Limitations and restrictionsEdit this page
Currently pglogical replication and administration requires superuser privileges. It may be later extended to more granular privileges.
TEMPORARY tables will not and cannot be replicated, much like
with physical streaming replication.
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.
DELETEs cannot be replicated for tables that lack a
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.
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
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.
On the downstream end pglogical does not support index-based constraints
DEFERRABLE. It will emit the error
if such an index is present when it attempts to apply changes to a table.
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.
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
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 constraints are not enforced for the replication process - what
succeeds on provider side gets applied to subscriber even if the
would be violated.
TRUNCATE ... CASCADE will only apply the
CASCADE option on the
(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.
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
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
bigint types for sequences
on multi-node systems as smaller sequences might reach end of the sequence
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
Apply process and the initial COPY process both run with
session_replication_role set to
replica which means that
ENABLE ALWAYS triggers will be fired.
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.
PGLogical does not support replication between databases with different
encoding. We recommend using
UTF-8 encoding in all replicated databases.
PostgreSQL's logical decoding facility does not support decoding changes to large objects, so pglogical cannot replicate large objects.
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
Postgres-XL changes defaults and available settings for
pglogical.use_spi configuration options.
- On this page
- Superuser is required
- UNLOGGED and TEMPORARY not replicated
- One database at a time
- PRIMARY KEY or REPLICA IDENTITY required
- Only one unique index/constraint/PK
- Unique constraints must not be deferrable
- No replication queue flush
- FOREIGN KEYS
- PostgreSQL Version differences
- Database encoding differences
- Large objects