# HTTP callbacks and asynchronous responses

*MuleSoft Developer II Study Guide · Expose production-ready Anypoint Platform-managed APIs from Mule applications · Last updated November 7, 2026 · https://www.certifyforce.com/study-guide/mulesoft-developer-ii/expose-production-ready-anypoint-platform-managed-apis-from-mule-applications/http-callbacks-and-asynchronous-responses*

Separate callback acceptance from processing, retain accepted work for the stated deployment, and match delivery retries and authentication to the partner contract. Configure outbound headers without multiplying HTTP clients.

- Certification: Salesforce MuleSoft Developer II
- Exam section: Expose production-ready Anypoint Platform-managed APIs from Mule applications
- Study guide: https://www.certifyforce.com/study-guide/mulesoft-developer-ii

**Official exam objective:**

> Implement HTTP callbacks (webhooks).

## What the exam expects

- Distinguish returning an acknowledgment from completing background work.
- Preserve accepted events under the stated runtime and queue configuration.
- Handle duplicate deliveries and retry only the outcomes the partner defines as retryable.
- Separate outbound client authentication, inbound protection and message signatures.
- Place changing header values at the operation boundary rather than multiplying client configurations.

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Async result | Main-flow execution does not receive the branch's result. | [Async](https://docs.mulesoft.com/mule-runtime/4.6/async-scope-reference) |
| Queue scope | Single-instance persistence uses disk; cluster persistence uses its memory grid. | [VM 2.0](https://docs.mulesoft.com/vm-connector/2.0/vm-reference) |
| Duplicate outcome | A repeated ID raises `MULE:DUPLICATE_MESSAGE`. | [Validator](https://docs.mulesoft.com/mule-runtime/4.6/idempotent-message-validator) |
| Retry unit | Every enclosed processor participates in another attempt. | [Until Successful](https://docs.mulesoft.com/mule-runtime/4.6/until-successful-scope) |
| MAC output | `HMACWith` returns lowercase hexadecimal text. | [HMACWith](https://docs.mulesoft.com/dataweave/2.6/dw-crypto-functions-hmacwith) |
| HTTP header placement | Per-message operation headers can coexist with fixed configuration defaults. | [Request headers](https://raw.githubusercontent.com/mulesoft/docs-connectors/33276ccdb80668dd8bd45102a529a5fcc0918eea/http/1.9/modules/ROOT/pages/http-request-ref.adoc) |

## Check yourself

- Why does changing the idempotent validator's Value Expression fail to change its behavior?
- Why should a test of `HMACWith` expect text rather than raw Binary?
- When request streaming mode is ALWAYS, what separate control does HTTP 1.9 document for avoiding whole-body buffering before sending?

## Sources

- [HTTP Request operation, HTTP Connector 1.9 historical source](https://raw.githubusercontent.com/mulesoft/docs-connectors/33276ccdb80668dd8bd45102a529a5fcc0918eea/http/1.9/modules/ROOT/pages/http-request-ref.adoc)
- [HTTP authentication, HTTP Connector 1.9 historical source](https://raw.githubusercontent.com/mulesoft/docs-connectors/33276ccdb80668dd8bd45102a529a5fcc0918eea/http/1.9/modules/ROOT/pages/http-authentication.adoc)
- [Async Scope, Mule 4.6](https://docs.mulesoft.com/mule-runtime/4.6/async-scope-reference)
- [VM Connector 2.0 Reference](https://docs.mulesoft.com/vm-connector/2.0/vm-reference)
- [Idempotent Message Validator, Mule 4.6](https://docs.mulesoft.com/mule-runtime/4.6/idempotent-message-validator)
- [On-Error Components, Mule 4.6](https://docs.mulesoft.com/mule-runtime/4.6/on-error-scope-concept)
- [Until Successful Scope, Mule 4.6](https://docs.mulesoft.com/mule-runtime/4.6/until-successful-scope)
- [HMACWith, DataWeave 2.6](https://docs.mulesoft.com/dataweave/2.6/dw-crypto-functions-hmacwith)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 8 linked practice questions, and unlimited timed mock exams are included with the MuleSoft Developer II pack (USD $19): https://www.certifyforce.com/practice-exams