# 6. Choosing, and when the answer is neither

Previous: [5. The limits](05-limits.md)

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## The decision, in one table

| If | Use | Because |
|---|---|---|
| Server pushes, client never speaks | **SSE** | A return type. No broker, no converter stack, no origin policy. Reconnect is free. |
| Clients talk to each other, or to the server, often | **STOMP over WebSocket** | Routing, presence, per-user destinations, and a path to a real broker without touching controllers. |
| One-off progress on an operation the client already started | **SSE** | The stream is scoped to the request. Nothing to clean up. |
| Payloads are binary, or large | **WebSocket** | SSE is line-oriented UTF-8; binary costs a third in base64. |
| The consumer is another service, not a browser | **neither** — see below | Browsers are the only reason to accept SSE's or STOMP's constraints. |
| You need the consumer to say "slow down" | **neither** | Covered in the next article. |

## The costs nobody puts in the table

**SSE costs a connection per stream, per tab.** Over HTTP/1.1 a browser allows about six
concurrent connections per origin, and an open `EventSource` is one of them. Three tabs of your
dashboard leaves three for everything else on the page. Over HTTP/2 they multiplex over one
connection and the limit effectively disappears — so "are we behind HTTP/2 end to end,
including the proxy?" is a real input to this decision, not a detail.

**SSE costs one serialisation per subscriber per event.** The fan-out in
[`EmitterRegistry.broadcast`](../src/main/java/com/ankurm/ssews/EmitterRegistry.java) is a loop:
a thousand open dashboards means a thousand `send()` calls on one scheduler thread, every tick.
The obvious fix — serialise once and write the bytes N times — is not something
`SseEmitter` offers.

**SSE does not pin threads, though.** That objection is wrong, and
[`sse-concurrency.txt`](output/sse-concurrency.txt) is the evidence:

```
=== server.tomcat.threads.max=10, clients=40 ===
emitters open        : 40
connector pool size  : 10 (max 10)
connector active     : 0
one broadcast reached: 40 streams
clients that read it : 40
```

**STOMP costs a stateful server.** Sessions live in one JVM's memory. Two instances behind a load
balancer do not share subscriptions, so a message published on instance A never reaches a
subscriber on instance B — which is the real reason to move to a broker relay, well before
you need persistence.

## Should you build this at all?

Three cases where the honest answer is no:

- **Updates arrive every few minutes.** Polling every thirty seconds is one endpoint, no
  lifecycle, no proxy configuration, no reconnect logic and no leaked emitters. The right answer
  more often than it is chosen.
- **You want a chat and you have one server and no plan for a second.** Everything in
  [chapter 4](04-stomp.md) is real work, and the first time you scale out you will be doing it
  again with a broker relay. Decide about the broker first.
- **The client is another service.** SSE's advantage is that browsers implement it. STOMP's
  advantage is that browsers can speak it. Neither advantage applies service-to-service, and
  neither protocol gives you flow control, deadlines or a schema — which is what the
  companion article on RSocket, gRPC and raw WebSocket is about.

## Everything else, one line each

- **Behind nginx**, a chunked SSE response is buffered by default and the client sees nothing
  until the buffer fills. `proxy_buffering off` or the `X-Accel-Buffering: no` response header.
- **`EventSource` cannot send an `Authorization` header.** Cookie, or a query parameter you then
  have to keep out of access logs.
- **Heartbeats.** SSE: send a `:` comment every N seconds. STOMP: `setHeartbeatValue` plus a
  `TaskScheduler`, and both sides must agree.
- **Compression.** `text/event-stream` compresses extremely well and Boot's HTTP compression is
  off by default; `server.compression.mime-types` must list it explicitly.
- **Surefire does not discover static nested test classes.** Seven of this module's seventeen
  tests silently did not run until they were split into top-level classes. Check the count, not
  the colour.

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Previous: [5. The limits](05-limits.md)