Try Scope

The Try scope enables you to handle errors that may occur when attempting to execute any of the components inside the Try scope. It also supports transactions. A Try scope wraps one or more operations, then catches and handles any exceptions that might be thrown by any of these enclosed operations. The behavior is as if you extracted those enclosed event components into a separate flow with its own error handling strategy, but inline, without having to actually define a new flow.

Error Handling with the Try Scope

When designing your flow, try to group those operations that are likely to experience errors inside a Try scope. The Try scope enables you to isolate potentially troublesome operations in your flow and assign them an error-handling method. You can also configure the operations inside the Try scope to be processed as a transaction.

The Try scope has an error handling strategy that you configure in the same way you configure error handling for a flow.

The Try scope can distinguish among various error type conditions and apply different behaviors. If an error is raised by a component inside a Try scope, then the Try scope’s error handler is executed. At this point, the error is available for inspection, so the handlers can execute and act accordingly:

On Error Continue

Executes and sends the result of the execution to its container Try scope, which uses that result to complete the execution successfully. Any transactions at this point are also committed.
On Error Propagate

Rolls back any transactions, then executes and uses that result to re-throw the existing error, causing its container Try scope’s execution to fail.

Example Try Scope Configuration

A flow showing a Try scope configuration including error handling options

In the previous example, any database connection errors (DB:CONNECTIVITY) are propagated because of the On Error Propagate (on-error-propagate) error handler. Propagation of this error causes the Try scope’s execution to fail and the flow’s error handler to execute. Other errors are handled through the On Error Continue (on-error-continue) error handler, so the Try scope’s execution is treated as successful when they occur, meaning that the next operation, an HTTP request, continues its execution.

If the Try scope has several components, then after a component raises an exception, subsequent components in the Try scope are not executed, regardless of the type of error handler that catches the exception. In the case of On Error Propagate, the error is propagated to the flow’s error handler, as if the Try scope did not exist. In the case of On Error Continue, processing continues outside the Try scope at the next flow component, as if the Try scope never threw an exception.

Handling Transactions

A transaction is a series of actions that should never be partially executed. Configure a Try scope so that it is a set of operations that are considered one unit that either succeeds or fails, depending on whether errors are propagated and the transaction rolled back, or handled and the transaction committed. In either case, the process flow in which the Try scope resides continues.

A configuration panel for a Try scope in Mule with transactional settings

The Try scope treats child operations as a transaction when the Transactional Action (transactionalAction) is set to ALWAYS_BEGIN or BEGIN_OR_JOIN. It can be configured in the following ways:

Ignore (INDIFFERENT)

Default. If a transaction is active, the scope joins it. If not, the scope does not create a transaction.
Always Begin (ALWAYS_BEGIN)

A new transaction is started every time the scope is executed.
Begin or Join (BEGIN_OR_JOIN)

Relevant only when execution order might vary (for example, due to asynchronous actions occurring outside the flow). If current flow processing has already started a transaction, the scope joins it. If not, the scope initiates a new transaction.

Analyzing Different Transactional Actions in Try Scope

The following example shows a jms:listener operation configured to initiate a transaction at flow level, a Try scope that tries to initiate or join the transaction (depending on its configuration), and a jms:publish operation configured to run outside of the transaction:

<flow name="someFlow">
	<jms:listener config-ref="JMS_Config" destination="test.in" transactionalAction="ALWAYS_BEGIN"/>
	<!-- Processors -->
	<try transactionalAction="${action}">
		<!-- Processors -->
		<!-- Join if possible is the default value for jms:publish operation -->
		<jms:publish config-ref="JMS_Config" destination="test.out" transactionalAction="NOT_SUPPORTED"/>
		<raise-error type="APP:SOME"/>
		<!-- More Processors -->
	</try>
	<!-- Processors -->
</flow>

If the operations within the Try scope do not produce an error, the scope finishes the execution and commits the transaction, independently of the configured transactionalAction value.

