What is a Sub-flow in Mule 4 used for?

• A. A Sub-flow is a reusable path invoked from flows using flow-ref.
• B. A Sub-flow is triggered directly via HTTP.
• C. A Sub-flow runs in a separate JVM process.
• D. A Sub-flow is used to configure security policies.

Answer: (C)

Sub-flow in Mule 4 is a reusable sequence of processing steps that you invoke from a flow using flow-ref. It has no inbound endpoint and cannot be triggered directly by HTTP. It runs inside the same Mule runtime and the same JVM as the calling flow, making it a lightweight way to share logic like transformations, validations, or error handling across multiple flows. When a flow calls a sub-flow, the sub-flow executes its steps and then control returns to the caller, with data passed back as needed. This approach improves maintainability by avoiding duplicated logic.

The other options don’t fit: a HTTP trigger is for flows with inbound endpoints, not sub-flows; running in a separate JVM process would imply a separate runtime, which sub-flows do not; and configuring security policies is not the role of a sub-flow.