A web service doesn't necessarily need to be called from "the web", meaning from the web browser or via API across the web. It can be called from another web service that's on a local network.

Typically, when called from the web, HTTPS protocol is used to ensure safety of that call. However, local networks are usually secure, meaning no one else has access to it but your own web services.

Thus, communication between local web services will be much faster if it doesn't use a secure protocol as it incurs the performance cost. Simple, fast and unburdened protocols, such as FastCGI, may be better.

FastCGI is interesting because it actually carries the same information as HTTP, so a web service can operate normally, using GET/POST/etc. request methods, passing parameters in URL, request body, environment variables etc. But at the same time, FastCGI is a fast binary protocol that doesn't incur cost of safety - and for local web-service to web-service communication, that's a good thing.

Just like HTTP, FastCGI can separate standard output from standard error, allowing both streams to be intermixed, but retrieved separately.

Overall, inter-web-service communication can be implemented with the aforementioned protocols in a way that preserves high-level HTTP functionality innate to web services, but with overall better performance.