As most software developers no doubt know, there are two primary models for API design: RPC and REST. Regardless of model, most modern APIs are implemented by mapping them in one way or another to the same HTTP protocol. It has also become common for RPC API designs to adopt one or two ideas from HTTP while staying within the RPC model, which has increased the range of choices that an API designer faces. This post tries to explain the choices, and give guidance on how to choose between them.