# Logical Decoding

PostgreSQL provides infrastructure to stream the modifications performed via SQL to external consumers. This functionality can be used for a variety of purposes, including replication solutions and auditing.

Changes are sent out in streams identified by logical replication slots.

The format in which those changes are streamed is determined by the output plugin used. An example plugin is provided in the PostgreSQL distribution. Additional plugins can be written to extend the choice of available formats without modifying any core code. Every output plugin has access to each individual new row produced by `INSERT` and the new row version created by `UPDATE`. Availability of old row versions for `UPDATE` and `DELETE` depends on the configured replica identity (see sql-altertable-replica-identity).

Changes can be consumed either using the streaming replication protocol (see protocol-replication and logicaldecoding-walsender), or by calling functions via SQL (see logicaldecoding-sql). It is also possible to write additional methods of consuming the output of a replication slot without modifying core code (see logicaldecoding-writer).

## Logical Decoding Examples

The following example demonstrates controlling logical decoding using the SQL interface.

Before you can use logical decoding, you must set guc-wal-level to `logical` and guc-max-replication-slots to at least 1. Then, you should connect to the target database (in the example below, `postgres`) as a superuser.

    postgres=# -- Create a slot named 'regression_slot' using the output plugin 'test_decoding'
    postgres=# SELECT * FROM pg_create_logical_replication_slot('regression_slot', 'test_decoding', false, true);
        slot_name    |    lsn
    -----------------+-----------
     regression_slot | 0/16B1970
    (1 row)

    postgres=# SELECT slot_name, plugin, slot_type, database, active, restart_lsn, confirmed_flush_lsn FROM pg_replication_slots;
        slot_name    |    plugin     | slot_type | database | active | restart_lsn | confirmed_flush_lsn
    -----------------+---------------+-----------+----------+--------+-------------+-----------------
     regression_slot | test_decoding | logical   | postgres | f      | 0/16A4408   | 0/16A4440
    (1 row)

    postgres=# -- There are no changes to see yet
    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
     lsn | xid | data
    -----+-----+------
    (0 rows)

    postgres=# CREATE TABLE data(id serial primary key, data text);
    CREATE TABLE

    postgres=# -- DDL isn't replicated, so all you'll see is the transaction
    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    |  xid  |     data
    -----------+-------+--------------
     0/BA2DA58 | 10297 | BEGIN 10297
     0/BA5A5A0 | 10297 | COMMIT 10297
    (2 rows)

    postgres=# -- Once changes are read, they're consumed and not emitted
    postgres=# -- in a subsequent call:
    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
     lsn | xid | data
    -----+-----+------
    (0 rows)

    postgres=# BEGIN;
    postgres=*# INSERT INTO data(data) VALUES('1');
    postgres=*# INSERT INTO data(data) VALUES('2');
    postgres=*# COMMIT;

    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    |  xid  |                          data
    -----------+-------+---------------------------------------------------------
     0/BA5A688 | 10298 | BEGIN 10298
     0/BA5A6F0 | 10298 | table public.data: INSERT: id[integer]:1 data[text]:'1'
     0/BA5A7F8 | 10298 | table public.data: INSERT: id[integer]:2 data[text]:'2'
     0/BA5A8A8 | 10298 | COMMIT 10298
    (4 rows)

    postgres=# INSERT INTO data(data) VALUES('3');

    postgres=# -- You can also peek ahead in the change stream without consuming changes
    postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL);
        lsn    |  xid  |                          data
    -----------+-------+---------------------------------------------------------
     0/BA5A8E0 | 10299 | BEGIN 10299
     0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/BA5A990 | 10299 | COMMIT 10299
    (3 rows)

    postgres=# -- The next call to pg_logical_slot_peek_changes() returns the same changes again
    postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL);
        lsn    |  xid  |                          data
    -----------+-------+---------------------------------------------------------
     0/BA5A8E0 | 10299 | BEGIN 10299
     0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/BA5A990 | 10299 | COMMIT 10299
    (3 rows)

    postgres=# -- options can be passed to output plugin, to influence the formatting
    postgres=# SELECT * FROM pg_logical_slot_peek_changes('regression_slot', NULL, NULL, 'include-timestamp', 'on');
        lsn    |  xid  |                          data
    -----------+-------+---------------------------------------------------------
     0/BA5A8E0 | 10299 | BEGIN 10299
     0/BA5A8E0 | 10299 | table public.data: INSERT: id[integer]:3 data[text]:'3'
     0/BA5A990 | 10299 | COMMIT 10299 (at 2017-05-10 12:07:21.272494-04)
    (3 rows)

    postgres=# -- Remember to destroy a slot you no longer need to stop it consuming
    postgres=# -- server resources:
    postgres=# SELECT pg_drop_replication_slot('regression_slot');
     pg_drop_replication_slot
    -----------------------

    (1 row)

The following examples show how logical decoding is controlled over the streaming replication protocol, using the program app-pgrecvlogical included in the PostgreSQL distribution. This requires that client authentication is set up to allow replication connections (see streaming-replication-authentication) and that `max_wal_senders` is set sufficiently high to allow an additional connection. The second example shows how to stream two-phase transactions. Before you use two-phase commands, you must set guc-max-prepared-transactions to at least 1.

