Logical decoding can be used to build
synchronous
replication solutions with the same user interface as synchronous
replication for streaming
replication. To do this, the streaming replication interface
(see Section 49.3) must be used to stream out
data. Clients have to send Standby status update (F)
(see Section 55.4) messages, just like streaming
replication clients do.
A synchronous replica receiving changes via logical decoding will work in
the scope of a single database. Since, in contrast to
that, synchronous_standby_names currently is
server wide, this means this technique will not work properly if more
than one database is actively used.
In synchronous replication setup, a deadlock can happen, if the transaction has locked [user] catalog tables exclusively. See Section 49.6.2 for information on user catalog tables. This is because logical decoding of transactions can lock catalog tables to access them. To avoid this users must refrain from taking an exclusive lock on [user] catalog tables. This can happen in the following ways:
Issuing an explicit LOCK on pg_class
in a transaction.
Perform CLUSTER on pg_class in
a transaction.
PREPARE TRANSACTION after LOCK command
on pg_class and allow logical decoding of two-phase
transactions.
PREPARE TRANSACTION after CLUSTER
command on pg_trigger and allow logical decoding of
two-phase transactions. This will lead to deadlock only when published table
have a trigger.
Executing TRUNCATE on [user] catalog table in a
transaction.
Note that these commands that can cause deadlock apply to not only explicitly indicated system catalog tables above but also to any other [user] catalog table.