Replication origins are intended to make it easier to implement logical replication solutions on top of logical decoding. They provide a solution to two common problems:
How to safely keep track of replication progress
How to change replication behavior based on the origin of a row; for example, to prevent loops in bi-directional replication setups
Replication origins have just two properties, a name and an OID. The name, which is what should be used to refer to the origin across systems, is free-form text. It should be used in a way that makes conflicts between replication origins created by different replication solutions unlikely; e.g. by prefixing the replication solution's name to it. The OID is used only to avoid having to store the long version in situations where space efficiency is important. It should never be shared across systems.
One nontrivial part of building a replication solution is to keep track of replay progress in a safe manner. When the applying process, or the whole cluster, dies, it needs to be possible to find out up to where data has successfully been replicated. Naive solutions to this, such as updating a row in a table for every replayed transaction, have problems like run-time overhead and database bloat.
Using the replication origin infrastructure a session can be
marked as replaying from a remote node (using the
Additionally the LSN and commit
time stamp of every source transaction can be configured on a per
transaction basis using
pg_replication_origin_xact_setup(). If that's done
replication progress will persist in a crash safe manner. Replay
progress for all replication origins can be seen in the pg_replication_origin_status view. An
individual origin's progress, e.g. when resuming replication, can
be acquired using
pg_replication_origin_progress() for any
pg_replication_origin_session_progress() for the origin
configured in the current session.
In replication topologies more complex than replication from
exactly one system to one other system, another problem can be that
it is hard to avoid replicating replayed rows again. That can lead
both to cycles in the replication and inefficiencies. Replication
origins provide an optional mechanism to recognize and prevent
that. When configured using the functions referenced in the
previous paragraph, every change and transaction passed to output
plugin callbacks (see Section 47.6) generated by
the session is tagged with the replication origin of the generating
session. This allows treating them differently in the output
plugin, e.g. ignoring all but locally-originating rows.
filter_by_origin_cb callback can
be used to filter the logical decoding change stream based on the
source. While less flexible, filtering via that callback is
considerably more efficient than doing it in the output plugin.
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