Handling Errors8% of the exam
Global and default error handlers
How Mule 4 handles a messaging error when nothing is configured, how to define a named global Error Handler and attach it with error-handler ref or the defaultErrorHandler-ref global configuration, and how to reuse a single named On-Error component with on-error ref.
Apply correct processors/syntax to implement global error handlers, and predict outcomes.
~7 min read · 5 practice questions ·
What the exam expects
- Predict what a caller receives when a flow fails and no error handling is configured (the default handling).
- Write the XML that defines a named global Error Handler and attaches it to chosen flows with
<error-handler ref="..."/>. - Make one global Error Handler the application default with a
<configuration defaultErrorHandler-ref="..."/>global element, and predict which handler runs when a flow also declares its own. - Reuse one named On-Error component inside several Error Handlers with
<on-error ref="..."/>. - Reject configurations that do nothing: a handler that is defined but never referenced, a handler called with Flow Reference, or a handler put in a subflow.
Key terms:
- Error Handler (
<error-handler>): the container that routes an error to the first matching On-Error component; built into every flow and Try scope, or defined globally with aname. - Global Error Handler: an
<error-handler name="...">defined at the root of a configuration file, outside any flow. - Default error handler: the handler named by
defaultErrorHandler-refon the<configuration>global element. - On-Error component: On Error Continue or On Error Propagate, the scopes that hold the processors that run for a matched error.
Key facts
Unhandled messaging error
- Detail
- Logged and propagated by the default handler (
OnErrorPropagateHandler)
HTTP Listener default responses
- Detail
- Success: 200 with the payload; failure: 500 with the error description
Reference a global handler from a flow
- Detail
<error-handler ref="name"/>inside the flow (XML-only)
Set the app default
- Detail
<configuration defaultErrorHandler-ref="name"/>; optional, default none
What
defaultErrorHandler-ref may point at- Detail
- Must be a reference to a global error handler
Studio path for the default
- Detail
- Global Elements > Create > Global Configuration > Configuration > Default Error Handler
Reuse one On-Error component
- Detail
<on-error ref="name"/>inside an<error-handler>
Who can reference a global handler
- Detail
- Flows and Try scopes
Subflows
- Detail
- No built-in error handling; use a Try scope inside
Flow handler versus default
- Detail
- Exam convention: a flow's own handler wins; the default is for flows with none
Sources for this section (5)
- What does a client calling an HTTP Listener flow receive when the flow fails and no error handling is configured?
- Why does the Studio console show an
OnErrorPropagateHandlerentry for a flow that has no Error Handler at all? - What may
defaultErrorHandler-refpoint at, and what is its default? - Why can no global or flow-level On-Error component handle an error of the
CRITICALtype?
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