WebSocket is firmly established as the default protocol for building realtime applications. But even if that saves you from choosing between other methods, there are still questions to answer when planning your realtime app.
That’s because although WebSocket gives us a protocol for two-way, low latency communication between clients and servers, there are a lot of things that WebSocket doesn’t solve by itself. For example, WebSocket doesn’t specify a mechanism for automatically reconnecting a broken connection. Similarly, as a messaging-focused protocol, WebSocket is a poor choice for streaming video and audio.
When you choose an approach to help you implement WebSocket in your application, you’ll come across broadly three options:
The default or basic implementation for your framework or language.
A more fully featured library that gives you WebSocket, as well as complementary functionality.
A realtime platform as a service (PaaS), which takes care of everything, including the delivery of the messages.
In this article, we focus on comparing two of those more fully featured libraries, Socket.IO and SockJS, delving into how they build upon WebSocket to address real-world application needs such as scalability and network resilience. To better understand them, we’ll compare:
The features they add on top of basic WebSocket.
How well they deal with real world challenges, such as scaling and unreliable connections.
The languages and frameworks they support.
We will also look at Ably, a realtime PaaS, that offers an alternative to tools such as Socket.IO and SockJS.
What is Socket.IO?
Socket.IO is a realtime messaging library designed for realtime, two-way communication between clients and servers, with an emphasis on low-latency, scalable architecture. It relies on its own transportation protocol, Engine.IO, to establish and manage realtime connections. While WebSocket is one of the protocols that Engine.IO uses, Socket.IO as a whole is not API-compatible with WebSocket. That’s particularly important because a standard WebSocket client cannot connect to a Socket.IO server and neither can a Socket.IO client work with a standard WebSocket server.
Socket.IO key features
Socket.IO sets out to offer a general purpose realtime messaging solution, whereas SockJS aims to emulate the WebSocket API. That gives Socket.IO greater scope to add functionality that would be harder for SockJS to achieve, including:
Protocol agnostic: At the lower level of Socket.IO is a transport layer called Engine.IO, which selects and manages the appropriate messaging technique (such as WebSocket or HTTP long polling) depending on what’s available. By providing a common interface, regardless of what underlying communication method is in use, Engine.IO saves you as a developer working with Socket.IO from having to consider how your messages are delivered. It also makes it easier for Socket.IO to add new protocols and techniques as they become available.
Event-based: Using Socket.IO, you can define and respond to application-specific, custom events. For example, if you’re implementing a chat application using Socket.IO, you could have it trigger an event each time a user starts typing. That could then form the basis of a typing indicator.
Thorough documentation: Socket.IO offers strong documentation, with a mix of tutorials and reference docs. Thanks to its popularity, there are also third-party educational materials on YouTube and elsewhere.
Selects the best technique for the conditions: Socket.IO detects which methods are available depending on the capabilities of the client and on network conditions. That could be WebSocket, HTTP long polling, or the WebTransport API. Socket.IO will also switch to a different protocol should changing network conditions make that necessary.
Automatic reconnections: Socket.IO actively manages each connection, selecting the most appropriate transport and automatically reconnecting in the event of a problem.
Efficient use of bandwidth: One of WebSocket’s advantages over older HTTP-based methods is that it establishes a single, ongoing connection. That reduces overhead from handshake packets. Socket.IO improves on this by using namespaces to divide a single connection into multiple virtual connections, thereby further reducing overhead.
Broadcast messages: Clients can join rooms according to any criteria you choose. For example, you could group all devices owned by a single user into one room, or add members of a particular chat channel to another room. This allows you to broadcast targeted messages to clients.
Some support for scaling: Scaling Socket.IO isn’t supported natively - but there are adapters that enable you to use a third-party tool, such as Redis, as a semi-persistent data store. This allows Socket.IO to coordinate between multiple servers. However, Socket.IO’s architecture is not designed for scaling in this way and, as we’ll see below, it can lead to reduced reliability.
