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49.6. Logical Decoding Output Plugins

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

49.6.1. 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, filter_by_origin_cb, truncate_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, stream_change_cb, and stream_prepare_cb are required, while stream_message_cb and stream_truncate_cb are optional.

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, stream_prepare_cb, commit_prepared_cb and rollback_prepared_cb callbacks are required, while filter_prepare_cb is optional.

49.6.2. 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().

49.6.3. 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.

49.6.4. 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.

Note

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.

49.6.4.1. 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 Section 49.6.3. 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.

49.6.4.2. 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);

49.6.4.3. 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.

49.6.4.4. 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);

49.6.4.5. 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.

Note

Only changes in user defined tables that are not unlogged (see UNLOGGED) and not temporary (see TEMPORARY or TEMP) can be extracted using logical decoding.

49.6.4.6. Truncate Callback

The 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 TRUNCATE statement for details.

49.6.4.7. 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.

49.6.4.8. 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.

49.6.4.9. 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.

49.6.4.10. 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);

49.6.4.11. 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);

49.6.4.12. 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);

49.6.4.13. 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);

49.6.4.14. Stream Start Callback

The 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);

49.6.4.15. Stream Stop Callback

The 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);

49.6.4.16. Stream Abort Callback

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

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

49.6.4.17. Stream Prepare Callback

The stream_prepare_cb callback is called to prepare a previously streamed transaction as part of a two-phase commit.

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

49.6.4.18. Stream Commit Callback

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

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

49.6.4.19. Stream Change Callback

The 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);

49.6.4.20. Stream Message Callback

The 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);

49.6.4.21. Stream Truncate Callback

The 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 TRUNCATE statement for details.

49.6.5. 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);