---
title: "Two-phase Commit Support for Logical Decoding"
id: logicaldecoding-two-phase-commits
pg_version: "20devel"
---

<a id="logicaldecoding-two-phase-commits"></a>

## 47.11. Two-phase Commit Support for Logical Decoding

With the basic output plugin callbacks (eg., `begin_cb`, `change_cb`, `commit_cb` and `message_cb`) two-phase commit commands like `PREPARE TRANSACTION`, `COMMIT PREPARED` and `ROLLBACK PREPARED` are not decoded. While the `PREPARE TRANSACTION` is ignored, `COMMIT PREPARED` is decoded as a `COMMIT` and `ROLLBACK PREPARED` is decoded as a `ROLLBACK`. 

To support the streaming of two-phase commands, an output plugin needs to provide additional callbacks. There are multiple two-phase commit callbacks that are required, (`begin_prepare_cb`, `prepare_cb`, `commit_prepared_cb`, `rollback_prepared_cb` and `stream_prepare_cb`) and an optional callback (`filter_prepare_cb`). 

If the output plugin callbacks for decoding two-phase commit commands are provided, then on `PREPARE TRANSACTION`, the changes of that transaction are decoded, passed to the output plugin, and the `prepare_cb` callback is invoked. This differs from the basic decoding setup where changes are only passed to the output plugin when a transaction is committed. The start of a prepared transaction is indicated by the `begin_prepare_cb` callback. 

When a prepared transaction is rolled back using the `ROLLBACK PREPARED`, then the `rollback_prepared_cb` callback is invoked and when the prepared transaction is committed using `COMMIT PREPARED`, then the `commit_prepared_cb` callback is invoked. 

Optionally the output plugin can define filtering rules via `filter_prepare_cb` to decode only specific transaction in two phases. This can be achieved by pattern matching on the `gid` or via lookups using the `xid`. 

The users that want to decode prepared transactions need to be careful about below mentioned points: 
-  If the prepared transaction has locked [user] catalog tables exclusively then decoding prepare can block till the main transaction is committed. 
-  The logical replication solution that builds distributed two phase commit using this feature can deadlock if the prepared transaction has locked [user] catalog tables exclusively. To avoid this users must refrain from having locks on catalog tables (e.g. explicit `LOCK` command) in such transactions. See [Section 47.9.2](logicaldecoding-synchronous.md#logicaldecoding-synchronous-caveats) for the details.