Use PostgreSQL logical decoding to read committed row changes from the write-ahead log (WAL) through a logical replication slot, then have a consumer such as Debezium deliver them to downstream systems. Your application no longer has to commit to PostgreSQL and separately publish the same change. The consumer still needs duplicate-safe processing and careful progress tracking.
Choose the event you actually need
Logical decoding is a fit when downstream systems need to react to database mutations: inserts, updates, and deletes in selected tables. PostgreSQL extracts persistent table changes from the WAL and presents them in a format a client can interpret. An output plugin shapes that stream; a logical replication slot tracks the stream’s position. PostgreSQL’s logical decoding documentation explains the mechanism.
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Not every row mutation is a useful business event. An update may reflect an internal state transition, maintenance, or a change that should not be interpreted as a business fact. If a consumer needs facts such as “order accepted” or “payment captured,” write an explicit event to an outbox table in the same transaction as the application state change, then capture that table with CDC. This keeps the event and state atomic without making a separate broker write from application code. Debezium’s Outbox Event Router documentation describes this pattern.
How the change moves from Postgres to a consumer
- PostgreSQL records the transaction. The database writes changes to its WAL as part of normal transaction processing.
- Logical decoding interprets the relevant changes. An output plugin makes changes available in a client-usable form.
- A logical slot preserves the stream position. Each slot tracks one change stream for one database, independently of whether a consumer is currently connected.
- A connector delivers events downstream. A CDC consumer such as Debezium reads the stream and forwards events to systems such as a message broker or search index.
With the pgoutput plugin, PostgreSQL publications define which tables are included. Use a separate slot for each independent consumer or connector; multiple consumers should not compete for one slot as though it were a shared subscription. Debezium’s PostgreSQL connector reference covers connector configuration and publication use.
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Configure decoding, access, and row identity
Enable logical decoding for the deployed service
Debezium’s PostgreSQL connector reference identifies wal_level=logical, sufficient replication-slot capacity, and a replication-capable user among the requirements. PostgreSQL includes pgoutput; managed database services may restrict plugins or require provider-specific configuration. Confirm the supported settings for the exact PostgreSQL version and hosting service before deployment. Grant only the replication and publication privileges the connector needs.
Decide whether consumers need old row values
Output plugins can expose the new row for an insert and the new version of a row after an update. Whether an update or delete also includes the old row values depends on the table’s REPLICA IDENTITY setting. Check the connector and consumer requirements before designing around before-images; changing replica identity can affect WAL volume and operations, so evaluate it for the workload rather than applying it by default. See PostgreSQL’s logical decoding documentation for the version-specific details.
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Bootstrap without leaving a gap
A one-time table dump followed by starting a live connector is not automatically complete: a change can occur between the snapshot and the stream’s starting point. The initial table state and ongoing WAL stream must correspond to the same database position.
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Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →PostgreSQL can export a snapshot when creating a logical replication slot. A client can use that snapshot for the initial load while the slot provides the matching point from which to continue streaming. Debezium’s initial snapshot workflow reads existing rows before continuing with changes from the corresponding position. Use the connector’s supported snapshot workflow or another method that explicitly coordinates the snapshot and WAL position; do not assume two independent operations line up. PostgreSQL’s logical decoding concepts describe exported snapshots and slots.
Make replay harmless
Plan for at-least-once delivery from the consumer’s perspective, not a guarantee that each event will be acted on exactly once. PostgreSQL documents that after a crash a slot may move back to an earlier log sequence number (LSN), causing recent changes to be emitted again. Its logical decoding guidance assigns clients responsibility for avoiding harmful effects from processing a message more than once.
- Use a stable event identity or source coordinates to recognize a replay.
- Make downstream writes idempotent where possible, so applying the same change again has no harmful effect.
- Persist consumer progress consistently with the side effects it represents; otherwise a restart can leave progress and downstream state out of sync.
These safeguards address duplicate delivery. They do not replace a correct initial snapshot or guarantee that every consumer interprets a row mutation as the right business event.
Manage slots as retained database resources
A logical slot persists independently of its connection and retains WAL, as well as catalog resources, needed by its consumer. If the consumer stops advancing, retained data can grow and create storage and maintenance risk. Monitor slot activity and lag, retained WAL, and catalog health. Remove a slot only when its consumer has been intentionally retired; an abandoned slot can continue retaining resources.
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For high availability, verify failover-slot behavior for the exact PostgreSQL version and managed-service implementation. Debezium documents failover-enabled logical slots for PostgreSQL 17 and later when configured; earlier versions and replica-slot workflows have different synchronization requirements. Check the connector reference against your deployment’s version and failover setup.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choose between CDC, an outbox, and notifications
| Approach | Best fit | Benefits | Boundaries and costs |
|---|---|---|---|
| Logical decoding / CDC | Replicating changes from existing tables to search, caches, analytics, or other consumers | Captures inserts, updates, and deletes without a separate application-side publish; supports a coordinated initial snapshot | Requires connector and slot operations, duplicate-safe consumers, and WAL-retention monitoring. Old-row visibility depends on replica identity. Row CDC is not a complete schema-change stream. |
| Transactional outbox plus CDC | Publishing explicit application-defined business events | Application state and event record commit together; the application controls event meaning and shape | The application must write the outbox row in the same transaction and manage its schema and retention. CDC infrastructure is still needed. |
LISTEN/NOTIFY |
Waking a process or signaling that it should check for work | Simple PostgreSQL-native interprocess signal; notifications issued in a transaction are sent only after commit, and a trigger can issue one automatically | It is not a durable row-change log. In the default configuration, payloads must be under 8,000 bytes, and identical notifications within one transaction can be folded together. Keep durable or large data in a table and treat the notification as a signal. |
PostgreSQL describes NOTIFY as a lightweight interprocess communication mechanism. Choose it when a wake-up signal is enough, not when a consumer must be able to recover a complete history of row changes.
The practical decision turns on event meaning, whether deletes and old row values matter, how the initial state will be aligned with live changes, duplicate handling, operational capacity for slots, and whether the database provider supports logical decoding.
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