Compression

Looking Glass decodes a compressed request body, so you can read it, and compresses its response when you ask, so you can see whether a proxy or a client decodes it.

At a Glance

• Looking Glass running (see Getting Started).
• gzip or deflate; Looking Glass handles nothing else.

Compressed Requests

A body sent with Content-Encoding: gzip or deflate is reflected decoded, and parsed like any other body:

echo '{"name":"ada"}' | gzip | curl 'localhost:8080/api' --data-binary @- \
  -H 'Content-Encoding: gzip' -H 'Content-Type: application/json' -H 'X-Glass-Format: json'

{
  "body": "{\"name\":\"ada\"}\n",
  "contentEncoding": "gzip",
  "json": { "name": "ada" },
  ...
}

• Bounded: decoding stops at the body limit, so a small upload can't expand past it. A decoded body that reaches the limit is cut off as usual.
• Left as is: a body that doesn't decode, or uses another encoding (br, zstd), is reflected as it arrived, and contentEncoding stays empty.

Compressed Responses

X-Glass-Encoding (or glass-encoding) compresses the response with gzip or deflate, whatever the request's Accept-Encoding says:

curl -si 'localhost:8080/?glass-encoding=gzip' | head -5

The response carries Content-Encoding and Vary: Accept-Encoding. Nothing is compressed unless you ask, so by default you see exactly what a client gets without one.

💡 TIP

A proxy can undo it. Traefik, like any proxy built on Go's HTTP client, adds Accept-Encoding: gzip when the client didn't send one, then decompresses the answer and drops its Content-Encoding. Send Accept-Encoding: gzip yourself (curl --compressed) to see the compressed response come through.

Any other value is answered with 400.

Next Steps

• Response Formats: how bodies are cut off and parsed.
• Configuration: every request option.