Logical replication behaves similarly to normal DML operations in that
the data will be updated even if it was changed locally on the subscriber
node. If incoming data violates any constraints the replication will
stop. This is referred to as a conflict. When
replicating UPDATE
or DELETE
operations, missing data will not produce a conflict and such operations
will simply be skipped.
Logical replication operations are performed with the privileges of the role
which owns the subscription. Permissions failures on target tables will
cause replication conflicts, as will enabled
row-level security on target tables
that the subscription owner is subject to, without regard to whether any
policy would ordinarily reject the INSERT
,
UPDATE
, DELETE
or
TRUNCATE
which is being replicated. This restriction on
row-level security may be lifted in a future version of
PostgreSQL.
A conflict will produce an error and will stop the replication; it must be resolved manually by the user. Details about the conflict can be found in the subscriber's server log.
The resolution can be done either by changing data or permissions on the subscriber so that it does not conflict with the incoming change or by skipping the transaction that conflicts with the existing data. When a conflict produces an error, the replication won't proceed, and the logical replication worker will emit the following kind of message to the subscriber's server log:
ERROR: duplicate key value violates unique constraint "test_pkey" DETAIL: Key (c)=(1) already exists. CONTEXT: processing remote data for replication origin "pg_16395" during "INSERT" for replication target relation "public.test" in transaction 725 finished at 0/14C0378
The LSN of the transaction that contains the change violating the constraint and
the replication origin name can be found from the server log (LSN 0/14C0378 and
replication origin pg_16395
in the above case). The
transaction that produced the conflict can be skipped by using
ALTER SUBSCRIPTION ... SKIP
with the finish LSN
(i.e., LSN 0/14C0378). The finish LSN could be an LSN at which the transaction
is committed or prepared on the publisher. Alternatively, the transaction can
also be skipped by calling the
pg_replication_origin_advance()
function.
Before using this function, the subscription needs to be disabled temporarily
either by
ALTER SUBSCRIPTION ... DISABLE
or, the
subscription can be used with the
disable_on_error
option. Then, you can use pg_replication_origin_advance()
function with the node_name
(i.e., pg_16395
)
and the next LSN of the finish LSN (i.e., 0/14C0379). The current position of
origins can be seen in the
pg_replication_origin_status
system view.
Please note that skipping the whole transaction includes skipping changes that
might not violate any constraint. This can easily make the subscriber
inconsistent.
When the
streaming
mode is parallel
, the finish LSN of failed transactions
may not be logged. In that case, it may be necessary to change the streaming
mode to on
or off
and cause the same
conflicts again so the finish LSN of the failed transaction will be written
to the server log. For the usage of finish LSN, please refer to ALTER SUBSCRIPTION ...
SKIP
.