Weak message guarantees: Unfortunately, Socket.IO’s connection management doesn’t extend to individual messages. Instead of offering exactly once delivery, Socket.IO defaults to at most once delivery. In other words, your message might never arrive and Socket.IO has nonetheless kept its promise. For those messages that do arrive, Socket.IO will deliver them in the intended order. If you want the additional development and maintenance burden, you can implement at least once delivery with extra client and server side code. Of course, there is also the risk that Socket.IO will send individual messages multiple times, so your application code will need to take care of that.
Single point of failure: If you scale Socket.IO vertically by adding more memory and CPU capacity, a maintenance window or a fault will take your messaging infrastructure offline. If you scale horizontally, by adding more Socket.IO servers, you’ll need a tool such as Redis to coordinate messages between clients and the different servers. That introduces another moving part to maintain, increases the complexity of your architecture, and shifts the single point of failure from Socket.IO to Redis. You could scale Redis horizontally but now you have two distributed systems to manage.
Single region: Socket.IO is, at its heart, a single server, single datacenter tool. Although there are workarounds, such as using Redis’s Pub/Sub functionality, to scale horizontally, there’s no formal tooling or advice for implementing a multi-region Socket.IO installation. That reduces your application’s resilience, as there’s no fallback region, and it means that end user experience will deteriorate in line with how fine those people are from the datacenter.
Memory leaks: Socket.IO has a history of memory leaks. Although there are ways to help avoid them, they rely on disabling crucial functionality. For example, disabling message compression or HTTP long polling are two potential solutions, but one reduces the efficiency of your connections and the other limits fallback options.
Incompatible with WebSocket: Socket.IO is not WebSocket and doesn’t claim to be. That means you cannot connect a pure WebSocket client to Socket.IO. Instead, you must use Socket.IO everywhere. That’s probably not too much of an inconvenience - but it does open the risk that Socket.IO could diverge significantly from WebSocket, while the rest of the industry standardizes on it.
What is SockJS?
Like Socket.IO, SockJS is a bidirectional messaging library that has both client and server elements. It too builds on WebSocket, offering fallback to methods such as HTTP long polling, but one of the standout differences is that SockJS emulates the WebSocket API. That means SockJS could act as a drop-in replacement for clients built using pure WebSocket libraries. But there is a potential pitfall. The SockJS project maintainers warn that it’s not possible for the library to implement the full WebSocket API.
However, the focus on emulating WebSocket does limit SockJS’s scope for additional functionality. SockJS offers no advanced functionality, such as Socket.IO’s rooms, custom events, or multiplexing. In a world of near-universal native support for WebSocket in browsers and other web clients, the case is weaker for a library whose only value-add is that it can fall back to other protocols should WebSocket be unavailable.
The relative popularity of SockJS compared to Socket.IO seems to reflect that. Although Google Trends tells just one part of the story, search volumes for SockJS have remained significantly lower than for Socket.IO over the years.
Interest (search volume) over time for Socket.IO as a search query vs SockJ as a search query.
SockJS key features
Designed to emulate WebSocket: SockJS aims to be easy to learn and use for developers already familiar with the WebSocket API.
Protocol agnostic: The driving idea behind SockJS is to shield you as a developer from having to plan for situations where WebSocket is unavailable. SockJS gives you the same API no matter which technique or protocol it uses to connect client and server.
Open source: SockJS is available free of charge under the MIT license.
Automatic fallback: Even though WebSocket is available almost everywhere, in those situations where you need to switch to an older HTTP-based technique, SockJS takes care of that automatically.
WebSocket API compatible: If you’re already familiar with WebSocket, SockJS will be easy to learn.
No multiplexing: If you need to separate messages into logical channels, SockJS offers no built-in way to achieve that. Instead, you’ll need to implement namespacing manually. There is a community developed library that implements multiplexing (similar to that offered by Socket.IO) but it was last maintained in 2017.
