How would you approach expanding the system to a new type of requests — for example, from FAS instead of FNS?

To expand the system to a new type of requests, for example, from FAS instead of FTS, I would approach it as follows:

1. Requirements analysis: study the specifics of requests from FAS, their format, business logic, and differences from FTS.
2. Modeling: determine which parts of the system need to be changed or extended — for example, adding new handlers, adapters, or services.
3. Architecture design: use extendability patterns, such as strategy or factory, to add support for the new request type without modifying existing code.
4. Implementation: develop a new module or component that will handle requests from FAS.
5. Testing: cover the new functionality with unit and integration tests.
6. Documentation and training: update documentation and, if necessary, train the team.

This approach minimizes risks and allows maintaining the system in an extendable and stable state.