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 LightDB 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 24.2.6). Additional temporary replication slots may be required for the initial data synchronization of pre-existing table data.
A logical replication subscription can be a standby for synchronous
replication (see Section 24.2.8). 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 lt_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 pg_subscription catalog.
The subscription is added using CREATE SUBSCRIPTION and can be stopped/resumed at any time using the ALTER SUBSCRIPTION command and removed using DROP SUBSCRIPTION.
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. 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 lt_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 should then be
dropped manually; otherwise it would continue to reserve WAL and might
eventually cause the disk to fill up. Such cases should be carefully
investigated.