Socket.IO vs. WebSocket: Key differences and which to use

WebSocket is a realtime technology that enables bi-directional, full-duplex communication between client and server over a persistent, single-socket connection.

A WebSocket connection starts as an HTTP request/response handshake. If this initial handshake is successful, the client and server have agreed to use the existing TCP connection that was established for the HTTP request as a WebSocket connection. This connection is kept alive for as long as needed (in theory, it can last forever), allowing the server and the client to independently send data at will.

The WebSocket technology includes two core building blocks:

• The WebSocket protocol. Standardized in December 2011 through RFC 6455, the WebSocket protocol enables real time communication between a WebSocket client and a WebSocket server over the web. It supports transmission of binary data and text strings.

• The WebSocket API. Allows you to perform necessary actions, like managing the WebSocket connection, sending and receiving messages, and listening for events triggered by the WebSocket server. Almost all modern browsers support the WebSocket API.

Socket.IO is a realtime library that enables low latency, bi-directional communication between web clients and servers. Socket.IO is built on top of the WebSocket protocol and provides additional capabilities such as automatic reconnections, broadcast support, or falling back to HTTP long polling.

Note that Socket.IO uses Engine.IO under the hood to establish the low-level connection and exchange data between client and server.

For a long time, Socket.IO officially only provided a Node.js server implementation, and a JavaScript client. More recently, the Socket.IO creators have also developed several other clients, in Java, C++, and Swift. 

Beyond the officially-supported implementations, there are multiple other Socket.IO client and server implementations, maintained by the community, although they don’t all support the latest version, which is version 4.  

WebSockets have many advantages, but they’re not without their limitations. Here’s a quick overview.

• Before WebSocket, HTTP techniques like AJAX long polling and Comet were the standard for building realtime apps. Compared to HTTP, WebSocket eliminates the need for a new connection with every request, drastically reducing the size of each message (no HTTP headers). This helps save bandwidth, improves latency, and makes WebSockets less taxing on the server side compared to HTTP.

• Flexibility is ingrained into the design of the WebSocket technology, which allows for the implementation of application-level protocols and extensions for additional functionality (such as pub/sub messaging).

• As an event-driven technology, WebSocket allows data to be transferred without the client requesting it. This characteristic is desirable in scenarios where the client needs to react quickly to an event (especially ones it cannot predict, such as a fraud alert).

• Unlike HTTP, WebSocket is stateful. This can be tricky to handle, especially at scale, because it requires the server layer to keep track of each individual WebSocket connection and maintain state information.

• WebSockets don’t automatically recover when connections are terminated – this is something you need to implement yourself, and is part of the reason why there are many WebSocket client-side libraries in existence.

• Certain environments (such as corporate networks with proxy servers) will block WebSocket connections.

By building on WebSockets, Socket.IO offers some unique advantages, although this comes with some downsides you should be aware of. Let’s take a closer look at how the pros and cons weigh up.

• Socket.IO supports multiplexing through namespaces. Making use of namespaces enables you to minimize the number of TCP connections used, and save socket ports on the server. 

• Socket.IO allows the server side to flexibly broadcast events to all the connected clients. You can also broadcast events to a subset of clients via the rooms feature.

• Socket.IO offers HTTP long-polling as a fallback option, which is useful in environments that don't support WebSockets.

• Socket.IO provides a configurable Ping/Pong heartbeat mechanism, allowing you to detect if a connection is alive or not. Additionally, if and when a client gets disconnected, it automatically reconnects.

• Socket.IO does not guarantee exactly-once messaging semantics. By default, an at-most-once guarantee is provided. Socket.IO can also be configured to provide at-least-once guarantees, although this brings additional engineering complexity – you have to use acknowledgments, timeouts, assign a unique ID to each event, and persist events in the database. 

• Socket.IO comes with limited native security features. For example, it doesn't provide end-to-end encryption, and it doesn't offer a mechanism to generate and renew tokens for authentication.

• Socket.IO is not compatible with any other WebSocket implementation. For example, you can't use a plain WebSocket client with a Socket.IO server; a Socket.IO client will not be able to connect to a plain WebSocket server either.

• Socket.IO is designed to work in a single region, rather than a multi-region architecture. This can lead to issues such as increased latency (if your users are in different regions), and even system downtime - what happens if the datacenter where you have your Socket.IO servers goes through an outage?

