG: background worker "pglogical apply 16404:2880255011" (PID 21783) exited with exit code 1
Query the
pg_replication_origin
function. Connect to the database on the subscriber node usingpsql
and query thepg_replication_origin
function, as follows.SELECT * FROM pg_replication_origin;
roident | roname ---------+-------- (0 rows)
The empty result set means that replication has been disrupted. Normally, you see output such as the following.
roident | roname ---------+---------------------------------------------------- 1 | pgl_labdb_docs_labcb4fa94_docs_lab3de412c (1 row)
Query the
pglogical.show_subscription_status
function as shown in the following example.SELECT subscription_name,status,slot_name FROM pglogical.show_subscription_status();
subscription_name | status | slot_name ---====----------------+--------+------------------------------------- docs_lab_subscription | down | pgl_labdb_docs_labcb4fa94_docs_lab3de412c (1 row)
This output shows that replication has been disrupted. Its status is
down
. Normally, the output shows the status asreplicating
.
If your logical replication process has been disrupted, you can re-establish replication by following these steps.
To reestablish logical replication between publisher and subscriber nodes
To re-establish replication, you first disconnect the subscriber from the publisher node and then re-establish the subscription, as outlined in these steps.
Connect to the subscriber node using
psql
as follows.psql --host=
222222222222
.aws-region
.rds.amazonaws.com --port=5432 --username=postgres
--password --dbname=labdb
Deactivate the subscription by using the
pglogical.alter_subscription_disable
function.SELECT pglogical.alter_subscription_disable('docs_lab_subscription',true);
alter_subscription_disable ---------------------------- t (1 row)
Get the publisher node's identifier by querying the
pg_replication_origin
, as follows.SELECT * FROM pg_replication_origin;
roident | roname ---------+------------------------------------- 1 | pgl_labdb_docs_labcb4fa94_docs_lab3de412c (1 row)
Use the response from the previous step with the
pg_replication_origin_create
command to assign the identifier that can be used by the subscription when re-established.SELECT pg_replication_origin_create('pgl_labdb_docs_labcb4fa94_docs_lab3de412c');
pg_replication_origin_create ------------------------------ 1 (1 row)
Turn on the subscription by passing its name with a status of
true
, as shown in the following example.SELECT pglogical.alter_subscription_enable('docs_lab_subscription',true);
alter_subscription_enable --------------------------- t (1 row)
Check the status of the node. Its status should be replicating
as shown in this example.
SELECT subscription_name,status,slot_name FROM pglogical.show_subscription_status();
subscription_name | status | slot_name -------------------------------+-------------+------------------------------------- docs_lab_subscription | replicating | pgl_labdb_docs_lab98f517b_docs_lab3de412c (1 row)
Check the status of the subscriber's replication slot on the publisher node. The
slot's active
column should return t
(true), indicating
that replication has been re-established.
SELECT slot_name,plugin,slot_type,active FROM pg_replication_slots;
slot_name | plugin | slot_type | active -------------------------------------------+------------------+-----------+-------- pgl_labdb_docs_lab98f517b_docs_lab3de412c | pglogical_output | logical | t (1 row)
Managing logical replication slots for RDS for PostgreSQL
Before you can perform a major version upgrade on an RDS for PostgreSQL DB instance that's serving as a publisher node in a logical replication scenario, you must drop the replication slots on the instance. The major version upgrade pre-check process notifies you that the upgrade can't proceed until the slots are dropped.
To drop slots from your RDS for PostgreSQL DB instance, first drop the subscription and then drop the slot.
To identify replication slots that were created using the pglogical
extension,
log in to each database and get the name of the nodes. When you query the subscriber node,
you get both the publisher and the subscriber nodes in the output, as shown in this example.
SELECT * FROM pglogical.node;
node_id | node_name ------------+------------------- 2182738256 | docs_lab_target 3410995529 | docs_lab_provider (2 rows)
You can get the details about the subscription with the following query.
SELECT sub_name,sub_slot_name,sub_target FROM pglogical.subscription;
sub_name | sub_slot_name | sub_target ----------+--------------------------------+------------ docs_lab_subscription | pgl_labdb_docs_labcb4fa94_docs_lab3de412c | 2182738256 (1 row)
You can now drop the subscription, as follows.
SELECT pglogical.drop_subscription(subscription_name := 'docs_lab_subscription');
drop_subscription ------------------- 1 (1 row)
After dropping the subscription, you can delete the node.
SELECT pglogical.drop_node(node_name := 'docs-lab-subscriber');
drop_node ----------- t (1 row)
You can verify that the node no longer exists, as follows.
SELECT * FROM pglogical.node;
node_id | node_name ---------+----------- (0 rows)
Parameter reference for the pglogical extension
In the table you can find parameters associated with the pglogical
extension. Parameters such as
pglogical.conflict_log_level
and pglogical.conflict_resolution
are used to handle update conflicts.
Conflicts can emerge when changes are made locally to the same tables that are subscribed to changes from the publisher.
Conflicts can also occur during various scenarios, such
as two-way replication or when multiple subscribers
are replicating from the same publisher. For more information, see
PostgreSQL bi-directional replication
using pglogical
Parameter | Description |
---|---|
pglogical.batch_inserts | Batch inserts if possible. Not set by default. Change to '1' to turn on, '0' to turn off. |
pglogical.conflict_log_level | Sets the log level to use for logging resolved conflicts. Supported string values are debug5, debug4, debug3, debug2, debug1, info, notice, warning, error, log, fatal, panic. |
pglogical.conflict_resolution | Sets method to use to resolve conflicts when conflicts are resolvable. Supported string values are error, apply_remote, keep_local, last_update_wins, first_update_wins. |
pglogical.extra_connection_options | Connection options to add to all peer node connections. |
pglogical.synchronous_commit | pglogical specific synchronous commit value |
pglogical.use_spi | Use SPI (server programming interface) instead of low-level API to apply changes. Set to '1' to turn on, '0' to turn off. For more information about
SPI, see Server Programming Interface |