WebSockets vs Server-Sent Events: Pick One

Many web applications need to show changes as they happen: new messages in a chat, status updates on an order, live dashboards or notifications. Browsers offer two main built-in ways to receive updates without the user refreshing: WebSockets and server-sent events. Both keep a connection open, but they differ in direction, protocol, browser behaviour and how they scale. Choosing between WebSockets vs server-sent events affects how much infrastructure you need, how reconnections work and how easily the feature fits your existing back end. Sometimes the right answer is neither, and simple polling works fine. This guide compares the options and explains when to use each.

How server-sent events work

Server-sent events, often shortened to SSE, let a server push updates to the browser over a long-lived HTTP connection. In the browser, the EventSource interface opens the connection and receives events. MDN notes that this is a one-way connection, so the client cannot send events to the server over it (MDN: Using server-sent events). If the browser needs to send data, it uses ordinary HTTP requests alongside the event stream.

Because SSE runs over standard HTTP, it often works with existing servers, proxies and authentication with modest changes.

How WebSockets work

The WebSocket API, as MDN describes it, makes it possible to open a two-way interactive communication session between the browser and a server, sending messages and receiving responses without polling (MDN: WebSocket API). The connection starts as an HTTP request and is then upgraded to the WebSocket protocol, after which either side can send messages at any time.

WebSockets suit features where the browser and server exchange messages frequently in both directions.

Direction of communication

The clearest difference is direction:

• SSE: server to browser only; browser sends via separate HTTP requests
• WebSockets: both directions over the same connection

If updates flow mostly from server to browser, such as notifications, progress updates or live feeds, SSE is often enough. If the browser sends frequent messages too, such as in chat, collaborative editing or multiplayer features, WebSockets are usually a better fit.

Reconnection and missed messages

Connections drop because of network changes, sleeping devices and server restarts. With SSE, MDN's description of the event stream format notes that if the connection is lost, the browser waits for a reconnection time before attempting to reconnect, and that an id field sets the EventSource object's last event ID. Assigning IDs to events makes it easier to design a way for reconnecting clients to catch up. With WebSockets, reconnection logic and recovery of missed messages are usually handled by your own code or a library.

Whichever you choose, design for missed messages. A client returning after a disconnection may need to fetch the current state through a normal API call before continuing with live updates.

Connection limits

MDN warns that when SSE is not used over HTTP/2, browsers limit the number of open connections per browser and domain to a very low number, six, which can be painful when users open several tabs. Over HTTP/2, many streams can share one connection. Check your hosting and proxies support HTTP/2 if you plan to use SSE widely.

Scaling and infrastructure

Both approaches keep many connections open at once, which affects servers and load balancers:

• Servers must handle many idle, long-lived connections efficiently
• Load balancers and proxies must allow long-lived connections and suitable timeouts
• When running several server instances, messages must reach whichever instance holds each user's connection, often through a shared message broker or publish-subscribe service

WebSockets may need more specific support from proxies and hosting platforms. SSE behaves more like ordinary HTTP, but long-lived responses still need configuration, such as disabling response buffering in proxies.

When polling is enough

Polling means the browser asks the server for updates at intervals. It is simple, works everywhere and uses standard caching and authentication. It is often good enough when:

• Updates are infrequent
• A delay of several seconds or more is acceptable
• User numbers are modest

Polling becomes inefficient when many users poll frequently for updates that rarely change. Our guide to polling vs webhooks discusses similar trade-offs for server-to-server integrations.

Worked example: WebSockets vs server-sent events by feature

The table below shows hypothetical choices for features in a business web application. Decisions depend on each application's needs and infrastructure.

FeatureDirectionUpdate frequencySuggested approachReasonOrder status pageServer to browserOccasionalPolling or SSEInfrequent updates; simple to implementNotification bellServer to browserOccasional to frequentSSEOne-way, works over standard HTTPLive operations dashboardServer to browserFrequentSSEContinuous server updates, no client messages neededCustomer support chatBoth directionsFrequentWebSocketsTwo-way messaging with low delayCollaborative document editingBoth directionsVery frequentWebSocketsContinuous exchange of small changesNightly report ready alertServer to browserRareEmail or polling on page loadReal-time connection not needed

Mixing approaches is common: use the simplest option that meets each feature's needs.

Consider managed real-time services

Running large numbers of real-time connections yourself means handling scaling, reconnections, message fan-out and monitoring. Managed real-time messaging services take on much of this work in exchange for a usage-based fee. They can make sense when real-time features are important but not central to your engineering focus, or when traffic is unpredictable. Compare their costs at your expected connection counts and message volumes, and check how they handle authentication and data residency.

Security considerations

Both connections need authentication and authorisation. Check that users can only receive updates they are allowed to see, validate every message received from browsers over WebSockets as you would any input, use encrypted connections and consider rate limits for clients that send messages.

Performance on the front end

Frequent updates can overwhelm the interface if every message triggers a re-render. Batch or throttle updates, update only the parts of the page that changed and pause non-essential streams for background tabs. Our guide to Next.js performance covers techniques for keeping interfaces responsive.

Test with realistic conditions

Test reconnection by switching networks, sleeping devices and restarting servers. Load test with realistic numbers of open connections and message rates, and confirm proxies and load balancers do not close connections unexpectedly.

Frequently Asked Questions

What is the difference between WebSockets and server-sent events?

Server-sent events provide one-way updates from server to browser over HTTP. WebSockets provide two-way messaging over a single upgraded connection.

When should I use server-sent events?

When updates flow mainly from server to browser, such as notifications, progress and live dashboards, and you prefer staying close to standard HTTP.

When are WebSockets the better choice?

When the browser and server exchange frequent messages in both directions, such as chat or collaborative editing.

Is polling ever better than real-time connections?

Yes, when updates are infrequent, some delay is acceptable and user numbers are modest. It is simpler and works with standard caching.

Add real-time features that hold up

We design and build real-time features with the right mix of WebSockets, server-sent events and polling, plus the infrastructure to scale them. See our custom app development services, or talk to us about real-time web features.

About the Author

Manav Sharma is the founder of Insyrge and a CRM solutions architect. He works across Zoho, Salesforce, HubSpot, Odoo and more, designing the CRM setups, integrations and automations that keep sales and operations teams working from the same data.