# Reactive execution engine, thread pools and back pressure

*MuleSoft Platform Integration Architect Study Guide · Designing for the runtime plane technology architecture · Last updated November 13, 2026 · https://www.certifyforce.com/study-guide/mulesoft-platform-integration-architect/designing-for-the-runtime-plane-technology-architecture/reactive-execution-engine-thread-pools-and-back-pressure*

How the Mule 4 reactive execution engine classifies work into processing types, runs it on the shared UBER pool, keeps transactions on one thread and applies back pressure, and what changes when you alter global or application-level thread pools or upgrade the runtime.

- Certification: Salesforce MuleSoft Platform Integration Architect
- Exam section: Designing for the runtime plane technology architecture
- Study guide: https://www.certifyforce.com/study-guide/mulesoft-platform-integration-architect

**Official exam objective:**

> Describe the characteristics and implications of the Mule 4 reactive event processing model.

## What the exam expects

- Describe the Mule 4 reactive, task-oriented execution model and the Proactor pattern behind it.
- Classify event processors as CPU-light, blocking I/O or CPU-intensive, and read those types in logs.
- Explain the scope of the UBER pool, the extra pools components create, and thread behavior inside a transaction.
- Predict the impact of global versus application-level thread pool configuration and plan testing for it.
- Explain what triggers back pressure and how each source reacts, and what to do with custom threading when upgrading.

**Processing type:** the kind of work an event processor tells Mule it does. **UBER pool:** Mule's single managed thread pool, shared by the applications of one Mule instance. **Proactor pattern:** an asynchronous design where tasks are submitted by category so threads return to the main loop quickly. **Back pressure:** what happens when Mule has no resources to accept a new event ([Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine)).

## Key facts

| Fact | Detail | Source |
|---|---|---|
| Default processing type | CPU Light when none is specified | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| UBER scope | One pool shared by all apps in a Mule instance, since 4.3 | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| Extra pools | NIO selectors and recurring-task pools can exist beside UBER | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| Transactions | Thread switches suspended; participants share one thread | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| Global file | `MULE_HOME/conf/schedulers-pools.conf`, on premises | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| UBER work queue | `workQueue.size=0` by default | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| App-level pools | New pool set for the app; defaults unchanged | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| Back-pressure triggers | Busy threads or `maxConcurrency` exceeded | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| Source reaction | Connector-specific: 503, wait, drop or disconnect | [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine) |
| LTS cadence | Periodic, scheduled for February, from Mule 4.6 | [Edge and LTS](https://docs.mulesoft.com/release-notes/mule-runtime/lts-edge-release-cadence) |

## Check yourself

- When did the UBER pool become Mule's single thread pool?
- Roughly how long is a CPU-light operation expected to take?
- When the scheduler service stops, what does `gracefulShutdownTimeout` control?
- Why should app-level pool sizes on CloudHub take the worker size into account?

## Sources

- [Execution Engine](https://docs.mulesoft.com/mule-runtime/4.6/execution-engine)
- [Tuning Recommendations](https://docs.mulesoft.com/mule-runtime/4.6/tuning-recommendations)
- [Edge and LTS Releases for Mule](https://docs.mulesoft.com/release-notes/mule-runtime/lts-edge-release-cadence)

## About the full unit

The "Core concepts", "Exam traps" and "Worked scenario" sections of this unit, its 11 linked practice questions, and unlimited timed mock exams are included with the MuleSoft Platform Integration Architect pack (USD $19): https://www.certifyforce.com/practice-exams