Limitations and restrictions#
Superuser is required#
Currently pglogical replication and administration requires superuser privileges. It may be later extended to more granular privileges.
UNLOGGED and TEMPORARY not replicated#
UNLOGGED and TEMPORARY tables will not and cannot be replicated,
much like with physical streaming replication.
One database at a time#
To replicate multiple databases you must set up individual provider/subscriber relationships for each. There is no way to configure replication for all databases in a PostgreSQL install at once.
PRIMARY KEY or REPLICA IDENTITY required#
UPDATE s and DELETE s cannot be replicated for tables that lack
a PRIMARY KEY or other valid replica identity such as using an
index, which must be unique, not partial, not deferrable, and include
only columns marked NOT NULL. Replication has no way to find the tuple
that should be updated/deleted since there is no unique identifier.
REPLICA IDENTITY FULL is not supported yet.
Only one unique index/constraint/PK#
If more than one upstream is configured or the downstream accepts local
writes then only one UNIQUE index should be present on downstream
replicated tables. Conflict resolution can only use one index at a time
so conflicting rows may ERROR if a row satisfies the PRIMARY KEY
but violates a UNIQUE constraint on the downstream side. This will
stop replication until the downstream table is modified to remove the
violation.
It’s fine to have extra unique constraints on an upstream if the downstream only gets writes from that upstream and nowhere else. The rule is that the downstream constraints must not be more restrictive than those on the upstream(s).
Partial secondary unique indexes are permitted, but will be ignored for conflict resolution purposes.
Unique constraints must not be deferrable#
On the downstream end pglogical does not support index-based constraints
defined as DEFERRABLE . It will emit the error
ERROR: pglogical doesnt support index rechecks needed for deferrable indexes
DETAIL: relation "public"."test_relation" has deferrable indexes: "index1", "index2"
if such an index is present when it attempts to apply changes to a table.
DDL#
Automatic DDL replication is not supported. Managing DDL so that the provider and subscriber database(s) remain compatible is the responsibility of the user.
pglogical provides the pglogical.replicate_ddl_command function to
allow DDL to be run on the provider and subscriber at a consistent
point.
No replication queue flush#
There’s no support for freezing transactions on the master and waiting until all pending queued xacts are replayed from slots. Support for making the upstream read-only for this will be added in a future release.
This means that care must be taken when applying table structure changes. If there are committed transactions that aren’t yet replicated and the table structure of the provider and subscriber are changed at the same time in a way that makes the subscriber table incompatible with the queued transactions replication will stop.
Administrators should either ensure that writes to the master are
stopped before making schema changes, or use the
pglogical.replicate_ddl_command function to queue schema changes so
they’re replayed at a consistent point on the replica.
Once multi-master replication support is added then using
pglogical.replicate_ddl_command will not be enough, as the
subscriber may be generating new xacts with the old structure after the
schema change is committed on the publisher. Users will have to ensure
writes are stopped on all nodes and all slots are caught up before
making schema changes.
FOREIGN KEYS#
Foreign keys constraints are not enforced for the replication process -
what succeeds on provider side gets applied to subscriber even if the
FOREIGN KEY would be violated.
TRUNCATE#
Using TRUNCATE ... CASCADE will only apply the CASCADE option on
the provider side.
(Properly handling this would probably require the addition of
ON TRUNCATE CASCADE support for foreign keys in PostgreSQL).
TRUNCATE ... RESTART IDENTITY is not supported. The identity restart
step is not replicated to the replica.
Sequences#
The state of sequences added to replication sets is replicated
periodically and not in real-time. Dynamic buffer is used for the value
being replicated so that the subscribers actually receive future state
of the sequence. This minimizes the chance of subscriber’s notion of
sequence’s last_value falling behind but does not completely
eliminate the possibility.
It might be desirable to call synchronize_sequence to ensure all
subscribers have up to date information about given sequence after “big
events” in the database such as data loading or during the online
upgrade.
It’s generally recommended to use bigserial and bigint types for
sequences on multi-node systems as smaller sequences might reach end of
the sequence space fast.
Users who want to have independent sequences on provider and subscriber
can avoid adding sequences to replication sets and create sequences with
step interval equal to or greater than the number of nodes. And then
setting a different offset on each node. Use the INCREMENT BY option
for CREATE SEQUENCE or ALTER SEQUENCE , and use setval(...)
to set the start point.
Triggers#
Apply process and the initial COPY process both run with
session_replication_role set to replica which means that
ENABLE REPLICA and ENABLE ALWAYS triggers will be fired.
PostgreSQL Version differences#
PGLogical can replicate across PostgreSQL major versions. Despite that, long term cross-version replication is not considered a design target, though it may often work. Issues where changes are valid on the provider but not on the subscriber are more likely to arise when replicating across versions.
It is safer to replicate from an old version to a newer version since PostgreSQL maintains solid backward compatibility but only limited forward compatibility. Initial schema synchronization is only supported when replicating between same version of PostgreSQL or from lower version to higher version.
Replicating between different minor versions makes no difference at all.
Database encoding differences#
PGLogical does not support replication between databases with different
encoding. We recommend using UTF-8 encoding in all replicated
databases.
Large objects#
PostgreSQL’s logical decoding facility does not support decoding changes to large objects, so pglogical cannot replicate large objects.
Postgres-XL#
Minimum supported version of Postgres-XL is 9.5r1.5.
Postgres-XL is only supported as subscriber (cannot be a provider). For workloads with many small transactions the performance of replication may suffer due to increased write latency. On the other hand large insert (or bulkcopy) transactions are heavily optimized to work very fast with Postgres-XL.
Also any DDL limitations apply so extra care need to be taken when using
replicate_ddl_command() .
Postgres-XL changes defaults and available settings for
pglogical.conflict_resolution and pglogical.use_spi
configuration options.