Code generation and microservices

2015 12 01

I’ve seen a lot of interest in code generation for microservices lately.

The mapping from an IDL definition to the programming language domain is roughly deterministic. That is, one could write an idl2code generation program, that takes an IDL definition as input, and produces a service definition (type FooService interface), as well as bindings (adapters) from the IDL’s transport to both the generated FooService and individual methods (endpoints). Most IDLs provide compilers that do this for supported target languages.

The mapping is loosely bidirectional. That is, one could also write a code2idl generation program, that takes a service definition (type FooService interface) as input, and produces an IDL definition to one or more transports, as well as bindings from those transports to both the provided FooService and individual methods (endpoints).

But, two notes on that:

• IDLs typically carry more restrictive type and structure semantics than programming languages. That is, it’s easy to write an interface in a programming language that can’t be expressed in a given IDL, whereas it’s hard to write an IDL definition that can’t be expressed in a given programming language.

• Contracts are always built against the service API (i.e. the IDL) and never the implementation (i.e. the code itself). That is, the API (IDL) defines the boundaries and behaviors of the service.

For these reasons, prefer “starting with” the IDL, treating it as the source of truth, and generating the code, rather than starting with the service definition in your programming language, and generating the IDL. (You’ll thank me later.)