In multicomponent applications, effective communication is key, and it heavily relies on well-defined contracts between components. Microservices predominantly employ two communication approaches: event-driven and API-first.
Event-Driven Architecture
Event-driven architecture is a paradigm where microservices communicate by generating and consuming events. Triggered by system internal or external changes, events serve as contracts in an event-driven structure.
API-First Approach
The API-first design is characterized by APIs serving as contracts that facilitate communication with a service. Essentially, the API is the intermediary between the system components. APIs, particularly HTTP APIs, are the primary means for external systems and developers to access system functionality and data.
HTTP APIs usually employ one of two protocols, REST or gRPC:
REST
Leverages a text-based format (typically JSON), offering more flexibility than gRPC but potentially leading to increased maintenance challenges.
gRPC
Utilizes a binary format (Protocol Buffers) and has a more rigid structure than REST. The API is specified in a contract (a .proto file) that outlines the request and response messages and permissible operations. Owing to its defined structure, gRPC APIs can be easier to maintain and more efficient due to their binary format.
A gRPC or REST API can be designed for either synchronous or asynchronous communication. The choice is dictated by its design and implementation.
In a synchronous communication model, the client sends a request and waits for a response before proceeding. The client is blocked until a response is received or a time-out occurs. While synchronous communication is straightforward to use, it may be less efficient as the client must wait for a response before continuing.
In contrast, asynchronous communication allows the client to send a request and continue processing while waiting for a response. The client is not blocked while waiting for the response before continuing. Asynchronous communication can be more efficient as it allows the client to continue processing while waiting for a response, but it might introduce complexity due to the asynchronous handling of responses.
Both REST and gRPC can be implemented in either synchronous or asynchronous styles depending on the requirements. However, due to ease of use and familiarity, REST is often chosen when exposing APIs externally, while gRPC might be preferred for internal microservices communication due to its performance and strong contract enforcement.
gRPC: Up and Running by Kasun Indrasiri and Danesh Kuruppu (O’Reilly) offers a comprehensive discussion on gRPC. API design, a vast subject in itself, is covered in many dedicated books.