    Example 1:
    $ pg_recvlogical -d postgres --slot=test --create-slot
    $ pg_recvlogical -d postgres --slot=test --start -f -
    ControlZ
    $ psql -d postgres -c "INSERT INTO data(data) VALUES('4');"
    $ fg
    BEGIN 693
    table public.data: INSERT: id[integer]:4 data[text]:'4'
    COMMIT 693
    ControlC
    $ pg_recvlogical -d postgres --slot=test --drop-slot

    Example 2:
    $ pg_recvlogical -d postgres --slot=test --create-slot --two-phase
    $ pg_recvlogical -d postgres --slot=test --start -f -
    ControlZ
    $ psql -d postgres -c "BEGIN;INSERT INTO data(data) VALUES('5');PREPARE TRANSACTION 'test';"
    $ fg
    BEGIN 694
    table public.data: INSERT: id[integer]:5 data[text]:'5'
    PREPARE TRANSACTION 'test', txid 694
    ControlZ
    $ psql -d postgres -c "COMMIT PREPARED 'test';"
    $ fg
    COMMIT PREPARED 'test', txid 694
    ControlC
    $ pg_recvlogical -d postgres --slot=test --drop-slot

The following example shows SQL interface that can be used to decode prepared transactions. Before you use two-phase commit commands, you must set `max_prepared_transactions` to at least 1. You must also have set the two-phase parameter as 'true' while creating the slot using `pg_create_logical_replication_slot` Note that we will stream the entire transaction after the commit if it is not already decoded.

    postgres=# BEGIN;
    postgres=*# INSERT INTO data(data) VALUES('5');
    postgres=*# PREPARE TRANSACTION 'test_prepared1';

    postgres=# SELECT * FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    | xid |                          data
    -----------+-----+---------------------------------------------------------
     0/1689DC0 | 529 | BEGIN 529
     0/1689DC0 | 529 | table public.data: INSERT: id[integer]:3 data[text]:'5'
     0/1689FC0 | 529 | PREPARE TRANSACTION 'test_prepared1', txid 529
    (3 rows)

    postgres=# COMMIT PREPARED 'test_prepared1';
    postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    | xid |                    data
    -----------+-----+--------------------------------------------
     0/168A060 | 529 | COMMIT PREPARED 'test_prepared1', txid 529
    (4 row)

    postgres=#-- you can also rollback a prepared transaction
    postgres=# BEGIN;
    postgres=*# INSERT INTO data(data) VALUES('6');
    postgres=*# PREPARE TRANSACTION 'test_prepared2';
    postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    | xid |                          data
    -----------+-----+---------------------------------------------------------
     0/168A180 | 530 | BEGIN 530
     0/168A1E8 | 530 | table public.data: INSERT: id[integer]:4 data[text]:'6'
     0/168A430 | 530 | PREPARE TRANSACTION 'test_prepared2', txid 530
    (3 rows)

    postgres=# ROLLBACK PREPARED 'test_prepared2';
    postgres=# select * from pg_logical_slot_get_changes('regression_slot', NULL, NULL);
        lsn    | xid |                     data
    -----------+-----+----------------------------------------------
     0/168A4B8 | 530 | ROLLBACK PREPARED 'test_prepared2', txid 530
    (1 row)

## Logical Decoding Concepts

### Logical Decoding

Logical decoding is the process of extracting all persistent changes to a database's tables into a coherent, easy to understand format which can be interpreted without detailed knowledge of the database's internal state.

In PostgreSQL, logical decoding is implemented by decoding the contents of the write-ahead log, which describe changes on a storage level, into an application-specific form such as a stream of tuples or SQL statements.

### Replication Slots

In the context of logical replication, a slot represents a stream of changes that can be replayed to a client in the order they were made on the origin server. Each slot streams a sequence of changes from a single database.

PostgreSQL also has streaming replication slots (see streaming-replication), but they are used somewhat differently there.

A replication slot has an identifier that is unique across all databases in a PostgreSQL cluster. Slots persist independently of the connection using them and are crash-safe.

A logical slot will emit each change just once in normal operation. The current position of each slot is persisted only at checkpoint, so in the case of a crash the slot might return to an earlier LSN, which will then cause recent changes to be sent again when the server restarts. Logical decoding clients are responsible for avoiding ill effects from handling the same message more than once. Clients may wish to record the last LSN they saw when decoding and skip over any repeated data or (when using the replication protocol) request that decoding start from that LSN rather than letting the server determine the start point. The Replication Progress Tracking feature is designed for this purpose, refer to replication origins.

Multiple independent slots may exist for a single database. Each slot has its own state, allowing different consumers to receive changes from different points in the database change stream. For most applications, a separate slot will be required for each consumer.

A logical replication slot knows nothing about the state of the receiver(s). It's even possible to have multiple different receivers using the same slot at different times; they'll just get the changes following on from when the last receiver stopped consuming them. Only one receiver may consume changes from a slot at any given time.

A logical replication slot can also be created on a hot standby. To prevent `VACUUM` from removing required rows from the system catalogs, `hot_standby_feedback` should be set on the standby. In spite of that, if any required rows get removed, the slot gets invalidated. It's highly recommended to use a physical slot between the primary and the standby. Otherwise, `hot_standby_feedback` will work but only while the connection is alive (for example a node restart would break it). Then, the primary may delete system catalog rows that could be needed by the logical decoding on the standby (as it does not know about the `catalog_xmin` on the standby). Existing logical slots on standby also get invalidated if `wal_level` on the primary is reduced to less than `logical`. This is done as soon as the standby detects such a change in the WAL stream. It means that, for walsenders that are lagging (if any), some WAL records up to the `wal_level` parameter change on the primary won't be decoded.

