## 47.6. Logical Decoding Output Plugins

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

### 47.6.1. 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;
        LogicalDecodeCommitCB commit_cb;
        LogicalDecodeMessageCB message_cb;
        LogicalDecodeFilterByOriginCB filter_by_origin_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` and `shutdown_cb` are optional.

### 47.6.2. 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()`.

### 47.6.3. 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.

### 47.6.4. 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.

### 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`.

#### 47.6.4.1. 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;
    } OutputPluginOptions;

`output_type` has to either be set to `OUTPUT_PLUGIN_TEXTUAL_OUTPUT` or `OUTPUT_PLUGIN_BINARY_OUTPUT`. See also Section 47.6.3.

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.

#### 47.6.4.2. 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);

#### 47.6.4.3. 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.

#### 47.6.4.4. 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);

#### 47.6.4.5. 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.

    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.

#### 47.6.4.6. 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.

#### 47.6.4.7. 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.

### 47.6.5. 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);