Other system is using Appian's Web API, In case of any process failure how would you show custom error message to requester.

You can handle and return custom error messages in a Web API by configuring the onError parameter of the a!startProcess() function. This ensures that if the system fails to initiate the process, the requester receives a meaningful JSON response instead of a generic server error.

Node-Level Implementation (Web API Expression)

When defining your Web API, you use a!startProcess() to trigger your business logic. You must configure both outcomes:

1. onSuccess: Returns a success code (typically HTTP 200 or 201) and any relevant process information.
2. onError: Catches failures in process initiation and returns your custom error message using the a!httpResponse() function.

Example Implementation Logic:

Use code with caution.

Handling Failures Inside a Running Process

If the Web API starts successfully but the process fails later (at a specific node like a database write), the Web API has already sent its response and cannot "pull it back". To handle this, use one of these best practices:

1. Synchronous Execution (For Short Processes): Use isSynchronous: true in a!startProcess(). This allows the API to wait for the process to complete (up to 30 seconds) and return a response based on the final status of process variables.
2. Validation Before Starting: Perform data validation within the Web API expression before calling a!startProcess(). If validation fails, return an a!httpResponse with a 400 Bad Request and your custom error details immediately.
3. Callback/Webhook Pattern: If the process is long-running, design the process to call an external API (a "callback") upon failure to notify the original system of the error asynchronously