Creation of a logical slot requires information about all the currently running transactions. On the primary, this information is available directly, but on a standby, this information has to be obtained from primary. Thus, slot creation may need to wait for some activity to happen on the primary. If the primary is idle, creating a logical slot on standby may take noticeable time. This can be sped up by calling the `pg_log_standby_snapshot` function on the primary.

Replication slots persist across crashes and know nothing about the state of their consumer(s). They will prevent removal of required resources even when there is no connection using them. This consumes storage because neither required WAL nor required rows from the system catalogs can be removed by `VACUUM` as long as they are required by a replication slot. In extreme cases this could cause the database to shut down to prevent transaction ID wraparound (see vacuum-for-wraparound). So if a slot is no longer required it should be dropped.

### Replication Slot Synchronization

The logical replication slots on the primary can be synchronized to the hot standby by using the `failover` parameter of `pg_create_logical_replication_slot`, or by using the `failover` option of `CREATE SUBSCRIPTION` during slot creation. Additionally, enabling `sync_replication_slots` on the standby is required. By enabling `sync_replication_slots` on the standby, the failover slots can be synchronized periodically in the slotsync worker. For the synchronization to work, it is mandatory to have a physical replication slot between the primary and the standby (i.e., `primary_slot_name` should be configured on the standby), and `hot_standby_feedback` must be enabled on the standby. It is also necessary to specify a valid `dbname` in the `primary_conninfo`. It's highly recommended that the said physical replication slot is named in `synchronized_standby_slots` list on the primary, to prevent the subscriber from consuming changes faster than the hot standby. Even when correctly configured, some latency is expected when sending changes to logical subscribers due to the waiting on slots named in `synchronized_standby_slots`. When `synchronized_standby_slots` is utilized, the primary server will not completely shut down until the corresponding standbys, associated with the physical replication slots specified in `synchronized_standby_slots`, have confirmed receiving the WAL up to the latest flushed position on the primary server.

While enabling `sync_replication_slots` allows for automatic periodic synchronization of failover slots, they can also be manually synchronized using the `pg_sync_replication_slots` function on the standby. However, this function is primarily intended for testing and debugging and should be used with caution. Unlike automatic synchronization, it does not include cyclic retries, making it more prone to synchronization failures, particularly during initial sync scenarios where the required WAL files or catalog rows for the slot might have already been removed or are at risk of being removed on the standby. In contrast, automatic synchronization via `sync_replication_slots` provides continuous slot updates, enabling seamless failover and supporting high availability. Therefore, it is the recommended method for synchronizing slots.

When slot synchronization is configured as recommended, and the initial synchronization is performed either automatically or manually via `pg_sync_replication_slots`, the standby can persist the synchronized slot only if the following condition is met: The logical replication slot on the primary must retain WALs and system catalog rows that are still available on the standby. This ensures data integrity and allows logical replication to continue smoothly after promotion. If the required WALs or catalog rows have already been purged from the standby, the slot will not be persisted to avoid data loss. In such cases, the following log message may appear:

    LOG:  could not synchronize replication slot "failover_slot"
    DETAIL:  Synchronization could lead to data loss, because the remote slot needs WAL at LSN 0/3003F28 and catalog xmin 754, but the standby has LSN 0/3003F28 and catalog xmin 756.

If the logical replication slot is actively used by a consumer, no manual intervention is needed; the slot will advance automatically, and synchronization will resume in the next cycle. However, if no consumer is configured, it is advisable to manually advance the slot on the primary using `pg_logical_slot_get_changes` or `pg_logical_slot_get_binary_changes`, allowing synchronization to proceed.

The ability to resume logical replication after failover depends upon the pg_replication_slots.synced value for the synchronized slots on the standby at the time of failover. Only persistent slots that have attained synced state as true on the standby before failover can be used for logical replication after failover. Temporary synced slots cannot be used for logical decoding, therefore logical replication for those slots cannot be resumed. For example, if the synchronized slot could not become persistent on the standby due to a disabled subscription, then the subscription cannot be resumed after failover even when it is enabled.

To resume logical replication after failover from the synced logical slots, the subscription's 'conninfo' must be altered to point to the new primary server. This is done using `ALTER SUBSCRIPTION ... CONNECTION`. It is recommended that subscriptions are first disabled before promoting the standby and are re-enabled after altering the connection string.

There is a chance that the old primary is up again during the promotion and if subscriptions are not disabled, the logical subscribers may continue to receive data from the old primary server even after promotion until the connection string is altered. This might result in data inconsistency issues, preventing the logical subscribers from being able to continue replication from the new primary server.

### Output Plugins

Output plugins transform the data from the write-ahead log's internal representation into the format the consumer of a replication slot desires.

### Exported Snapshots

When a new replication slot is created using the streaming replication interface (see protocol-replication-create-replication-slot), a snapshot is exported (see functions-snapshot-synchronization), which will show exactly the state of the database after which all changes will be included in the change stream. This can be used to create a new replica by using `SET TRANSACTION SNAPSHOT` to read the state of the database at the moment the slot was created. This transaction can then be used to dump the database's state at that point in time, which afterwards can be updated using the slot's contents without losing any changes.

