Postgres Maintenance

Logical Replication Slots

Postgres logical replication slots are used to keep track of replication progress (recorded as a LSN).

Every time a new version of Sync Streams or Sync Rules are deployed, PowerSync creates a new replication slot, then switches over and deletes the old replication slot when the reprocessing of the new Sync Streams/Rules version is done.

The replication slots can be viewed using this query:

select slot_name, confirmed_flush_lsn, active, pg_size_pretty(pg_wal_lsn_diff(pg_current_wal_lsn(), restart_lsn)) as lag from pg_replication_slots;

Example output:

slot_name	confirmed_flush_lsn	active	lag
powersync_1_c3c8cf21	0/70D8240	1	56 bytes
powersync_2_e62d7e0f	0/70D8240	1	56 bytes

In some cases, a replication slot may remain without being used. In this case, the slot prevents Postgres from deleting older WAL entries. One such example is when a PowerSync instance has been deprovisioned.

While this is desired behavior for slot replication downtime, it could result in excessive disk usage if the slot is not used anymore.

Inactive slots can be dropped using:

select slot_name, pg_drop_replication_slot(slot_name) from pg_replication_slots where active = false;

Postgres prevents active slots from being dropped. If it does happen (e.g. while a PowerSync instance is disconnected), PowerSync would automatically re-create the slot, and restart replication.

Maximum Replication Slots

Postgres is configured with a maximum number of replication slots per server. Since each PowerSync instance uses one replication slot for replication and an additional one while deploying a new Sync Streams/Rules version, the maximum number of PowerSync instances connected to one Postgres server is equal to the maximum number of replication slots, minus 1.

If other clients are also using replication slots, this number is reduced further.

The maximum number of slots can be configured by setting max_replication_slots (not all hosting providers expose this), and checked using:

select current_setting('max_replication_slots')

If this number is exceeded, you’ll see an error such as “all replication slots are in use”.