Unpredictable latency: If you’ve built your application with the tolerances of WebSocket in mind, SockJS’s ability to fall back to slower transport mechanisms could lead to a diminished user experience.
Basic horizontal scaling support: SockJS offers some features that will help you establish sticky sessions through a load balancer, such as HAProxy, but a SockJS cluster is really just a collection of independent SockJS servers. There’s no communication between those servers and so any attempt to horizontally scale SockJS will add to your planning work and to your maintenance schedule.
Should I use Socket.IO or SockJS?
The decision on how to transport data between your application's frontend and backend is a crucial architectural choice that will impact both your application's performance and development process. Opting for a less suitable technology could result in reduced uptime, increased latency, and the need to manually implement features that could be included with other options.
Choosing between Socket.IO and SockJS comes down to six questions:
How important is WebSocket API compatibility to your project?
Do you need more advanced functionality such as automatic reconnections, connection multiplexing, and the option of using new protocols such as WebTransport?
What server-side language or framework are you using?
What is your appetite for risk when it comes to the longevity of the project?
Which performs best?
Which option will get you up and running faster?
Let’s look at each of those in turn.
1. WebSocket compatibility
In some respects, opting for WebSocket API compatibility is the lower risk choice. That’s because it reduces how much you’d need to rearchitect your application should you want to switch to another WebSocket compatible solution.
On the other hand, strict adherence to WebSocket’s API limits the scope for innovative features in your realtime communication layer. And those are features that you will probably want, such as automatic reconnection. So, you’ll end up implementing and maintaining your own solution.
If WebSocket API compatibility is a deal breaker, then out of the two projects we’re comparing here, SockJS would be the solution. However, SockJS has serious limitations and you’ll need to evaluate whether those trade-offs are worth that API compatibility.
2. High level functionality
There’s a clear difference between Socket.IO and SockJS when it comes to value added functionality. SockJS’s promise to mirror the WebSocket API limits the library’s offering, whereas - Socket.IO is free to innovate in ways that the underlying transport protocols might not.
Let’s recap what Socket.IO and SockJS offer when it comes to higher level features:
Multiplexing: Socket.IO uses namespaces to send more than one virtual channel across a single connection. The unofficial SockJS’ add-on for multiplexing is no longer maintained.
Broadcast data: Socket.IO clients can join one or more rooms, which enable broadcast of data to each member client. SockJS does not enable data broadcast without additional work.
Automatic reconnections: Socket.IO’s connection manager automatically handles reconnections and will select the best communication method according to what it is available at the time. SockJS triggers an onclose event if the connection is lost, enabling you to manually reconnect.
Although Socket.IO offers the richer feature set out of the two libraries we’re comparing here, it is still quite limited compared to options such as a realtime platform as a service (PaaS). Ably’s PaaS, for example, goes further by offering presence, message history, native push notifications, and more.
3. Languages and frameworks
Both projects have community supported libraries for other languages and frameworks. However, they are not part of the official maintenance or release process of either project. For example, Socket.IO’s Java server library is primarily the work of a single open source contributor. As an open source project, obviously you could pick up the slack should development slow on that library, but such a commitment will distract from your other roadmap priorities.
4. Support and project maintenance
Socket.IO is still actively developed, with major releases introducing new functionality such as support for WebTransport, which is positioned as an HTTP/3-based update to WebSocket. Frequent minor releases address bugs and vulnerabilities.
5. Socket.IO vs SockJS performance
Let’s look at three performance factors for both Socket.IO and SockJS:
How long it takes to establish a connection.
Capacity for concurrent clients.
Time to send or receive a message.
Research from 2020 put plain WebSocket, Socket.IO, and SockJS head to head. You can read our article on Socket.IO performance to get the full run down. Although some time has passed since that research was conducted, it’s worth noting that SockJS has seen very little development in the intervening years, meaning it is unlikely to have changed.