The transaction and the messages are handled differently when a <raise-error/> component is added, depending on the transactionalAction of the Try scope:

Ignore (INDIFFERENT)

In this case, the transaction continues while executing the operations inside the Try scope. When the error is raised, it is propagated to the source (which does not handle it with an <on-error-continue> error handler). The transaction is rolled back, and the message is available again in the JMS queue. This rollback does not affect the completed jms:publish operation, which was outside of the transaction scope because transactionAction was set to its default, NOT_SUPPORTED.

Had transactionAction been set to JOIN_IF_POSSIBLE, the jms:publish operation would have been rolled back.
Always Begin (ALWAYS_BEGIN)

This raises an error because an active transaction already exists.
Begin or Join (BEGIN_OR_JOIN)

In this case, a transaction was already initiated so the scope joins the active transaction. The result is the same as in INDIFFERENT.

Error Handler at Flow Level

In the following example, an error handler is added at flow level:

<flow name="someFlow">
	<jms:listener config-ref="JMS_Config" destination="test.in" transactionalAction="ALWAYS_BEGIN"/>
	<!-- Processors -->
	<try transactionalAction="${action}">
		<!-- Processors -->
		<!-- Join if possible is the default value for jms:publish operation -->
		<jms:publish config-ref="JMS_Config" destination="test.out"/>
		<raise-error type="APP:SOME"/>
		<!-- More Processors -->
	</try>
	<!-- Processors -->
	<error-handler>
		<on-error-continue/>
	</error-handler>
</flow>

The behavior in this example is:

Ignore (INDIFFERENT)

The transaction continues. Because the error is handled by an on-error-continue error handler, the transaction is committed. The message read from the jms:listener source is consumed, and the message processed by the jms:publish operation is actually sent.
Always Begin (ALWAYS_BEGIN)

Raises an error because an active transaction already exists.
Begin or Join (BEGIN_OR_JOIN)

Displays the same behavior as INDIFFERENT.

Error Handler Inside the Try Scope

In this case, the error handler is inside the Try scope and the error occurs after the execution of the scope:

<flow name="someFlow">
	<jms:listener config-ref="JMS_Config" destination="test.in" transactionalAction="ALWAYS_BEGIN"/>
	<!-- Processors -->
	<try transactionalAction="${action}">
		<!-- Processors -->
		<!-- Join if possible is the default value for jms:publish operation -->
		<jms:publish config-ref="JMS_Config" destination="test.out"/>
		<!-- More Processors -->
		<!-- There could be a component that raises an error, it will be handled by the error handler -->
		<error-handler>
			<on-error-continue/>
		</error-handler>
	</try>
	<!-- Processors -->
	<raise-error type="APP:SOME"/>
</flow>

Depending on the configured transactionalAction, the behavior in the Try scope is one of the following:

Ignore (INDIFFERENT)

The transaction continues but the error is not handled by an on-error-continue at the flow level, causing the transaction to be rolled back, and the message to not be sent.
Always Begin (ALWAYS_BEGIN)

Raises an error because an active transaction already exists.
Begin or Join (BEGIN_OR_JOIN)

Displays the same behavior as INDIFFERENT.

Configuring Local or XA Transactions

In addition to configuring the Transactional Action, you can also configure the Transaction Type to be Local (single Resource) or XA Transaction.

The Try scope uses the Transaction Type configuration only when a new transaction is created. The type cannot change while a transaction executes.

For each Transactional Action, the behavior changes:

Ignore (INDIFFERENT)

The Transaction used is the one already created (if there is one). This means that setting the Transactional Type makes no difference for this Transactional Action.
Always Begin (ALWAYS_BEGIN)

The Transaction created is of the type set in the Transaction Type configuration. Remember that ALWAYS_BEGIN is an invalid configuration when already running within a Single Resource transaction.
Begin or Join (BEGIN_OR_JOIN)

If there is a Transaction already created, then the Transactional Type makes no difference, as in the case of INDIFFERENT. If there is no Transaction, it creates one of the Type configured in Transaction Type, as in the case of ALWAYS_BEGIN.

Variable and Payload Propagation

Every payload modification or variable addition, modification, or removal is propagated through the rest of the execution. This propagation includes modifications that take place inside the error handlers.