Messaging and contracts

In monolithic applications, messaging between components is mostly achieved using function calls, whereas for microservices, it is achieved using lightweight messaging systems, often HTTP(S). Using a lightweight messaging system is one of the most promising features of microservices and makes it easier to adopt and scale, as compared to service-oriented architecture (SOA) that uses a complex messaging system with multiple protocols. Microservices are more about keeping the endpoints smart and the communication channels as simple as possible.

In a microservices architecture, often multiple microservices need to interact with each other to achieve a particular task. These interactions can be either direct, via request-response-based (https://en.wikipedia.org/wiki/Request-response) communication, or through a lightweight message-oriented middleware (MOM) (https://en.wikipedia.org/wiki/Message-oriented_middleware). Direct messaging is synchronous, that is, the requester waits for the response to be returned, whereas a message-oriented middleware is primarily used for asynchronous communication.