A subscription is the downstream side of logical replication. The node where a subscription is defined is referred to as the subscriber. A subscription defines the connection to another database and set of publications (one or more) to which it wants to subscribe.
The subscriber database behaves in the same way as any other PostgreSQL instance and can be used as a publisher for other databases by defining its own publications.
A subscriber node may have multiple subscriptions if desired. It is possible to define multiple subscriptions between a single publisher-subscriber pair, in which case care must be taken to ensure that the subscribed publication objects don't overlap.
Each subscription will receive changes via one replication slot (see Section 27.2.6). Additional replication slots may be required for the initial data synchronization of pre-existing table data and those will be dropped at the end of data synchronization.
A logical replication subscription can be a standby for synchronous replication (see Section 27.2.8). The standby name is by default the subscription name. An alternative name can be specified as
application_name in the connection information of the subscription.
Subscriptions are dumped by
pg_dump if the current user is a superuser. Otherwise a warning is written and subscriptions are skipped, because non-superusers cannot read all subscription information from the
When a subscription is dropped and recreated, the synchronization information is lost. This means that the data has to be resynchronized afterwards.
The schema definitions are not replicated, and the published tables must exist on the subscriber. Only regular tables may be the target of replication. For example, you can't replicate to a view.
The tables are matched between the publisher and the subscriber using the fully qualified table name. Replication to differently-named tables on the subscriber is not supported.
Columns of a table are also matched by name. The order of columns in the subscriber table does not need to match that of the publisher. The data types of the columns do not need to match, as long as the text representation of the data can be converted to the target type. For example, you can replicate from a column of type
integer to a column of type
bigint. The target table can also have additional columns not provided by the published table. Any such columns will be filled with the default value as specified in the definition of the target table.
As mentioned earlier, each (active) subscription receives changes from a replication slot on the remote (publishing) side.
Additional table synchronization slots are normally transient, created internally to perform initial table synchronization and dropped automatically when they are no longer needed. These table synchronization slots have generated names: “
pg_%u_sync_%u_%llu” (parameters: Subscription
relid, system identifier
Normally, the remote replication slot is created automatically when the subscription is created using
CREATE SUBSCRIPTION and it is dropped automatically when the subscription is dropped using
DROP SUBSCRIPTION. In some situations, however, it can be useful or necessary to manipulate the subscription and the underlying replication slot separately. Here are some scenarios:
When creating a subscription, the replication slot already exists. In that case, the subscription can be created using the
create_slot = false option to associate with the existing slot.
When creating a subscription, the remote host is not reachable or in an unclear state. In that case, the subscription can be created using the
connect = false option. The remote host will then not be contacted at all. This is what pg_dump uses. The remote replication slot will then have to be created manually before the subscription can be activated.
When dropping a subscription, the replication slot should be kept. This could be useful when the subscriber database is being moved to a different host and will be activated from there. In that case, disassociate the slot from the subscription using
ALTER SUBSCRIPTION before attempting to drop the subscription.
When dropping a subscription, the remote host is not reachable. In that case, disassociate the slot from the subscription using
ALTER SUBSCRIPTION before attempting to drop the subscription. If the remote database instance no longer exists, no further action is then necessary. If, however, the remote database instance is just unreachable, the replication slot (and any still remaining table synchronization slots) should then be dropped manually; otherwise it/they would continue to reserve WAL and might eventually cause the disk to fill up. Such cases should be carefully investigated.
Create some test tables on the publisher.
test_pub=# CREATE TABLE t1(a int, b text, PRIMARY KEY(a)); CREATE TABLE test_pub=# CREATE TABLE t2(c int, d text, PRIMARY KEY(c)); CREATE TABLE test_pub=# CREATE TABLE t3(e int, f text, PRIMARY KEY(e)); CREATE TABLE
Create the same tables on the subscriber.
test_sub=# CREATE TABLE t1(a int, b text, PRIMARY KEY(a)); CREATE TABLE test_sub=# CREATE TABLE t2(c int, d text, PRIMARY KEY(c)); CREATE TABLE test_sub=# CREATE TABLE t3(e int, f text, PRIMARY KEY(e)); CREATE TABLE
Insert data to the tables at the publisher side.
