PostgreSQL PGVector logical replication slots

To continuously replicate data from one database to another, you can configure logical replication using a logical replication slot.

Replication slots must always have a consumer. If there is no consumer, the volume of data in the slot will grow. You can check if replication slots have a consumer using an SQL query or the replication slot status.

If you no longer use a replication slot, delete it.

For more information on logical replication, see the Logical Replication section in the PostgreSQL documentation.

1. Create a logical replication slot

We recommend creating logical replication slots in the Control Panel or via the Managed Databases API. If you create a slot using a client connected to the database, we cannot guarantee that the slot will work properly.

The maximum number of logical replication slots is 26.

Only a user with the dbaas_replication role can create slots — this role is automatically assigned to the database owner and cannot be assigned to other users.

In the Control Panel, in the top menu, click Products and select Managed Databases.
Open the tab Active.
Open the cluster page → tab Databases.
Open the database card.
In the Replication slots section, click Add replication slot.
Enter a slot name or leave the default name.
Click Create.

2. Configure logical replication using a slot

After creating a slot, configure logical replication between the source database and the target database. The target database can be located in Selectel Managed Databases or in external storage.

Create a publication in the source database:
```
CREATE PUBLICATION <publication_name> FOR TABLE <table_name>;
```
Specify:
- <publication_name> — publication name;
- <table_name> — table name.
Optional: if required, add additional tables to the publication:
```
ALTER PUBLICATION <publication_name> ADD TABLE <extra_table_name>;
```
Specify <extra_table_name> — table name.
Create the data schema for all replicated tables in the target database and dump the schema in the source database using the utility pg_dump:
```
pg_dump \
"host=<host> \
port=<port> \
dbname=<database_name> \
user=<user_name>" \
--schema-only \
--no-privileges \
--no-subscriptions \
--no-publications \
-Fd -f <dump_directory>
```
Specify:
- <host> — IP address or DNS name of the master node of the cluster where the source database is located;
- <port> — port;
- <database_name> — source database name;
- <user_name> — username of the source database owner;
- <dump_directory> — dump directory.
Restore the schema from the dump in the target database using the utility pg_restore:
```
pg_restore \
-Fd -v \
--single-transaction -s \
--no-privileges -O \
-h <host> \
-U <user_name> \
-p <port> \
-d <database_name> \
<dump_directory>
```
Specify:
- <host> — IP address or DNS name of the master node of the cluster where the target database is located;
- <user_name> — target database username;
- <database_name> — target database name;
- <port> — port of the cluster where the target database is located;
- <dump_directory> — dump directory.
Create a subscription in the target database on behalf of a user with the role dbaas_replication:
```
CREATE SUBSCRIPTION <subscription_name> CONNECTION
   'host=<host>
   port=<port>
   dbname=<database_name>
   user=<user_name>
   password=<password>
   sslmode=verify-ca'
PUBLICATION <publication_name>
WITH (copy_data=true, create_slot=false, enabled=true, slot_name=<logical_slot_name>);
```
Specify:
- <subscription_name> — subscription name;
- <host> — IP address or DNS name of the master node of the cluster where the source database is located;
- <port> — port of the cluster where the source database is located;
- <user_name> — username of the source database;
- <password> — user password;
- <database_name> — source database name;
- <logical_slot_name> — logical replication slot name.
Existing data will appear in the target database table.

If new data is added to the source database table, it will be automatically replicated.
To stop logical replication, disable the subscription, detach the slot from it, and delete the subscription:
```
ALTER SUBSCRIPTION <subscription_name> DISABLE;
ALTER SUBSCRIPTION <subscription_name> SET (slot_name=NONE);
DROP SUBSCRIPTION <subscription_name>;
```
If you disconnect all subscriptions from a slot, information will accumulate in the slot and take up disk space. If the slot is no longer needed, delete it.

View logical replication slot status

In the Control Panel, in the top menu, click Products and select Managed Databases.
Open the tab Active.
Open the cluster page → tab Databases.
Open the database card.

In the Replication slots section, check the status in the slot row.

CREATING	The slot is being created
ACTIVE	The slot is in use — the slot has a consumer, and data is transmitted to the target database
UNUSED	The slot is not in use — the slot has no consumer, data is not transmitted to the target database, accumulates in the replication slot, and takes up additional disk space. If you will no longer use the replication slot, delete it.
DELETING	The slot is being deleted

Check consumers of logical replication slots using an SQL query

To check whether logical replication slots have consumers, run an SQL query against the view pg_replication_slots:

SELECT slot_name, pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(),restart_lsn)) AS replicationSlotLag,active
FROM pg_replication_slots;

Sample output:

slot_name        | replicationslotlag | active
-----------------+--------------------+--------
myslot1          | 129 GB             | f
myslot2          | 704 MB             | t
myslot3          | 624 MB             | t

Where:

slot_name — logical replication slot name;
replicationslotlag — size of WAL files that will not be automatically deleted during checkpoints and that logical replication slot consumers can use;
active — boolean value indicating whether the logical replication slot is in use:
- f — the slot has no consumer;
- t — the slot has a consumer.

If you have PostgreSQL version 13 or higher, you can limit the maximum size of stored WAL files using the max_slot_wal_keep_size parameter. Note that when using this parameter, the write-ahead log may be deleted before the consumer reads changes in the logical replication slot.

Delete a logical replication slot

We recommend deleting logical replication slots in the Control Panel or via the Managed Databases API. If you delete a slot using a client connected to the database, the slot may not be deleted correctly.

In the Control Panel, in the top menu, click Products and select Managed Databases.
Open the tab Active.
Open the cluster page → tab Databases.
Open the database card.
In the Replication slots section, in the slot row, click .