An example output plugin can be found in the contrib/test_decoding subdirectory of the PostgreSQL source tree.
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.
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.
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
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 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.
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.
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.
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.
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:
output_type has to either be set to OUTPUT_PLUGIN_TEXTUAL_OUTPUT or OUTPUT_PLUGIN_BINARY_OUTPUT. See also Section 48.6.3. 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.
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.
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.
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 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 UNLOGGED) and not temporary (see TEMPORARY or TEMP) can be extracted using logical decoding.
The truncate_cb callback is called for a TRUNCATE command.
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.
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.
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. 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.
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: