Which of the following is the best scenario for using a custom transformer?

• A. You want to create a basic workspace that converts files from Esri Shapefile to Esri File Geodatabase.
• B. You want to create a reusable and user-friendly way to query an API not officially supported by any FME formats or transformers.
• C. You want to read a custom coordinate system.
• D. You want to write out your data to multiple tables based on attribute values you don't know in advance.

Answer: (B)

Custom transformers are most useful when you need to package and reuse a specialized piece of logic that isn’t covered by built-in transformers. In this scenario, wrapping the process of querying an API that isn’t officially supported by any FME format or transformer into a single reusable unit makes sense. You can define a custom transformer that handles the API calls, authentication, rate limiting, parsing responses, and error handling, while exposing parameters like the API endpoint, credentials, and query options. This enables non-technical users or other workflows to access the API without needing to understand the underlying workspace, and it centralizes maintenance so any changes to the API interaction are made in one place.

The other scenarios are typically achievable with existing FME tools: converting between formats is a straightforward use of readers and writers; reading a custom coordinate system can be done with FME’s CRS definitions; and routing to multiple tables based on attributes can be handled with standard routing and writers.