Feature Request: Add Public API for Processor Pre/Post Hooks to Aid Debugging

[andreas-habel]

Hi,

Description

To improve the developer experience when writing and testing complex pipelines, one of my most requested features is the ability to properly debug a pipeline.

This would ideally include the ability to:

• Step through each processor's execution
• Run to a specific pipeline step and halt (Breakpoints)
• Inspect the input message before a processor executes
• Inspect the output message after a processor executes (very helpful for branch processors!)

Currently, there doesn't appear to be a public API that allows for this level of introspection, which is essential for building a step-through debugger or a visual pipeline tracer.

Alternatives Considered

My first approach was to try and implement this externally:

• Parse an existing pipeline configuration
• Manually wrap every single processor in a custom "debug" processor (which would record I/O or halt execution)
• Feed this new, enriched pipeline configuration into a StreamBuilder.

This approach failed for two main reasons:

• Complexity: Programmatically parsing and reliably wrapping all processors, especially nested ones (like in branch or switch processors), is extremely difficult and fragile.
• Context Loss: This wrapping "mangles" the original processor paths (e.g., processors.0 becomes processors.0.debug_wrapper.original or something similar). This makes it very difficult to map the execution step back to the original pipeline configuration for visualization.

Proposed Solution

I request a new public API to inject a wrapper or hook around each processor during its construction, without altering the user-defined configuration path.

This could be conceptually similar to the existing BuildTraced() function on the StreamBuilder, but allows for custom pre- and post hook functions.
The pre- and post functions would provide informations about the wrapped processor (name, type, path, config - basically a WalkedComponent) and access to the currect message batch.
Using the return value of the hooks, one could influence the execution and stop the pipeline if desired.