Applications that do not require snapshot export may suppress it with the `SNAPSHOT 'nothing'` option.

## Streaming Replication Protocol Interface

The commands

- `CREATE_REPLICATION_SLOT `slot_name` LOGICAL `output_plugin``

- `DROP_REPLICATION_SLOT `slot_name`` `WAIT`

- `START_REPLICATION SLOT `slot_name` LOGICAL ...`

are used to create, drop, and stream changes from a replication slot, respectively. These commands are only available over a replication connection; they cannot be used via SQL. See protocol-replication for details on these commands.

The command app-pgrecvlogical can be used to control logical decoding over a streaming replication connection. (It uses these commands internally.)

## Logical Decoding SQL Interface

See functions-replication for detailed documentation on the SQL-level API for interacting with logical decoding.

Synchronous replication (see synchronous-replication) is only supported on replication slots used over the streaming replication interface. The function interface and additional, non-core interfaces do not support synchronous replication.

## System Catalogs Related to Logical Decoding

The pg_replication_slots view and the pg_stat_replication view provide information about the current state of replication slots and streaming replication connections respectively. These views apply to both physical and logical replication. The pg_stat_replication_slots view provides statistics information about the logical replication slots.

## Logical Decoding Output Plugins

An example output plugin can be found in the `contrib/test_decoding` subdirectory of the PostgreSQL source tree.

### Initialization Function

An output plugin is loaded by dynamically loading a shared library with the output plugin's name as the library base name. The normal library search path is used to locate the library. To provide the required output plugin callbacks and to indicate that the library is actually an output plugin it needs to provide a function named `_PG_output_plugin_init`. This function is passed a struct that needs to be filled with the callback function pointers for individual actions.

    typedef struct OutputPluginCallbacks
    {
        LogicalDecodeStartupCB startup_cb;
        LogicalDecodeBeginCB begin_cb;
        LogicalDecodeChangeCB change_cb;
        LogicalDecodeTruncateCB truncate_cb;
        LogicalDecodeCommitCB commit_cb;
        LogicalDecodeMessageCB message_cb;
        LogicalDecodeFilterByOriginCB filter_by_origin_cb;
        LogicalDecodeShutdownCB shutdown_cb;
        LogicalDecodeFilterPrepareCB filter_prepare_cb;
        LogicalDecodeBeginPrepareCB begin_prepare_cb;
        LogicalDecodePrepareCB prepare_cb;
        LogicalDecodeCommitPreparedCB commit_prepared_cb;
        LogicalDecodeRollbackPreparedCB rollback_prepared_cb;
        LogicalDecodeStreamStartCB stream_start_cb;
        LogicalDecodeStreamStopCB stream_stop_cb;
        LogicalDecodeStreamAbortCB stream_abort_cb;
        LogicalDecodeStreamPrepareCB stream_prepare_cb;
        LogicalDecodeStreamCommitCB stream_commit_cb;
        LogicalDecodeStreamChangeCB stream_change_cb;
        LogicalDecodeStreamMessageCB stream_message_cb;
        LogicalDecodeStreamTruncateCB stream_truncate_cb;
    } OutputPluginCallbacks;

    typedef void (*LogicalOutputPluginInit) (struct OutputPluginCallbacks *cb);

The `begin_cb`, `change_cb` and `commit_cb` callbacks are required, while `startup_cb`, `truncate_cb`, `message_cb`, `filter_by_origin_cb`, and `shutdown_cb` are optional. If `truncate_cb` is not set but a `TRUNCATE` is to be decoded, the action will be ignored.

An output plugin may also define functions to support streaming of large, in-progress transactions. The `stream_start_cb`, `stream_stop_cb`, `stream_abort_cb`, `stream_commit_cb`, and `stream_change_cb` are required, while `stream_message_cb` and `stream_truncate_cb` are optional. The `stream_prepare_cb` is also required if the output plugin also support two-phase commits.

An output plugin may also define functions to support two-phase commits, which allows actions to be decoded on the `PREPARE TRANSACTION`. The `begin_prepare_cb`, `prepare_cb`, `commit_prepared_cb` and `rollback_prepared_cb` callbacks are required, while `filter_prepare_cb` is optional. The `stream_prepare_cb` is also required if the output plugin also supports the streaming of large in-progress transactions.

### Capabilities

To decode, format and output changes, output plugins can use most of the backend's normal infrastructure, including calling output functions. Read only access to relations is permitted as long as only relations are accessed that either have been created by `initdb` in the `pg_catalog` schema, or have been marked as user provided catalog tables using

    ALTER TABLE user_catalog_table SET (user_catalog_table = true);
    CREATE TABLE another_catalog_table(data text) WITH (user_catalog_table = true);

Note that access to user catalog tables or regular system catalog tables in the output plugins has to be done via the `systable_*` scan APIs only. Access via the `heap_*` scan APIs will error out. Additionally, any actions leading to transaction ID assignment are prohibited. That, among others, includes writing to tables, performing DDL changes, and calling `pg_current_xact_id()`.

### Output Modes

Output plugin callbacks can pass data to the consumer in nearly arbitrary formats. For some use cases, like viewing the changes via SQL, returning data in a data type that can contain arbitrary data (e.g., `bytea`) is cumbersome. If the output plugin only outputs textual data in the server's encoding, it can declare that by setting `OutputPluginOptions.output_type` to `OUTPUT_PLUGIN_TEXTUAL_OUTPUT` instead of `OUTPUT_PLUGIN_BINARY_OUTPUT` in the startup callback. In that case, all the data has to be in the server's encoding so that a `text` datum can contain it. This is checked in assertion-enabled builds.

