Mule-Dev-201 Tests Predicting Exactly What the Mule Event Holds — Not Drag-and-Drop Familiarity
For developers building Mule 4 apps in Anypoint Studio. Most items show a flow, RAML fragment or DataWeave script and ask what happens next, so trace payload, attributes and variables processor by processor.
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Twelve official sections weighted 5% to 10% are grouped here into seven. Questions are prediction puzzles: read a short flow, RAML or script and choose the logged value, payload or HTTP status. The guide aligns to the Spring '24 release, and Mule 3 syntax is always a wrong answer.
The parent's error handler runs next
The child completed successfully from the parent's view, so the parent carries on after the Flow Reference with the child handler's payload. Had the child used On Error Propagate, the parent's handlers would see the error.
APIkit returns 404 Not Found
The router matches the resource, so it raises APIKIT:METHOD_NOT_ALLOWED, mapped to 405. Unknown paths give 404, invalid parameters or body give 400, and a declared method without a flow gives 501.
The other routes are cancelled immediately
Mule waits for every route, then raises MULE:COMPOSITE_ROUTING carrying successes and failures. On success the result is an object keyed by route index, so read a body as payload['0'].payload.
The more specific NOT_FOUND scope runs
On Error scopes are evaluated top to bottom and the first match wins, so ANY catches it. Put ANY last. A flow with its own handler ignores the global default handler for unmatched errors.
Debug the implementation's DataWeave logic
Gatekeeper blocks traffic until API Manager policies download. Check that autodiscovery uses the API instance ID with valid environment client credentials; the listener should bind 0.0.0.0 on ${http.port}.
For Each restores the original collection when it finishes. Collect results in a variable, or use Parallel For Each, which outputs a collection of results, or a DataWeave map. vars.counter starts at 1.
targetValue defaults to #[message], so the body sits at vars.result.payload. Set targetValue to #[payload] to store the body only. A target leaves the current payload and attributes untouched.
Select always returns an array of objects, so the value is payload[0].email. Bind request values through Input Parameters with :name placeholders rather than concatenating SQL, and add the vendor JDBC driver as a dependency.
Subflows have no source and no error handler and run in the caller's error scope. Use a private flow, which can define one, or wrap steps in a Try scope. Flow Reference shares payload, attributes and variables both ways.
The default accept policy NO_FAILURES skips it; ONLY_FAILURES or ALL change that. Max failed records 0 stops at the first failure and -1 means unlimited. Batch Jobs run asynchronously, so the calling flow continues immediately.
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Mule-Dev-201 asks what the event holds after each processor. Test your tracing accuracy now.