Both WebSockets and Server-Sent Events keep a connection open so a server can push data to a client without being asked each time. That shared description hides how different they are, and picking the wrong one usually shows up months later as accidental complexity.
Here is the decision, and then how to actually test whichever you land on.
The one difference that decides it
WebSockets are bidirectional. SSE is server-to-client only.
Almost every other difference follows from that. If both sides need to send messages over the same connection at the same time, you need WebSockets. If the server does the talking and the client mostly listens, SSE does the job with less machinery.
| WebSockets | Server-Sent Events | |
|---|---|---|
| Direction | Both ways | Server to client only |
| Protocol | ws:// / wss:// — its own protocol after an HTTP upgrade | Plain HTTP, text/event-stream |
| Payload | Text or binary | Text only (UTF-8) |
| Reconnection | You implement it | Built in, automatic |
| Resume after drop | You implement it | Built in, via Last-Event-ID |
| Proxies and CDNs | Sometimes need configuration | Works like any HTTP response |
| Browser API | WebSocket | EventSource |
The reconnection row is the one people underestimate. EventSource reconnects on its own
and tells the server where the client left off. With WebSockets, that is your code, your
backoff strategy, and your bug.
When to reach for SSE
SSE is the right default for anything that reads like a feed:
- Live dashboards and metrics
- Notification and activity streams
- Progress on a long-running job
- Log tailing
- Streaming tokens from a model response
All of these are one-directional. The client's occasional writes — dismissing a
notification, cancelling a job — go over a normal HTTP request, which is fine. You do not
need a persistent bidirectional channel to send one POST every few minutes.
The operational argument is stronger than the API argument: SSE is just an HTTP response that stays open. Your existing auth, logging, rate limiting and CDN behaviour apply unchanged, because as far as the infrastructure is concerned it is a slow download.
When you genuinely need WebSockets
Reach for WebSockets when the client is a real participant, not an audience:
- Chat and collaborative editing, where both sides emit constantly
- Multiplayer state synchronisation
- Anything sending binary frames
- Interactive sessions where round-trip latency on every client message matters
The tell is symmetry. If you find yourself describing the client's messages as "and it also sends…", SSE plus normal requests is probably enough. If client messages are as frequent and as important as server messages, use WebSockets.
Testing a WebSocket endpoint
The mental shift is that there is no single response to assert on. There is a connection, and then a sequence of frames in both directions.
A useful test does four things:
- Connects and confirms the handshake succeeds — including auth, which usually rides in a header or a query parameter on the upgrade request.
- Sends a frame and confirms the server responds with the expected frame.
- Validates the shape of each frame, in both directions, rather than eyeballing it.
- Closes cleanly and checks the close code.
That third step is where most WebSocket bugs actually live. A frame arrives, it looks roughly right, and a field silently changed type three weeks ago. Validating every frame against a schema catches that the day it happens rather than when a client crashes.
In SchemaClient, a WebSocket request connects to a ws:// or wss:// endpoint, shows the
live message log in order, and validates frames in both directions against a schema — the
WebSocket testing guide walks through it.
One practical caveat worth knowing before you design around it: native WebSocket connections do not travel through SchemaClient's localhost tunnel. Point the client at your local server directly for WebSocket work.
Testing an SSE endpoint
SSE is easier to test precisely because it is ordinary HTTP. The endpoint returns
Content-Type: text/event-stream and writes events in a simple text format:
event: price
id: 1042
data: {"symbol":"ACME","price":41.20}
event: price
id: 1043
data: {"symbol":"ACME","price":41.35}
Three things are worth asserting:
The events parse. Each data: line should be valid against whatever shape you promised.
Because events are named, you can validate different event types against different schemas —
a price event and an error event are not the same object.
Resumption works. Kill the connection, reconnect with Last-Event-ID set to the last
event you saw, and confirm the server resumes rather than replaying from the start or
skipping. This is the SSE feature teams get wrong most often, because it works fine until a
client's network blips in production.
The stream ends the way you expect. Some endpoints close after a terminal event; others stay open indefinitely. Know which yours is.
SchemaClient watches events stream in live, supports reconnecting with Last-Event-ID, and
validates events by name — see testing Server-Sent Events.
Unlike WebSockets, SSE does relay through the tunnel, so you can point a hosted service at a
local streaming endpoint.
A reasonable default
If you are unsure, start with SSE. It is less code, it reconnects for free, it behaves like
the rest of your HTTP stack, and it is easier to debug because you can read the stream with
curl. Move to WebSockets when you have a concrete need for client-to-server messages on the
same connection — not before.
The cost of starting with SSE and switching later is one endpoint rewrite. The cost of starting with WebSockets you did not need is a reconnection strategy you maintain forever.