### Output Plugin Callbacks

An output plugin gets notified about changes that are happening via various callbacks it needs to provide.

Concurrent transactions are decoded in commit order, and only changes belonging to a specific transaction are decoded between the `begin` and `commit` callbacks. Transactions that were rolled back explicitly or implicitly never get decoded. Successful savepoints are folded into the transaction containing them in the order they were executed within that transaction. A transaction that is prepared for a two-phase commit using `PREPARE TRANSACTION` will also be decoded if the output plugin callbacks needed for decoding them are provided. It is possible that the current prepared transaction which is being decoded is aborted concurrently via a `ROLLBACK PREPARED` command. In that case, the logical decoding of this transaction will be aborted too. All the changes of such a transaction are skipped once the abort is detected and the `prepare_cb` callback is invoked. Thus even in case of a concurrent abort, enough information is provided to the output plugin for it to properly deal with `ROLLBACK PREPARED` once that is decoded.

Only transactions that have already safely been flushed to disk will be decoded. That can lead to a `COMMIT` not immediately being decoded in a directly following `pg_logical_slot_get_changes()` when `synchronous_commit` is set to `off`.

#### Startup Callback

The optional `startup_cb` callback is called whenever a replication slot is created or asked to stream changes, independent of the number of changes that are ready to be put out.

    typedef void (*LogicalDecodeStartupCB) (struct LogicalDecodingContext *ctx,
                                            OutputPluginOptions *options,
                                            bool is_init);

The `is_init` parameter will be true when the replication slot is being created and false otherwise. `options` points to a struct of options that output plugins can set:

    typedef struct OutputPluginOptions
    {
        OutputPluginOutputType output_type;
        bool        receive_rewrites;
    } OutputPluginOptions;

`output_type` has to either be set to `OUTPUT_PLUGIN_TEXTUAL_OUTPUT` or `OUTPUT_PLUGIN_BINARY_OUTPUT`. See also logicaldecoding-output-mode. If `receive_rewrites` is true, the output plugin will also be called for changes made by heap rewrites during certain DDL operations. These are of interest to plugins that handle DDL replication, but they require special handling.

The startup callback should validate the options present in `ctx->output_plugin_options`. If the output plugin needs to have a state, it can use `ctx->output_plugin_private` to store it.

#### Shutdown Callback

The optional `shutdown_cb` callback is called whenever a formerly active replication slot is not used anymore and can be used to deallocate resources private to the output plugin. The slot isn't necessarily being dropped, streaming is just being stopped.

    typedef void (*LogicalDecodeShutdownCB) (struct LogicalDecodingContext *ctx);

#### Transaction Begin Callback

The required `begin_cb` callback is called whenever a start of a committed transaction has been decoded. Aborted transactions and their contents never get decoded.

    typedef void (*LogicalDecodeBeginCB) (struct LogicalDecodingContext *ctx,
                                          ReorderBufferTXN *txn);

The `txn` parameter contains meta information about the transaction, like the time stamp at which it has been committed and its XID.

#### Transaction End Callback

The required `commit_cb` callback is called whenever a transaction commit has been decoded. The `change_cb` callbacks for all modified rows will have been called before this, if there have been any modified rows.

    typedef void (*LogicalDecodeCommitCB) (struct LogicalDecodingContext *ctx,
                                           ReorderBufferTXN *txn,
                                           XLogRecPtr commit_lsn);

#### Change Callback

The required `change_cb` callback is called for every individual row modification inside a transaction, may it be an `INSERT`, `UPDATE`, or `DELETE`. Even if the original command modified several rows at once the callback will be called individually for each row. The `change_cb` callback may access system or user catalog tables to aid in the process of outputting the row modification details. In case of decoding a prepared (but yet uncommitted) transaction or decoding of an uncommitted transaction, this change callback might also error out due to simultaneous rollback of this very same transaction. In that case, the logical decoding of this aborted transaction is stopped gracefully.

    typedef void (*LogicalDecodeChangeCB) (struct LogicalDecodingContext *ctx,
                                           ReorderBufferTXN *txn,
                                           Relation relation,
                                           ReorderBufferChange *change);

The `ctx` and `txn` parameters have the same contents as for the `begin_cb` and `commit_cb` callbacks, but additionally the relation descriptor `relation` points to the relation the row belongs to and a struct `change` describing the row modification are passed in.

Only changes in user defined tables that are not unlogged (see sql-createtable-unlogged) and not temporary (see sql-createtable-temporary) can be extracted using logical decoding.

#### Truncate Callback

The optional `truncate_cb` callback is called for a `TRUNCATE` command.

    typedef void (*LogicalDecodeTruncateCB) (struct LogicalDecodingContext *ctx,
                                             ReorderBufferTXN *txn,
                                             int nrelations,
                                             Relation relations[],
                                             ReorderBufferChange *change);

The parameters are analogous to the `change_cb` callback. However, because `TRUNCATE` actions on tables connected by foreign keys need to be executed together, this callback receives an array of relations instead of just a single one. See the description of the sql-truncate statement for details.