As similar technologies, Socket.IO and WebSocket can both be used to deliver realtime features for end-users. We can broadly group WebSocket and Socket.IO use cases into two 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 sports updates, alerts, realtime dashboards, or live location tracking.

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

WebSocket is a technology that enables two-way realtime communication between client and server. In contrast, Socket.IO is a library that provides an abstraction layer on top of WebSockets, making it easier to create realtime applications.

It seems that WebSockets have an edge in terms of performance over Socket.IO; for example, WebSockets have a much lower memory requirement compared to Socket.IO. However, some differences are to be expected. After all, Socket.IO is a more complex (and demanding) solution than raw WebSockets.

Socket.IO provides features such as auto-reconnect, rooms, and fallback to long polling. With vanilla WebSockets, you don’t benefit from such features out of the box; you have to build all these capabilities yourself if they're relevant for your use case.

Generally, Socket.IO enables you to implement production-ready realtime features more quickly, though it’s not as flexible. 

As a reminder, Socket.IO is a library built on WebSockets. It adds features you would certainly need to implement yourself anyway, like a heartbeat and auto-reconnect, as well as features you will probably need - like multiplexing, rooms, and the ability to fallback to long polling in case the browser can’t establish a WebSocket connection. Socket.IO has ready-made libraries for web, Android, and iOS, further adding to the convenience factor and overall appeal. 

If none of these features matter for your project, they might be considered bloat. To shave some kb from your client, and potentially reduce latency a fraction by optimizing around your specific use case, you might consider building on WebSockets directly. However, you should carefully weigh this decision against the time and effort required to essentially recreate the same core functionality offered by Socket.IO across platforms. 

Another consideration when comparing Socket.IO vs WebSocket is server-side scalability. 

If you’re using Socket.IO with Node on the server side, some additional tools are available to help you scale horizontally. That said, scaling WebSockets or Socket.IO is a path fraught with challenges.

In the next section, we’ll touch on the challenges you will experience scaling WebSockets, and offer an alternative that might just solve all your problems. 

Murphy's law, which says “whatever can go wrong, will go wrong,” remains true even in the world of cloud computing. Servers can go offline for various reasons, such as routine updates, bugs, or provider outages. When something inevitably goes wrong, it's your responsibility to ensure a smooth recovery without data loss or disruption to your users.

Recovering from outages is relatively straightforward for traditional HTTP websites. By deploying code on multiple servers and using a proxy to redirect requests, you can handle outages efficiently. However, with WebSockets, the stateful nature complicates matters. If a server goes down, rebalancing the traffic becomes challenging. You would need to terminate all connections, which causes the clients to come thundering home, leading to a congestion point.

Socket.IO provides some resources to assist with these challenges, but they still require significant effort to implement and may encounter intermittent production issues that are difficult to debug. Additionally, Socket.IO is primarily designed for single-region usage, making it time-consuming and complex to create a globally-distributed architecture with high uptime and low latency around the clock.

While WebSockets offer total flexibility, you probably don’t want to reinvent the wheel if you can help it. Instead, it would be better to spend that time on features that matter most to your application.

Socket.IO is a step in the right direction. It builds on WebSockets to make them more resilient and offers convenient features like rooms.

While adapters are available to help scale Socket.IO, the challenge of orchestrating and managing realtime infrastructure that dynamically scales to ensure high throughput and low latency cannot be understated. 

If this isn’t something you have experience with already, the time and effort required will make the time you saved using a library like Socket.IO seem insignificant. 

Thankfully, there are hosted services like Ably available that give you all the benefits of Socket.IO (plus, additional features like user presence) and take care of scaling for you.

Ably is a realtime experience infrastructure, and an alternative to using raw WebSockets or Socket.IO. 

Our APIs and SDKs help developers build and deliver realtime experiences without having to worry about maintaining and scaling messy WebSocket infrastructure. 

Key Ably features and capabilities:

• Pub/sub messaging over serverless WebSockets, with rich features such as message delta compression, automatic reconnections with continuity, user presence, message history, and message interactions.

• A globally-distributed network of datacenters and edge acceleration points-of-presence. 

• Guaranteed message ordering and delivery. 

• Global fault tolerance and a 99.999% uptime SLA.

• < 65ms round-trip latency (P99).

• Dynamic elasticity, so we can quickly scale to handle any demand (billions of WebSocket messages sent to millions of pub/sub channels and WebSocket connections). 

Explore our documentation to find out more and get started with a free Ably account.