PolarDB:decoderbufs (decode to ProtoBuf)

Prerequisites

Before you begin, ensure that you have:

• A PolarDB for PostgreSQL cluster running a supported version (see Applicability)

• The wal_level parameter set to logical in the cluster parameters

Set wal_level in the console. For instructions, see Set cluster parameters. Changing this parameter restarts the cluster—plan your maintenance window accordingly.

Applicability

decoderbufs is supported on the following PolarDB for PostgreSQL versions:

To check your minor engine version, run SHOW polardb_version; or view it in the console. If your version doesn't meet the requirement, upgrade the minor engine version.

The decoderbufs extension requires the following libraries:

How it works

decoderbufs is loaded automatically—no manual setup is required. When wal_level is set to logical, PostgreSQL writes additional metadata to the WAL that enables logical decoding. decoderbufs reads this metadata and emits each row change as a ProtoBuf-encoded message.

A logical replication slot holds the WAL stream for a given consumer. Two functions let you interact with the slot:

• pg_logical_slot_peek_changes — reads change events without advancing the WAL position. Use this to inspect events during debugging without consuming them.

• pg_logical_slot_get_changes — reads change events and advances the WAL position. Use this for normal consumption, where processed events should not be replayed.

Set up logical replication

1. Create a logical replication slot with decoderbufs as the output plugin.

   SELECT * FROM pg_create_logical_replication_slot('decoderbufs_demo', 'decoderbufs');

2. On the publisher instance, modify the table you want to replicate.

3. Inspect the WAL log without advancing the position (useful for debugging).

   SELECT data FROM pg_logical_slot_peek_changes('decoderbufs_demo', NULL, NULL, 'debug-mode', '1');

4. Consume the WAL changes and advance the position.

   SELECT data FROM pg_logical_slot_get_changes('decoderbufs_demo', NULL, NULL, 'debug-mode', '1');

5. Verify the current WAL position of the replication slot.

   SELECT * FROM pg_replication_slots WHERE slot_type = 'logical';

Type mapping

decoderbufs maps PostgreSQL types to the following ProtoBuf fields: