47.6. Logical Decoding Output Plugins

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 47.6.3 . 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.

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);

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.

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);

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.

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 TRUNCATE statement for details.

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.

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.

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.

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);

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);

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);

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);

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);

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);

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);

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);

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);

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);

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);

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 TRUNCATE statement for details.