test_pub=# INSERT INTO t1 VALUES (1, 'one'), (2, 'two'), (3, 'three'); INSERT 0 3 test_pub=# INSERT INTO t2 VALUES (1, 'A'), (2, 'B'), (3, 'C'); INSERT 0 3 test_pub=# INSERT INTO t3 VALUES (1, 'i'), (2, 'ii'), (3, 'iii'); INSERT 0 3
Create publications for the tables. The publications
pub3a disallow some
publish operations. The publication
pub3b has a row filter (see Section 31.3).
test_pub=# CREATE PUBLICATION pub1 FOR TABLE t1; CREATE PUBLICATION test_pub=# CREATE PUBLICATION pub2 FOR TABLE t2 WITH (publish = 'truncate'); CREATE PUBLICATION test_pub=# CREATE PUBLICATION pub3a FOR TABLE t3 WITH (publish = 'truncate'); CREATE PUBLICATION test_pub=# CREATE PUBLICATION pub3b FOR TABLE t3 WHERE (e > 5); CREATE PUBLICATION
Create subscriptions for the publications. The subscription
sub3 subscribes to both
pub3b. All subscriptions will copy initial data by default.
test_sub=# CREATE SUBSCRIPTION sub1 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub1' test_sub-# PUBLICATION pub1; CREATE SUBSCRIPTION test_sub=# CREATE SUBSCRIPTION sub2 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub2' test_sub-# PUBLICATION pub2; CREATE SUBSCRIPTION test_sub=# CREATE SUBSCRIPTION sub3 test_sub-# CONNECTION 'host=localhost dbname=test_pub application_name=sub3' test_sub-# PUBLICATION pub3a, pub3b; CREATE SUBSCRIPTION
Observe that initial table data is copied, regardless of the
publish operation of the publication.
test_sub=# SELECT * FROM t1; a | b ---+------- 1 | one 2 | two 3 | three (3 rows) test_sub=# SELECT * FROM t2; c | d ---+--- 1 | A 2 | B 3 | C (3 rows)
Furthermore, because the initial data copy ignores the
publish operation, and because publication
pub3a has no row filter, it means the copied table
t3 contains all rows even when they do not match the row filter of publication
test_sub=# SELECT * FROM t3; e | f ---+----- 1 | i 2 | ii 3 | iii (3 rows)
Insert more data to the tables at the publisher side.
test_pub=# INSERT INTO t1 VALUES (4, 'four'), (5, 'five'), (6, 'six'); INSERT 0 3 test_pub=# INSERT INTO t2 VALUES (4, 'D'), (5, 'E'), (6, 'F'); INSERT 0 3 test_pub=# INSERT INTO t3 VALUES (4, 'iv'), (5, 'v'), (6, 'vi'); INSERT 0 3
Now the publisher side data looks like:
test_pub=# SELECT * FROM t1; a | b ---+------- 1 | one 2 | two 3 | three 4 | four 5 | five 6 | six (6 rows) test_pub=# SELECT * FROM t2; c | d ---+--- 1 | A 2 | B 3 | C 4 | D 5 | E 6 | F (6 rows) test_pub=# SELECT * FROM t3; e | f ---+----- 1 | i 2 | ii 3 | iii 4 | iv 5 | v 6 | vi (6 rows)
Observe that during normal replication the appropriate
publish operations are used. This means publications
pub3a will not replicate the
INSERT. Also, publication
pub3b will only replicate data that matches the row filter of
pub3b. Now the subscriber side data looks like:
test_sub=# SELECT * FROM t1; a | b ---+------- 1 | one 2 | two 3 | three 4 | four 5 | five 6 | six (6 rows) test_sub=# SELECT * FROM t2; c | d ---+--- 1 | A 2 | B 3 | C (3 rows) test_sub=# SELECT * FROM t3; e | f ---+----- 1 | i 2 | ii 3 | iii 6 | vi (4 rows)
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