WebSocket vs REST: Key differences and which to use

WebSocket is a realtime protocol that enables full-duplex, bidirectional communication between a web client and a web server over a persistent, single-socket connection.

Similar to HTTP, WebSocket works on top of TCP. Unlike HTTP, WebSockets are stateful. They're suitable for event-driven apps and services that require high-frequency, low-latency communication between client and server. Use cases include live chat, realtime dashboards, streaming live score updates, multi-user document collaboration, multiplayer gaming, and many more. 

The WebSocket API extends the WebSocket protocol to web clients. Almost all browsers support the WebSocket API natively, making it convenient and easy to start building browser-based realtime apps with WebSockets. 

To communicate via the WebSocket API, the client needs to instantiate a new WebSocket object using the WebSocket constructor - as shown below. The WebSocket object attempts to establish a connection to the server. 

const socket = new WebSocket('wss://example.org', 'myCustomProtocol');

The WebSocket constructor contains a required parameter — the url of the WebSocket server ('wss://example.org' in our example). Additionally, the optional protocols parameter may also be included, to indicate one or more WebSocket subprotocols (application-level protocols) that can be used during the client-server communication. 

Once the connection is established, the client and the server can exchange data. The snippet below shows how the client can send data to the server, using the send() method:

socket.send("Here's some text that the server is urgently awaiting!");

And here’s how the client listens to updates from the server:

socket.onmessage = (event) => {
  console.log(event.data);
}

REST is a software architectural style - probably the most widespread and commonly used one. Unlike WebSocket, REST is not a protocol, but a set of architectural constraints. For a web service to be truly RESTful, it must adhere to the following constraints: 

• Stateless. Web servers don't keep track of previous communication. Every client request must contain the details needed to complete itself effectively.

• Layered system. Client calls and server responses pass through several intermediary layers. 

• Client-Server. REST imposes the client-server design pattern to enforce the separation of concerns. The client app should only know the requested resource's URI (Uniform Resource Identifier). The server application should only return the appropriate response and be unconcerned about the frontend UI. 

• Cacheable. Clients (and intermediaries) must be able to cache resources. Every response should define itself as cacheable or non-cacheable. 

• Uniform interface. There are several guiding principles: individual resources are identified in requests using URIs as identifiers, clients can manipulate resources through representations, messages should be self-descriptive (include enough information about how to process them), and HATEOAS. 

• Code on demand (optional). Servers can temporarily extend or customize the functionality of a client by transferring executable code.

A REST API is a middleman for transferring data between a web client and a web server. It uses the HTTP communication protocol and provides a lightweight way for web clients to perform CRUD (Create, Read, Update and Delete) operations via different methods (or HTTP verbs). The most commonly-used methods are GET, PUT, POST,  DELETE, and PATCH.  

In a nutshell, REST APIs use: 

• GET requests to retrieve resources 

• PUT requests to update resources

• PATCH requests to apply partial modifications to resources

• POST requests to create resources

• DELETE requests to delete resources

For instance, a GET request to Ably's homepage informs the server that a web client wants to view the information on the homepage. If the client request is valid and successful, the web server will return the information requested alongside a status code of 200. 

In this section, we'll directly compare WebSocket vs REST and then summarize their key differences. 

The first key difference is the WebSocket vs REST performance. As they use HTTP, REST APIs have a bigger overhead per message than WebSockets, leading to increased latency and higher CPU usage. This means that REST APIs aren’t optimized for realtime applications that involve low-latency and often frequent messages being sent - WebSockets are a much better choice in these scenarios. 

Another key difference is that WebSocket is stateful, while HTTP, the underlying transport protocol used by RESTful services, is stateless. Arguably, a stateless protocol is easier to implement compared to stateful WebSockets. 

The last key difference we’ll cover is the nature of the communication. HTTP/REST is request-driven and half-duplex; in contrast, WebSockets are event-driven and full-duplex, enabling both the client and the server to send new information as soon as it becomes available, without any need to request updates. 

Whether you should use a REST architecture or use event-driven WebSockets depends on your use case. As mentioned before, WebSockets are generally ideal for web apps that require realtime communication, while REST is tailored for retrieving, creating, and managing resources. 

In the next sections, we'll outline specific use cases for both WebSockets and REST.

We can broadly group Web Sockets use cases into two distinct categories:

• Realtime updates, where the communication is unidirectional, and the server is streaming low-latency (and often frequent) updates to the client. Think of live score updates or alerts and notifications, to name just a few use cases.

• Bidirectional communication, where both the client and the server send and receive messages. Examples include chat, virtual events, and virtual classrooms (the last two usually involve features like live polls, quizzes, and Q&As). WebSockets can also be used to underpin multi-user synchronized collaboration functionality, such as multiple people editing the same document simultaneously.

REST is ideal for:

• Applications that need to perform actions on resources (e.g. retrieve, update) on an occasional, ad-hoc basis.

• Request-driven, non-realtime, and stateless systems.

• CRUD operations.

• Cloud apps, microservices, and web apps.

We hope this article has helped you gain a good understanding of the differences between WebSocket and REST. It’s worth noting that REST and WebSocket are not competing technologies, as they are designed and optimized for different use cases. In fact, they are complementary technologies, that are often used together to engineer web apps. 

For example, let’s say you’ve developed a food delivery app. You can use REST APIs to handle account management operations (e.g., create an account, sign in, reset password), order food, and retrieve your order history for browsing. WebSockets help you embed realtime functionality into your food delivery app, such as live asset tracking, so customers are always aware of where their order is, and when it will arrive.

Ably is a serverless WebSocket platform designed for last-mile messaging. We make it easy for developers to build realtime experiences like chat, live dashboards, and asset tracking for millions of users, without the hassle of managing and scaling infrastructure.

Ably provides:

• Robust and diverse features, including pub/sub messaging, automatic reconnections with continuity, and presence.

• Dependable guarantees: <65 ms round trip latency for 99th percentile, guaranteed ordering and delivery, global fault tolerance, and a 99.999% uptime SLA.

• An elastically-scalable, globally-distributed edge network capable of streaming billions of messages to millions of concurrently-connected devices.

• 25+ client SDKs targeting every major programming language.

In addition to realtime (WebSocket) APIs, we also offer a stateless REST client library. This allows you to easily integrate Ably with a RESTful backend: you can have server-side REST publishes, while Ably clients (end-user devices) can subscribe to realtime updates over WebSockets.