#### Origin Filter Callback

The optional `filter_by_origin_cb` callback is called to determine whether data that has been replayed from `origin_id` is of interest to the output plugin.

    typedef bool (*LogicalDecodeFilterByOriginCB) (struct LogicalDecodingContext *ctx,
                                                   RepOriginId origin_id);

The `ctx` parameter has the same contents as for the other callbacks. No information but the origin is available. To signal that changes originating on the passed in node are irrelevant, return true, causing them to be filtered away; false otherwise. The other callbacks will not be called for transactions and changes that have been filtered away.

This is useful when implementing cascading or multidirectional replication solutions. Filtering by the origin allows to prevent replicating the same changes back and forth in such setups. While transactions and changes also carry information about the origin, filtering via this callback is noticeably more efficient.

#### Generic Message Callback

The optional `message_cb` callback is called whenever a logical decoding message has been decoded.

    typedef void (*LogicalDecodeMessageCB) (struct LogicalDecodingContext *ctx,
                                            ReorderBufferTXN *txn,
                                            XLogRecPtr message_lsn,
                                            bool transactional,
                                            const char *prefix,
                                            Size message_size,
                                            const char *message);

The `txn` parameter contains meta information about the transaction, like the time stamp at which it has been committed and its XID. Note however that it can be NULL when the message is non-transactional and the XID was not assigned yet in the transaction which logged the message. The `lsn` has WAL location of the message. The `transactional` says if the message was sent as transactional or not. Similar to the change callback, in case of decoding a prepared (but yet uncommitted) transaction or decoding of an uncommitted transaction, this message callback might also error out due to simultaneous rollback of this very same transaction. In that case, the logical decoding of this aborted transaction is stopped gracefully. The `prefix` is arbitrary null-terminated prefix which can be used for identifying interesting messages for the current plugin. And finally the `message` parameter holds the actual message of `message_size` size.

Extra care should be taken to ensure that the prefix the output plugin considers interesting is unique. Using name of the extension or the output plugin itself is often a good choice.

#### Prepare Filter Callback

The optional `filter_prepare_cb` callback is called to determine whether data that is part of the current two-phase commit transaction should be considered for decoding at this prepare stage or later as a regular one-phase transaction at `COMMIT PREPARED` time. To signal that decoding should be skipped, return `true`; `false` otherwise. When the callback is not defined, `false` is assumed (i.e. no filtering, all transactions using two-phase commit are decoded in two phases as well).

    typedef bool (*LogicalDecodeFilterPrepareCB) (struct LogicalDecodingContext *ctx,
                                                  TransactionId xid,
                                                  const char *gid);

The `ctx` parameter has the same contents as for the other callbacks. The parameters `xid` and `gid` provide two different ways to identify the transaction. The later `COMMIT PREPARED` or `ROLLBACK PREPARED` carries both identifiers, providing an output plugin the choice of what to use.

The callback may be invoked multiple times per transaction to decode and must provide the same static answer for a given pair of `xid` and `gid` every time it is called.

#### Transaction Begin Prepare Callback

The required `begin_prepare_cb` callback is called whenever the start of a prepared transaction has been decoded. The `gid` field, which is part of the `txn` parameter, can be used in this callback to check if the plugin has already received this `PREPARE` in which case it can either error out or skip the remaining changes of the transaction.

    typedef void (*LogicalDecodeBeginPrepareCB) (struct LogicalDecodingContext *ctx,
                                                 ReorderBufferTXN *txn);

#### Transaction Prepare Callback

The required `prepare_cb` callback is called whenever a transaction which is prepared for two-phase commit has been decoded. The `change_cb` callback for all modified rows will have been called before this, if there have been any modified rows. The `gid` field, which is part of the `txn` parameter, can be used in this callback.

    typedef void (*LogicalDecodePrepareCB) (struct LogicalDecodingContext *ctx,
                                            ReorderBufferTXN *txn,
                                            XLogRecPtr prepare_lsn);

#### Transaction Commit Prepared Callback

The required `commit_prepared_cb` callback is called whenever a transaction `COMMIT PREPARED` has been decoded. The `gid` field, which is part of the `txn` parameter, can be used in this callback.

    typedef void (*LogicalDecodeCommitPreparedCB) (struct LogicalDecodingContext *ctx,
                                                   ReorderBufferTXN *txn,
                                                   XLogRecPtr commit_lsn);

#### Transaction Rollback Prepared Callback

The required `rollback_prepared_cb` callback is called whenever a transaction `ROLLBACK PREPARED` has been decoded. The `gid` field, which is part of the `txn` parameter, can be used in this callback. The parameters `prepare_end_lsn` and `prepare_time` can be used to check if the plugin has received this `PREPARE TRANSACTION` in which case it can apply the rollback, otherwise, it can skip the rollback operation. The `gid` alone is not sufficient because the downstream node can have a prepared transaction with same identifier.

    typedef void (*LogicalDecodeRollbackPreparedCB) (struct LogicalDecodingContext *ctx,
                                                     ReorderBufferTXN *txn,
                                                     XLogRecPtr prepare_end_lsn,
                                                     TimestampTz prepare_time);

#### Stream Start Callback

The required `stream_start_cb` callback is called when opening a block of streamed changes from an in-progress transaction.

    typedef void (*LogicalDecodeStreamStartCB) (struct LogicalDecodingContext *ctx,
                                                ReorderBufferTXN *txn);

#### Stream Stop Callback

The required `stream_stop_cb` callback is called when closing a block of streamed changes from an in-progress transaction.

