REST is often one of the first architecture and API design approaches that come to mind when application development professionals talk about building microservices. There are many reasons for this, but it mainly comes down to its ability to support stateless communication processes and cultivate independent application services.
There are, however, a few issues that those new to REST may encounter and need to overcome. Learning how REST operates -- and how to implement it properly -- is an essential part of successful microservices development.
What is REST?
REST is an architectural style that facilitates continuous, multichannel communication between services and other associated software components. REST is not a type of development technology, per se, nor is it a strictly defined set of design standards. Instead, it's better defined as a set of universal design principles that provide project constraints for an architectural build.
At its core, REST establishes a client-server architectural structure focused on stateless connectivity. Most RESTful API implementations and processes operate through a series of HTTP and HTTPS methods. However, fundamentally, REST is protocol-agnostic, and can be used alongside alternatives like the Constrained Application Protocol.
The role of REST in a microservices architecture
The principles of microservices architecture dictate that small, autonomous development teams should build, test, deploy and scale their respective application services independently and continuously. To that end, a successful microservices architecture requires solid definitions of API schemas, contextual boundaries and parameters. Back-end infrastructure components must also be able to communicate with those services appropriately, when needed.
The most universally accepted standard for RESTful API development is the OpenAPI Specification standard, which is a collection of documents and contracts that detail how services and components should communicate. With those guidelines, developers can use RESTful APIs to build interfaces that facilitate microservices-based interactions. REST is also an ideal choice if individual microservices need to perform simultaneous operations or require synchronous responses to execute their respective processes.
The value of combining REST and microservices
In a traditional, monolithic application structure, individual application services typically require access to each other's operational back-end data in order to communicate. One of the most compelling benefits of REST in a microservices architecture is that various services can interact without concern for the other's underlying mechanics.
As a result, developers can build, test, deploy and scale each microservice independently without impacting other services or systems. This is what makes REST an ideal architectural choice for building microservices-based applications: The services in a REST-based microservices architecture can evolve independently of one another and efficiently communicate in a stateless manner.
REST also helps promote resource efficiency. Because REST APIs are lightweight, they help microservices communicate quickly with minimum overhead and network latency. RESTful APIs support content negotiation, and you can use several different serialization formats in your code for better performance. For example, it's possible to create a REST API that can handle both a human-readable format like JSON and a binary representation format like Google's protocol buffers.
A minimal RESTful API implementation
The code example below illustrates a basic REST-based API implementation. Note how the RESTful API action named GetAllEmployee acquires the HttpGet attribute, ensuring that it only interacts with other HttpGet requests. The GetAllEmployee action calls a method that queries the EmployeeRepository class, retrieves all employee records, and then returns a list of those records.
Pitfalls of REST, and possible alternatives
REST-based microservices often present development teams with a few notable hurdles, mainly surrounding the challenges of orchestrating multiple, distributed services. Most often, these issues come in the form of increased latency when new services are added, complex error handling processes, tangled code that is hard to debug and blocked messages during synchronous communication.
There are several ways to deal with these pitfalls, such as implementing the pipeline pattern, which adds a layer that processes data based on a predetermined, sequential order of operations. Another remedial approach is to switch from message-driven to event-driven communication, in which service interactions are coordinated through a series of event triggers and commands.
There are some other API design options besides REST that developers can use to build microservices architecture, including GraphQL, gRPC and OData. Some of these emerged to deal with the REST issues listed above, though each also comes with its own set of benefits and drawbacks.
GraphQL is the most commonly discussed alternative to REST. It is a query language that enables a client to send requests that only pull the information they need from a repository, rather than querying all the data within that repository. This can significantly minimize the amount of duplicate or superfluous data fetched during request processes, as well as reduce the number of requests required. As such, it is a good way to avoid underfetching and overfetching problems.
Another open source RPC architecture designed by Google, gRPC, uses HTTP/2 as its underlying transport protocol. As opposed to the text-based data format provided by REST and GraphQL, gRPC uses the protocol buffers data exchange approach. However, unlike REST and GraphQL, gRPC requires developers to create clear and explicit API use documentation (especially API contracts), and demands that both the client and requesting server have access to that documentation.
Open Data Protocol (OData) is an application-level protocol that defines how you can interact with data in RESTful services. OData enables developers to focus on the business logic of a REST-based application without worrying about underlying HTTP methods, how to specify request and response headers, payload formats, query parameters and other back-end concerns.