# Compression in the HTTP Stack

StarView writes SSE through the Plug adapter. It does not set
`Content-Encoding`, negotiate Brotli, or install a compression middleware.
Compression may already be active in your HTTP server, reverse proxy or CDN.
Check the deployed response before adding another layer.

## Check each response type and hostname

Check HTML, the fingerprinted JavaScript/CSS referenced by that HTML, and a
read-only SSE endpoint. Tenant domains may take different paths through your
infrastructure even when they share one Phoenix application.

```sh
curl -sS -o /dev/null -D - -H 'Accept-Encoding: br' https://example.com/
curl -sS -o /dev/null -D - -H 'Accept-Encoding: gzip' https://example.com/
curl -sS -o /dev/null -D - -H 'Accept-Encoding: zstd' https://example.com/
curl -sS -o /dev/null -D - -H 'Accept-Encoding: identity' https://example.com/
```

Look for `Content-Encoding`, `Vary: Accept-Encoding`, the response type and
cache policy. Measure the actual transferred bytes: an omitted
`Content-Length` on a streamed response does not mean the response is empty.
Never label an uncompressed body as compressed, and retain a fallback for
clients that do not advertise the preferred encoding.

## Keep static assets efficient

Run your normal production asset build, minification and Phoenix digest task.
Serve fingerprinted assets with a long immutable cache policy, and enable the
matching `Plug.Static` precompressed-file options only for files your build
actually produces. Phoenix's Gzip output does not also create Brotli files.

A proxy or CDN can negotiate compression for assets without adding a Brotli
dependency to StarView. Do not apply the static asset cache policy to
authenticated HTML or event streams.

## Preserve streaming latency

SSE must arrive while the connection remains open. Test several timed events
through the complete production path; a small response that finishes quickly
does not prove a persistent stream flushes promptly.

If compressing a stream, use a streaming compressor and flush after each event
without ending the compression context. Watch for proxy buffering and minimum
body-size thresholds that delay the first event. Also check clean disconnects
and reconnect snapshots. Do not add unconditional Brotli encoding based only
on an example from another language or adapter.

[Caddy's reverse proxy](https://caddyserver.com/docs/caddyfile/directives/reverse_proxy)
documents immediate flushing for `text/event-stream`, and its
[encode directive](https://caddyserver.com/docs/caddyfile/directives/encode)
documents Gzip and Zstandard support. Verify the deployed Caddy version and
configuration rather than assuming the latest documentation matches it.
[Cloudflare](https://developers.cloudflare.com/speed/optimization/content/compression/)
can negotiate Brotli, Gzip and Zstandard for supported content types; ordinary
HTML compression is not evidence that `text/event-stream` uses the same path.

Choose compression based on measured bandwidth, CPU cost and event latency.
A working Gzip or Zstandard setup may already provide most of the benefit.