    typedef void (*LogicalDecodeStreamStopCB) (struct LogicalDecodingContext *ctx,
                                               ReorderBufferTXN *txn);

#### Stream Abort Callback

The required `stream_abort_cb` callback is called to abort a previously streamed transaction.

    typedef void (*LogicalDecodeStreamAbortCB) (struct LogicalDecodingContext *ctx,
                                                ReorderBufferTXN *txn,
                                                XLogRecPtr abort_lsn);

#### Stream Prepare Callback

The `stream_prepare_cb` callback is called to prepare a previously streamed transaction as part of a two-phase commit. This callback is required when the output plugin supports both the streaming of large in-progress transactions and two-phase commits.

    typedef void (*LogicalDecodeStreamPrepareCB) (struct LogicalDecodingContext *ctx,
                                                  ReorderBufferTXN *txn,
                                                  XLogRecPtr prepare_lsn);

#### Stream Commit Callback

The required `stream_commit_cb` callback is called to commit a previously streamed transaction.

    typedef void (*LogicalDecodeStreamCommitCB) (struct LogicalDecodingContext *ctx,
                                                 ReorderBufferTXN *txn,
                                                 XLogRecPtr commit_lsn);

#### Stream Change Callback

The required `stream_change_cb` callback is called when sending a change in a block of streamed changes (demarcated by `stream_start_cb` and `stream_stop_cb` calls). The actual changes are not displayed as the transaction can abort at a later point in time and we don't decode changes for aborted transactions.

    typedef void (*LogicalDecodeStreamChangeCB) (struct LogicalDecodingContext *ctx,
                                                 ReorderBufferTXN *txn,
                                                 Relation relation,
                                                 ReorderBufferChange *change);

#### Stream Message Callback

The optional `stream_message_cb` callback is called when sending a generic message in a block of streamed changes (demarcated by `stream_start_cb` and `stream_stop_cb` calls). The message contents for transactional messages are not displayed as the transaction can abort at a later point in time and we don't decode changes for aborted transactions.

    typedef void (*LogicalDecodeStreamMessageCB) (struct LogicalDecodingContext *ctx,
                                                  ReorderBufferTXN *txn,
                                                  XLogRecPtr message_lsn,
                                                  bool transactional,
                                                  const char *prefix,
                                                  Size message_size,
                                                  const char *message);

#### Stream Truncate Callback

The optional `stream_truncate_cb` callback is called for a `TRUNCATE` command in a block of streamed changes (demarcated by `stream_start_cb` and `stream_stop_cb` calls).

    typedef void (*LogicalDecodeStreamTruncateCB) (struct LogicalDecodingContext *ctx,
                                                   ReorderBufferTXN *txn,
                                                   int nrelations,
                                                   Relation relations[],
                                                   ReorderBufferChange *change);

The parameters are analogous to the `stream_change_cb` callback. However, because `TRUNCATE` actions on tables connected by foreign keys need to be executed together, this callback receives an array of relations instead of just a single one. See the description of the sql-truncate statement for details.

### Functions for Producing Output

To actually produce output, output plugins can write data to the `StringInfo` output buffer in `ctx->out` when inside the `begin_cb`, `commit_cb`, or `change_cb` callbacks. Before writing to the output buffer, `OutputPluginPrepareWrite(ctx, last_write)` has to be called, and after finishing writing to the buffer, `OutputPluginWrite(ctx, last_write)` has to be called to perform the write. The `last_write` indicates whether a particular write was the callback's last write.

The following example shows how to output data to the consumer of an output plugin:

    OutputPluginPrepareWrite(ctx, true);
    appendStringInfo(ctx->out, "BEGIN %u", txn->xid);
    OutputPluginWrite(ctx, true);

## Logical Decoding Output Writers

It is possible to add more output methods for logical decoding. For details, see `src/backend/replication/logical/logicalfuncs.c`. Essentially, three functions need to be provided: one to read WAL, one to prepare writing output, and one to write the output (see logicaldecoding-output-plugin-output).

## Synchronous Replication Support for Logical Decoding

### Overview

Logical decoding can be used to build synchronous replication solutions with the same user interface as synchronous replication for streaming replication. To do this, the streaming replication interface (see logicaldecoding-walsender) must be used to stream out data. Clients have to send `Standby status update (F)` (see protocol-replication) messages, just like streaming replication clients do.

A synchronous replica receiving changes via logical decoding will work in the scope of a single database. Since, in contrast to that, `synchronous_standby_names` currently is server wide, this means this technique will not work properly if more than one database is actively used.

### Caveats

In synchronous replication setup, a deadlock can happen, if the transaction has locked [user] catalog tables exclusively. See logicaldecoding-capabilities for information on user catalog tables. This is because logical decoding of transactions can lock catalog tables to access them. To avoid this users must refrain from taking an exclusive lock on [user] catalog tables. This can happen in the following ways:

- Issuing an explicit `LOCK` on pg_class in a transaction.

- Perform `CLUSTER` on pg_class in a transaction.

- `PREPARE TRANSACTION` after `LOCK` command on pg_class and allow logical decoding of two-phase transactions.

- `PREPARE TRANSACTION` after `CLUSTER` command on pg_trigger and allow logical decoding of two-phase transactions. This will lead to deadlock only when published table have a trigger.

- Executing `TRUNCATE` on [user] catalog table in a transaction.

