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Use PostgreSQL logical decoding to read committed table changes from the write-ahead log (WAL) through a logical replication slot, then let a consumer publish or process them. The application writes to PostgreSQL once instead of trying to commit a database change and a separate message together. Build for replay, though: after a crash, a consumer may receive recent changes again, so downstream effects must be duplicate-safe.
Choose between row changes and business events
Logical decoding extracts persistent table changes from PostgreSQL’s WAL in a form an application can consume. A logical replication slot tracks the stream for one database, and an output plugin exposes the changes. A connector such as Debezium can consume that stream and route events to downstream systems. This is change data capture (CDC): PostgreSQL remains the write path, while a separate consumer reads the resulting changes. See PostgreSQL’s logical decoding concepts.
First decide what a downstream system should learn. A row-level stream reports mutations to tables. A business event expresses a domain fact, such as “order accepted” or “payment captured.” Those are not necessarily the same: one business action may update several rows, and a row update may not represent a meaningful business event by itself.
- Use CDC on the application tables when consumers need the current row mutations—for example, to replicate data, update a search index or refresh a cache.
- Use a transactional outbox plus CDC when consumers need explicit business events. In the same database transaction as the application’s state change, write an outbox record describing the event. CDC publishes that record later, so the state change and event record commit together without a separate broker write in application code. See Debezium’s Outbox Event Router.
The outbox does not remove the need to operate CDC; it changes what the application records and what the stream means. The application must write the outbox row atomically with its business state and maintain the outbox schema and retention.
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How logical decoding avoids the dual-write gap
In a dual-write design, application code commits a database transaction and separately sends a message. If one operation succeeds and the other fails, the database and the downstream system disagree. With CDC, the database transaction is the application’s write. PostgreSQL records persistent changes in WAL, and a consumer reads them through a logical replication slot. The consumer can then publish or apply the changes elsewhere.
This avoids having application code coordinate two independent writes, but it does not make delivery exactly once. A slot persists independently of a connection, and PostgreSQL may replay recent changes after a crash. The consumer must therefore handle duplicates safely and keep its progress tracking consistent with its side effects. PostgreSQL explicitly assigns logical decoding clients responsibility for avoiding harmful effects from processing a message more than once in its logical decoding documentation.
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Configure the source and connector
Logical decoding requires compatible PostgreSQL configuration and a consumer that can use the selected output plugin. Debezium’s PostgreSQL connector reference describes requirements including wal_level=logical, replication-slot capacity and a suitable replication user. PostgreSQL’s built-in pgoutput plugin is an option where supported. Hosted database services may impose provider-specific settings or restrictions, so check the requirements for the deployed PostgreSQL version and service rather than assuming every host exposes the same capabilities.
- Confirm support and configuration. Check the PostgreSQL version and host documentation for logical decoding, the chosen plugin and slot requirements. Plan the required WAL and slot capacity before enabling a connector.
- Set up the replication user and permissions. Grant only the replication and publication privileges the connector needs. Avoid using a broadly privileged application account as a shortcut.
- Define the publication. For a
pgoutputconnector, a publication defines which tables’ changes are streamed. Include the tables the consumer actually needs. - Allocate a slot for each independent consumer. A logical slot tracks one stream for one database. Multiple independent consumers normally need separate slots, so one consumer’s progress does not stand in for another’s.
- Validate the stream before relying on it. Confirm that the selected tables and operations reach the consumer, and verify how the connector represents the row values your application needs.
Check what row values the stream contains
Output plugins can expose new row versions for INSERT and UPDATE. Whether an UPDATE or DELETE includes old row values depends on the table’s REPLICA IDENTITY setting. Do not assume a delete event contains the full deleted row, or that an update includes its before-image. Verify the consumer’s needs against the table configuration and the behavior documented for the PostgreSQL version in use. PostgreSQL’s logical decoding documentation describes the role of replica identity.
