Comment by codebeaker | Hacker News Reader

Comment by codebeaker | Hacker News Reader

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Also, according to the spec HTTP servers may not always honour the value in the `Accept-Encoding` header[0].

> Even if both the client and the server supports the same compression algorithms, the server may choose not to compress the body of a response, if the identity value is also acceptable.

I've actually run into this twice in my career and it has been a surprise to those around me in both cases. Both times in the context of small payloads where the server is applying some heuristic about whether to encode or not. (e.g status page stops sending gzipped output when the server is becoming "unhealthy")

[0]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Ac...

Makes more sense to use the verb "negotiating" with Accept-* headers rather than "honoring".

This makes obvious sense once you consider that the client tells the server which compression formats it supports in every request yet not every data format is compressible, nor does the server necessary support any candidate compression format.

For example, the server wouldn't gzip a jpeg since it's already compressed.

All Accept-* headers are like this. e.g. the server doesn't necessarily support any of the languages requested in the Accept-Language header, but it doesn't hurt to ask. You always have to inspect the response headers to see the result of negotiation.

"Accept-Encoding" means only that the client also understands specific encoding (in this case compression) it is still up to the server to chose what to dot. There was a time when browsers didn't support any compression. This header was introduced to signal to server what is acceptable by the client, that's why the header allows specifying multiple compression algorithms.

Similar thing is with header that's quite useful, but for some reason very few sites honor it: "Accept-Language" browser can specify which languages are preferred, but it is up to server to honor it (for example given language version is not available).

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