The key findings are:
Establishing a connection: At 7,000 concurrent connections, SockJS took 470ms to open a new connection, with Socket.IO closer to 400ms.
Concurrent capacity: The study tested a maximum of 10,000 concurrent connections. Socket.IO was able to manage that level of load but SockJS topped out at 7,000 concurrent connections.
Time to receive a message: With 1,000 concurrent connections, it took SockJS 47ms to receive a message from the server, whereas Socket.IO took more than ten times as long at 525ms.
Memory usage: Socket.IO’s memory requirements grew out of scale with the number of concurrent connections. At 5,000 connections it took 500MB of RAM, whereas at 10,000 connections it used 2GB. SockJS’s memory usage was linear up until it hit its apparent limit of 7,000 connections.
6. Will Socket.IO or SockJS get you to market faster?
Documentation, API consistency, error message quality, and SDK coverage are just a few of the factors that impact how fast it is to get something up and running with a library such as Socket.IO or SockJS.
Let’s look at the factors that will impact your long term developer experience and time to market:
Ease of development:
SockJS: If you’re already familiar with the WebSocket API, you should feel at home with SockJS. However, documentation is limited and SockJS no longer has a project website of its own. You’ll also need to implement separately features that SockJS lacks, such as automatic reconnections, which are available as standard in Socket.IO.
Socket.IO: Thanks to its comprehensive documentation, Socket.IO offers a smoother learning experience than SockJS. Its advanced functionality will also save you from having to code those aspects manually.
SockJS: The key risk here is that SockJS development has come almost to a halt in recent years. That could lead to the project becoming unmaintained, which would put the burden of vulnerability and bug fixes on your shoulders.
Socket.IO: Widely used and with active new feature development, Socket.IO appears to be the less risky choice in terms of how much maintenance you’ll need to do yourself.
Community and support: Even before development slowed, it appears that SockJS had less of a community than Socket.IO. The sockjs tag on StackOverflow has 650 questions, compared to almost 21,000 for the socket.io tag. That could mean answers to your development questions are less likely to be easy to find when it comes to SockJS.
Scalability and performance: As the research mentioned above shows, SockJS is quite performant at smaller scales, but in the specific circumstances of that test hit an upper limit of 7,000 concurrent connections. Socket.IO was slower but could handle more connections, albeit with non-linear increases in RAM usage. Neither library is well suited to operating at scale, with half-hearted support for scaling within the same location and no support for multi-region setups.
Ably: a low latency, globally scalable alternative
If it’s a choice between Socket.IO and SockJS, then Socket.IO offers a better solution in every category we’ve looked at in this article. But there’s only so much that an open source library can offer you. Both Socket.IO and SockJS give you the code to run realtime messaging but not the infrastructure.
Building and managing your own realtime infrastructure is a substantial upfront and ongoing investment, which will divert resources from your core functionality. The alternative is to choose a realtime PaaS, such as Ably, that takes care of the full realtime stack.
Delivering your realtime messaging using Ably will give you:
Global network reach: Scale globally thanks to an extensive network of edge locations.
The right protocol: Ably selects the right protocol, whether that’s WebSocket, Server Sent Events, MQTT, or something else. That broadens the reach of your application but offers greater sophistication than the simple fallback offered by SockJS.
<65 ms latency: Create seamless experiences with quick response times.
Resilient delivery: Ably guarantees that messages arrive in order and on time.
99.999% uptime: Ably’s dedicated team works 24/7, so you can have peace of mind even during high-demand periods.
Elastic scaling: From thousands to billions of messages, Ably meets your application’s demands seamlessly.
A great developer experience: with SDKs targeting more than 25 languages and frameworks, integrations with common tooling, and industry-leading documentation, Ably’s developer experience gives you the tools to become productive quickly.
Discover how Ably empowers you to integrate global, scalable, and efficient realtime experiences in your apps, with a developer experience built by developers just like you.