WebSockets vs. SSE vs. Long Polling: Choosing the Right Real-Time Strategy for Your SaaS
Your users do not want to refresh a page to see updated data. Whether it is a live dashboard, a collaborative editor, a chat thread, or a payment status update — real-time feedback is no longer a premium feature. It is the baseline expectation. The question is not whether to implement real-time, but which transport layer to build on.
Choose wrong, and you will either be rewriting your infrastructure at 10,000 concurrent users or paying cloud bills that make no sense for your revenue stage.
Here is a practical decision framework for SaaS teams.
The Three Contenders at a Glance
| Mechanism | Direction | Protocol | Connection Overhead |
|---|---|---|---|
| WebSockets | Bidirectional | TCP (ws/wss) | Low (persistent) |
| Server-Sent Events (SSE) | Server → Client | HTTP/1.1 or HTTP/2 | Low (persistent) |
| Long Polling | Simulated push | HTTP | High (repeated) |
These are not interchangeable tools. Each has a sweet spot, and each carries hidden costs when used outside of it.
Long Polling: The Pragmatic Fallback
Long polling works by having the client send an HTTP request, the server holds the connection open until new data is available (or a timeout occurs), then responds and the client immediately fires another request.
It is the oldest trick in the book, but it is not without merit.
When it makes sense:
- You need real-time behavior but your infrastructure cannot support persistent connections (some serverless platforms, shared hosting)
- Message frequency is low — think order status updates or infrequent notifications
- You want maximum compatibility with legacy proxies and firewalls
The hidden costs: Every "poll" is a full HTTP round trip. At scale, this means your server is handling a flood of near-empty requests. If you have 5,000 concurrent users polling every 3 seconds, that is over 1.6 million requests per hour — most of which return nothing. Your load balancer, database connection pool, and compute all absorb that overhead.
// Simple long-poll client loop
async function poll() {
try {
const res = await fetch('/api/events?lastId=' + lastEventId);
const data = await res.json();
handleUpdate(data);
lastEventId = data.id;
} catch (e) {
await sleep(2000); // backoff on error
} finally {
poll(); // immediately re-poll
}
}
Long polling is a valid starting point, not a destination. Build it to ship fast, but plan to replace it.
Server-Sent Events (SSE): The Underrated Middle Ground
SSE is a native browser API that keeps a single HTTP connection open and lets the server push newline-delimited text events down the wire. It is unidirectional — server to client only — but that covers the majority of SaaS real-time use cases.
When it makes sense:
- Live dashboards, analytics feeds, activity logs
- Notification systems where the client does not need to send data back on the same channel
- Teams that want simplicity: SSE is just HTTP, so it works naturally with existing reverse proxies, load balancers, and CDNs
- HTTP/2 multiplexing makes SSE even more efficient — multiple event streams share a single TCP connection
Infrastructure advantage: Because SSE is pure HTTP, you do not need sticky sessions or a special WebSocket-aware proxy configuration. Standard Nginx or Caddy setups handle it without modification.
The limits:
- Browsers cap SSE connections per domain (typically 6 under HTTP/1.1 — solved by HTTP/2)
- You cannot push binary data natively
- No built-in client-to-server messaging on the same channel
For SaaS products where the server is the source of truth and clients are consumers — think analytics platforms, CMS publishing, or monitoring tools — SSE often delivers WebSocket-level responsiveness with half the operational complexity.
WebSockets: High Power, High Responsibility
WebSockets establish a persistent, full-duplex TCP connection. Once the HTTP handshake upgrades the connection, both client and server can send frames at any time with minimal overhead.
When it makes sense:
- True bidirectional, low-latency communication: multiplayer features, collaborative editing (think Google Docs-style), real-time trading interfaces, live support chat
- High message frequency in both directions
- You need sub-100ms perceived latency consistently
The infrastructure tax: WebSockets are stateful. Each open connection lives on a specific server process. This breaks the standard horizontal scaling model where any request can hit any server. You need:
- Sticky sessions at the load balancer, or
- A pub/sub broker (Redis, NATS, or a managed service like Ably or Pusher) to relay messages between server instances
Serverless functions — Lambda, Cloudflare Workers in their base form — do not natively support persistent WebSocket connections without gateway-level workarounds. This is a meaningful architectural constraint for teams building on serverless-first stacks.
Also consider: WebSocket connections that sit idle still consume file descriptors and memory. At 50,000 concurrent idle connections, that is real RAM on your server.
Decision Framework: Which One Do You Actually Need?
Work through these questions:
-
Does the client need to send data to the server in real time?
- Yes → WebSockets
- No → SSE or long polling
-
How frequent are server-side events?
- Multiple times per second → WebSockets or SSE
- Occasional (minutes apart) → Long polling is fine
-
What is your deployment target?
- Serverless/edge → SSE (with streaming responses) or managed WebSocket services
- Traditional servers / containers → Any of the three, with WebSockets requiring pub/sub for horizontal scale
-
What is your team's operational maturity?
- Early-stage, small team → SSE covers most use cases with the least ops overhead
- Mature platform team → WebSockets with a proper broker if the product genuinely needs it
-
Do you have corporate/enterprise users behind strict firewalls?
- Some enterprise proxies block WebSocket upgrades. SSE and long polling over HTTPS port 443 are almost universally permitted.
A Note on Latency in Practice
Theoretical latency for all three approaches can be under 50ms on a well-tuned server. In practice:
- WebSockets consistently deliver 10–50ms message delivery after the handshake
- SSE delivers comparable latency (20–60ms) for server-push scenarios — the extra overhead versus WebSockets is negligible for most use cases
- Long polling introduces artificial latency tied to your poll interval and server hold timeout, often 500ms–3 seconds in real deployments
The latency gap between WebSockets and SSE is rarely the deciding factor. The deciding factor is almost always the operational complexity your team can sustain.
Why This Matters for Your Project
If you are building a SaaS product and adding real-time features, the transport layer you choose today becomes load-bearing infrastructure tomorrow. Starting with long polling is defensible for an MVP. Migrating from SSE to WebSockets later is a contained refactor. But building a WebSocket-first architecture on a serverless platform without a broker is the kind of decision that creates 3 a.m. incidents at scale. Match the tool to your product's actual communication pattern, your team's infrastructure expertise, and the deployment model you are committed to — and you will avoid the most expensive category of real-time architecture mistakes.
