Limitations and restrictions

Superuser privileges are required

Currently, the Spock extension requires superuser privileges to configure replication and administration.

UNLOGGED and TEMPORARY tables are 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

UPDATEs and DELETEs 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, 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.

You can have additional unique constraints upstream if the downstream consumer 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.

spock.check_all_uc_indexes is an experimental GUC that adds INSERT conflict resolution by allowing Spock to consider all unique constraints, not just the primary key or replica identity. For more information, see Configuring Spock.

Unique constraints must not be deferrable

On the downstream end spock does not support index-based constraints defined as DEFERRABLE. It will emit the error:

ERROR: spock doesn't 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.

No replication queue flush

There is no support for freezing transactions on the master and waiting until all pending queued xacts are replayed from slots.

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 spock.replicate_ddl 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 spock.replicate_ddl 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.

TRUNCATE ... RESTART IDENTITY is not supported. The identity restart step is not replicated to the replica.

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

Spock 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

Spock does not support replication between databases with different encoding. We recommend using UTF-8 encoding in all replicated databases.