## 50.6. Logical Decoding Output Plugins

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

### 50.6.1. 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;
        LogicalDecodeAbortCB abort_cb;
        LogicalDecodeMessageCB message_cb;
        LogicalDecodeFilterPrepareCB filter_prepare_cb;
        LogicalDecodePrepareCB prepare_cb;
        LogicalDecodeCommitPreparedCB commit_prepared_cb;
        LogicalDecodeAbortPreparedCB abort_prepared_cb;
        LogicalDecodeFilterByOriginCB filter_by_origin_cb;
        LogicalDecodeFilterDecodeTxnCB filter_decode_txn_cb;
        LogicalDecodeShutdownCB shutdown_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.

### 50.6.2. 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);

Any actions leading to transaction ID assignment are prohibited. That, among others, includes writing to tables, performing DDL changes, and calling `txid_current()`.

### 50.6.3. 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.

### 50.6.4. 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 transaction which is being decoded is aborted concurrently via a `ROLLBACK PREPARED` command. In that case, the logical decoding will be aborted midways.

### 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`.

#### 50.6.4.1. 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 50.6.3. 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.

#### 50.6.4.2. 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);

#### 50.6.4.3. 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.

#### 50.6.4.4. 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);

#### 50.6.4.5. Transaction Prepare Callback

The optional `prepare_cb` callback is called whenever a transaction which is prepared for two-phase 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 (*LogicalDecodePrepareCB) (struct LogicalDecodingContext *ctx,
                                            ReorderBufferTXN *txn,
                                            XLogRecPtr prepare_lsn);

#### 50.6.4.6. Commit Prepared Transaction Callback

The optional `commit_prepared_cb` callback is called whenever a commit prepared transaction 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);

#### 50.6.4.7. Rollback Prepared Transaction Callback

The optional `abort_prepared_cb` callback is called whenever a rollback prepared transaction has been decoded. The `gid` field, which is part of the `txn` parameter can be used in this callback.

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

#### 50.6.4.8. Transaction Abort Callback

The required `abort_cb` callback is called whenever a transaction abort has to be initiated. This can happen if we are decoding a transaction that has been prepared for two-phase commit and a concurrent rollback happens while we are decoding it. It might make sense, even before we commence decoding, in such cases to check if the rollback happened even before we start looking at the changes to completely avoid the decoding of such transactions.

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

#### 50.6.4.9. 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.

    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.

#### 50.6.4.10. 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.

#### 50.6.4.11. 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.

#### 50.6.4.12. Decode Filter Callback

The optional `filter_decode_txn_cb` callback is called to determine whether data that is part of the current transaction should be continued to be decoded.

    typedef bool (*LogicalDecodeFilterDecodeTxnCB) (struct LogicalDecodingContext *ctx,
                                                    ReorderBufferTXN *txn);

The `ctx` parameter has the same contents as for the other callbacks. The `txn` parameter contains meta information about the transaction, like its XID. Note however that it can be NULL in some cases. To signal that decoding process should terminate, return true; false otherwise.

#### 50.6.4.13. 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 decode at this prepare stage or as a regular one-phase transaction at `COMMIT PREPARED` time later.

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

The `ctx` parameter has the same contents as for the other callbacks. The `txn` parameter contains meta information about the transaction. The `xid` contains the XID because `txn` can be NULL in some cases. The `gid` is the identifier that later identifies this transaction for `COMMIT PREPARED` or `ROLLBACK PREPARED`. The callback has to provide the same static answer for a given combination of `xid` and `gid` every time it is called. To signal that decoding should be skipped, return true; false otherwise.

#### 50.6.4.14. 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. 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.

### 50.6.5. 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);