Transaction Management

You can demarcate a transaction by applying a transaction to an endpoint. If a Mule flow begins with a transactional resource (i.e. inbound endpoint), Mule can start a new transaction and manage the entire flow as a transaction. If your flow includes a transactional outbound endpoint, Mule manages the outgoing operation as a transaction. With both a transactional inbound and outbound endpoint, Mule executes the outgoing operation as part of the transaction initiated by the inbound endpoint.

The following endpoints support transactional demarcation:

• JMS
• JDBC
• VM

However, there may be situations in which a Mule flow begins with a non-transactional inbound endpoint – such as HTTP or SFTP – but which requires the use of a transaction within the flow.  For example, a Mule flow may accept information from an external Web service,  then transform the data, before charging a credit card and saving invoice information to a database. In such a situation, you can demarcate a transaction by wrapping the credit card charge and database commit operations within a transaction to ensure either complete success or complete failure and rollback.

Transaction Types

Mule supports three different types of transactions: single resource, multiple resource and extended architecture (XA). The following table describes some of the characteristics of each type.

Multi-source vs. XA While XA Transactions offer similar functionality, Multiple Resource transactions use less overhead.  XA transactions, on the other hand, are more reliable and can include JDBC resources. For a discussion of different approaches – including the 1.5 phase commit concept which Multiple Resource transactions use – see the JavaWorld article on distributed transactions.

Assumptions

This document assumes that you are familiar with Mule ESB and/or the Mule Studio interface. To increase your familiarity with Mule Studio, consider completing one or more Mule Studio Tutorials. Further, this document assumes you have some knowledge of transaction processing and are familiar with Mule endpoints.  

Configuring Transactions

You can demarcate a transaction by either applying a transactional configuration to an endpoint, or by wrapping several elements in a transactional wrapper.

• Apply a transaction to an inbound endpoint when you want Mule to handle the complete flow as a transaction.
• Apply a transaction to an outbound endpoint when you want Mule to handle the outgoing operation as part of an existing transaction.
• Apply a transaction as a wrapper (known as a scope in Studio) when a you want to apply a transaction to elements within a flow which does not begin with a inbound endpoint configured as a transaction.

 The following subsections outline the steps to apply transactions in your flow.

Applying a Transaction to an Endpoint

You can apply a transaction to any of the following inbound or outbound endpoints:

• JMS
• VM
• JDBC 

Applying a Transaction as Wrapper

Configuration Tips and Tricks

• Operations that occur inside a transaction execute synchronously. You cannot build an asynchronous flow inside a transaction.
• Mule creates a transaction for the first outbound endpoint that can be part of a transaction (JMS, JDBC, VM). All the outbound endpoints in the flow which appear after the first outbound endpoint and which use the same type of resource then participate in the transaction. Where such a following endpoint does not use the same type of resource (i.e. where a JDBC endpoint follows a JMS endpoint), the transaction initiated by the first outbound endpoint fails. To avoid execution failure in such a situation, configure the secondary outbound endpoint outside the transaction by setting the action attribute to NOT_SUPPORTED.
• If you apply an XA transaction to multiple endpoints in your flow, be sure to configure the connectors to use XA-enabled resources.
• If you apply an XA transaction to a JMS inbound endpoint in your flow, you have the option of specifying the polling frequency of the queue. Access XA Transactions for configuration details.
• Mule can manage non-transactional outbound endpoints. By default, an outbound endpoint from a non-transactional transport ignores an active transaction rather than rejecting it. In other words, the default transactional action for such endpoints is no longer NONE. The example code below  illustrates this  behavior. Mule processes messages it receives from the VM queue synchronously and transactionally. The file transport in the code example is not transactional thus, writing to the file is not part of the transaction. However, if a message throws an exception while Mule is creating the file, Mule will roll back the transaction and reprocess the message. This example is, in effect, a multiple resource transaction.

Transaction Exception Strategies 

To handle exceptions Mule throws while processing transactions, you have three options:

1. Configure no exception strategies for the flow or transaction, thus employing Mule's default exception strategy by default. 
2. Configure an exception strategy for the flow in which a transaction exists. The flow's exception strategy handles all exceptions Mule throws while processing the transaction.
3. Configure an exception strategy for the scope of an individual transaction. The transaction's exception strategy handles all exceptions Mule throws while processing the transaction. If you wish to manage a transactional exception differently from all other exceptions thrown, consider applying an exception strategy to your transaction.

Refer to the Error Handling documentation to learn more about Mule's default exception strategy and how to apply exception strategies to flows.  Follow the steps below to apply an exception strategy to an individual transaction. 

Go Further

• Read more about Single Resource Transactions, Multiple Resource Transaction, and XA Transactions. 
• Read more about Exception Strategies.
• Consider reading Distributed transactions in Spring, with and without XA, an article on distributed transactions using both XA and non-XA approaches.  The multi-resource transaction support described in the example above is related to the Best Efforts 1PC pattern described in the article.
• Use the Transactions Configuration Reference for quick reference to transactional attributes.