Note that these commands can cause deadlocks not only for the system catalog tables listed above but for other catalog tables.

## Streaming of Large Transactions for Logical Decoding

The basic output plugin callbacks (e.g., `begin_cb`, `change_cb`, `commit_cb` and `message_cb`) are only invoked when the transaction actually commits. The changes are still decoded from the transaction log, but are only passed to the output plugin at commit (and discarded if the transaction aborts).

This means that while the decoding happens incrementally, and may spill to disk to keep memory usage under control, all the decoded changes have to be transmitted when the transaction finally commits (or more precisely, when the commit is decoded from the transaction log). Depending on the size of the transaction and network bandwidth, the transfer time may significantly increase the apply lag.

To reduce the apply lag caused by large transactions, an output plugin may provide additional callback to support incremental streaming of in-progress transactions. There are multiple required streaming callbacks (`stream_start_cb`, `stream_stop_cb`, `stream_abort_cb`, `stream_commit_cb` and `stream_change_cb`) and two optional callbacks (`stream_message_cb` and `stream_truncate_cb`). Also, if streaming of two-phase commands is to be supported, then additional callbacks must be provided. (See logicaldecoding-two-phase-commits for details).

When streaming an in-progress transaction, the changes (and messages) are streamed in blocks demarcated by `stream_start_cb` and `stream_stop_cb` callbacks. Once all the decoded changes are transmitted, the transaction can be committed using the `stream_commit_cb` callback (or possibly aborted using the `stream_abort_cb` callback). If two-phase commits are supported, the transaction can be prepared using the `stream_prepare_cb` callback, `COMMIT PREPARED` using the `commit_prepared_cb` callback or aborted using the `rollback_prepared_cb`.

One example sequence of streaming callback calls for one transaction may look like this:

    stream_start_cb(...);   <-- start of first block of changes
      stream_change_cb(...);
      stream_change_cb(...);
      stream_message_cb(...);
      stream_change_cb(...);
      ...
      stream_change_cb(...);
    stream_stop_cb(...);    <-- end of first block of changes

    stream_start_cb(...);   <-- start of second block of changes
      stream_change_cb(...);
      stream_change_cb(...);
      stream_change_cb(...);
      ...
      stream_message_cb(...);
      stream_change_cb(...);
    stream_stop_cb(...);    <-- end of second block of changes

    [a. when using normal commit]
    stream_commit_cb(...);    <-- commit of the streamed transaction

    [b. when using two-phase commit]
    stream_prepare_cb(...);   <-- prepare the streamed transaction
    commit_prepared_cb(...);  <-- commit of the prepared transaction

The actual sequence of callback calls may be more complicated, of course. There may be blocks for multiple streamed transactions, some of the transactions may get aborted, etc.

Similar to spill-to-disk behavior, streaming is triggered when the total amount of changes decoded from the WAL (for all in-progress transactions) exceeds the limit defined by `logical_decoding_work_mem` setting. At that point, the largest top-level transaction (measured by the amount of memory currently used for decoded changes) is selected and streamed. However, in some cases we still have to spill to disk even if streaming is enabled because we exceed the memory threshold but still have not decoded the complete tuple e.g., only decoded toast table insert but not the main table insert.

Even when streaming large transactions, the changes are still applied in commit order, preserving the same guarantees as the non-streaming mode.

## Two-phase Commit Support for Logical Decoding

With the basic output plugin callbacks (eg., `begin_cb`, `change_cb`, `commit_cb` and `message_cb`) two-phase commit commands like `PREPARE TRANSACTION`, `COMMIT PREPARED` and `ROLLBACK PREPARED` are not decoded. While the `PREPARE TRANSACTION` is ignored, `COMMIT PREPARED` is decoded as a `COMMIT` and `ROLLBACK PREPARED` is decoded as a `ROLLBACK`.

To support the streaming of two-phase commands, an output plugin needs to provide additional callbacks. There are multiple two-phase commit callbacks that are required, (`begin_prepare_cb`, `prepare_cb`, `commit_prepared_cb`, `rollback_prepared_cb` and `stream_prepare_cb`) and an optional callback (`filter_prepare_cb`).

If the output plugin callbacks for decoding two-phase commit commands are provided, then on `PREPARE TRANSACTION`, the changes of that transaction are decoded, passed to the output plugin, and the `prepare_cb` callback is invoked. This differs from the basic decoding setup where changes are only passed to the output plugin when a transaction is committed. The start of a prepared transaction is indicated by the `begin_prepare_cb` callback.

When a prepared transaction is rolled back using the `ROLLBACK PREPARED`, then the `rollback_prepared_cb` callback is invoked and when the prepared transaction is committed using `COMMIT PREPARED`, then the `commit_prepared_cb` callback is invoked.

Optionally the output plugin can define filtering rules via `filter_prepare_cb` to decode only specific transaction in two phases. This can be achieved by pattern matching on the `gid` or via lookups using the `xid`.

The users that want to decode prepared transactions need to be careful about below mentioned points:

- If the prepared transaction has locked [user] catalog tables exclusively then decoding prepare can block till the main transaction is committed.

- The logical replication solution that builds distributed two phase commit using this feature can deadlock if the prepared transaction has locked [user] catalog tables exclusively. To avoid this users must refrain from having locks on catalog tables (e.g. explicit `LOCK` command) in such transactions. See logicaldecoding-synchronous-caveats for the details.