# Fault tolerance for HTTP API calls

*MuleSoft Developer II Study Guide · Implement performant and reliable Mule applications · Last updated November 7, 2026 · https://www.certifyforce.com/study-guide/mulesoft-developer-ii/implement-performant-and-reliable-mule-applications/fault-tolerance-for-http-api-calls*

Separate HTTP transport retries from flow retry scopes, preserve failure signals, and predict response, variable and completion behaviour at each boundary.

- Certification: Salesforce MuleSoft Developer II
- Exam section: Implement performant and reliable Mule applications
- Study guide: https://www.certifyforce.com/study-guide/mulesoft-developer-ii

**Official exam objective:**

> Implement fault-tolerant invocations of HTTP-based APIs, reacting correctly to HTTP status codes.

## What the exam expects

- Separate transport retries, response classification and business-state polling.
- Place error propagation and fallback handling at the intended retry boundary.
- Predict variables across failed and successful attempts.
- Interpret synchronous execution, retry pacing and attempt logs correctly.

A **response validator** classifies a received status; a **transport retry** repeats a request after a transport failure; **Until Successful** retries a bounded processor scope; **propagation** exposes an error to its owner; **exhaustion** means the configured retries failed ([HTTP retries](https://docs.mulesoft.com/http-connector/latest/http-request-ref), [Until Successful](https://docs.mulesoft.com/mule-runtime/4.6/until-successful-scope)).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Request reconnection | SDK reconnection configuration does not govern HTTP Request retries. | [HTTP retries](https://docs.mulesoft.com/http-connector/latest/http-request-ref) |
| Built-in disable control | `mule.http.client.maxRetries=0` disables that mechanism. | [HTTP retries](https://docs.mulesoft.com/http-connector/latest/http-request-ref) |
| Default classification | Received statuses 400 and above cause HTTP errors. | [Validator](https://docs.mulesoft.com/http-connector/latest/http-request-ref) |
| Redirect inspection | Follow Redirects governs whether the original redirect is exposed. | [Reference](https://docs.mulesoft.com/http-connector/latest/http-documentation) |
| Business-state polling | A pending 200 needs a failure signal inside the retry scope. | [Scope](https://docs.mulesoft.com/mule-runtime/4.6/until-successful-scope), [Raise Error](https://docs.mulesoft.com/mule-runtime/4.6/raise-error-component-reference) |
| Exhaustion | The final scope failure is `MULE:RETRY_EXHAUSTED`. | [Scope](https://docs.mulesoft.com/mule-runtime/4.6/until-successful-scope) |
| Inner Continue | A handled owner completes successfully and can hide the retry signal. | [On-Error](https://docs.mulesoft.com/mule-runtime/4.6/on-error-scope-concept) |
| Attempt state | Failed variable changes reset; successful output propagates. | [Variables](https://docs.mulesoft.com/mule-runtime/4.6/until-successful-scope) |
| Retry diagnostics | Attempt-level ERROR logs can precede later success. | [Logging](https://docs.mulesoft.com/mule-runtime/4.6/until-successful-scope) |

## Check yourself

- Why does the following processor wait while Until Successful retries?
- When attempt timestamps are uneven, why inspect attempt duration before blaming the interval configuration?
- Does one ERROR-level router retry entry establish the final business outcome?

## Sources

- [Configure HTTP Request Operation](https://docs.mulesoft.com/http-connector/latest/http-request-ref)
- [HTTP Connector Reference](https://docs.mulesoft.com/http-connector/latest/http-documentation)
- [Mule 4.6 Until Successful Scope](https://docs.mulesoft.com/mule-runtime/4.6/until-successful-scope)
- [Mule 4.6 Raise Error Component](https://docs.mulesoft.com/mule-runtime/4.6/raise-error-component-reference)
- [Mule 4.6 Try Scope](https://docs.mulesoft.com/mule-runtime/4.6/try-scope-concept)
- [Mule 4.6 On-Error Components](https://docs.mulesoft.com/mule-runtime/4.6/on-error-scope-concept)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 6 linked practice questions, and unlimited timed mock exams are included with the MuleSoft Developer II pack (USD $19): https://www.certifyforce.com/practice-exams