If consumers require old values, evaluate the appropriate replica identity for the workload before depending on them. Changing replica identity can affect WAL volume and operations; it is not a substitute for checking the actual event requirements. Also distinguish row CDC from schema-change handling: streaming row mutations does not, by itself, make DDL changes equivalent to row events.
Start with a consistent snapshot, not a dump followed by a stream
A consumer often needs both the rows that already exist and the changes that happen while it is starting. An ordinary table dump followed by starting a connector can leave a gap: a row may change between the dump and the start of streaming.
Coordinate the initial snapshot with the live stream. PostgreSQL can export a snapshot when a logical slot is created; that snapshot represents the database state corresponding to the point after which the slot’s streamed changes are included. Debezium’s initial workflow snapshots existing rows before continuing from the matching stream position. Use the connector’s documented snapshot workflow rather than stitching together an unrelated dump and WAL position. See PostgreSQL’s snapshot and slot explanation and the Debezium connector reference.
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Treat the stream as at-least-once from the consumer’s perspective. If the consumer processes a change but crashes before its progress is safely recorded, the change may be delivered again. A robust design makes handling the same change twice harmless.
- Use stable event identity or source coordinates to recognize a replay.
- Make downstream writes idempotent where possible, so applying the same event again does not create an additional harmful effect.
- Record consumer progress consistently with the side effects. Avoid acknowledging progress before the corresponding work is durable.
- For an outbox, give events stable identifiers and make downstream handling duplicate-safe just as you would for direct row CDC.
These practices reduce duplicate effects; they do not turn the database-to-consumer path into an exactly-once guarantee.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Operate slots as ongoing resource reservations
A logical replication slot retains WAL and catalog resources PostgreSQL still needs for the consumer. If a consumer stalls or disappears while its slot remains, retained resources can grow and create storage or maintenance risk. Monitor slot activity, restart lag, retained WAL and catalog health. Remove a slot only when its consumer is intentionally retired; an unused slot should not be left behind indefinitely. PostgreSQL details slot behavior and replay considerations in its logical decoding concepts.
For high availability, check slot failover behavior against the exact PostgreSQL version and provider. Debezium documents failover-enabled slots for PostgreSQL 17 and later when configured; earlier versions and replica-slot workflows have different synchronization requirements. Do not assume that a slot will be available or synchronized after failover without validating the deployed setup in the connector documentation.
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Compare CDC, an outbox and LISTEN/NOTIFY
| Approach | Best fit | What it provides | Key boundaries |
|---|---|---|---|
| Logical decoding / CDC | Changes to existing tables for replication, indexing, caches, analytics or other consumers | Streams inserts, updates and deletes without an application-side second write; can pair an initial snapshot with the live stream | Requires connector and slot operations; consumers must handle duplicates; stalled slots retain WAL and catalog resources; old-row visibility depends on replica identity; row CDC is not a complete DDL event stream. |
| Transactional outbox plus CDC | Explicit business events that must commit atomically with application state | Application controls event meaning and shape; the state change and outbox record commit in one database transaction | Application must write and maintain outbox records; CDC infrastructure and duplicate-safe consumers are still required. |
| LISTEN/NOTIFY | A lightweight signal to wake a process or indicate that attention is needed | PostgreSQL-native interprocess notification; a notification issued in a transaction is sent only after commit, and a trigger can issue it automatically | Not a durable row-change log; the default payload limit is under 8,000 bytes; identical notifications in one transaction can be folded. Store durable or larger data in a table and use the notification as a signal. |
PostgreSQL describes NOTIFY as an interprocess communication mechanism. It can tell a process to check for work, but the notification itself should not be treated as the record of that work. For a durable stream, use a table-backed design such as CDC or an outbox.
Choose by event meaning, whether deletes and old values matter, how the initial state will be loaded, duplicate handling, operational capacity and whether the database provider supports the required logical decoding configuration.
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