[mozey]

Have to admit I've never used this code, and didn't know what it was about. Quickly read up about it. So ETag is a hash of the resource. You must provide it with requests that modify the resource. If your hash doesn't match the server hash, then 412 Precondition Failed is returned?

https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/412

https://www.rfc-editor.org/rfc/rfc9110#status.412

[alganet]

You can provide all sorts of conditions using HTTP headers such as "If-Match", "If-None-Match", "If-Modified-Since", "If-Range", etc. The server can chose an HTTP code to indicate some sort of cache invalidation signal.

304 means "you're good, your cached version satisfies the conditions"

412 means "you're not good, your cached version does not satisfy the conditions"

412 usually applies to modifications, but it could be for reading too, in the case of ranged requests (getting a specific range of bytes from a large representation). See the "Range" header.

These are all interconected. The headers, the codes, etc. They are very useful for caching and can save a lot of bandwidth. Browsers and CDNs use them extensively. Server-to-server communcation could use them as well, but I haven't seen popular implementations (let's say, a web framework that provides abstraction over these mechanisms).

[alganet]

Also, other popular HTTP codes have cache implications.

For example, 404 implies "No indication is given of whether the condition is temporary or permanent". Cache invalidation headers don't apply to this code because a 404 means there's something about the _resource_ and not only the _representation_ that could not be found. That client cannot cache the 404 result, not even for a fraction of a second.

404's brother 410 implies "This condition is expected to be considered permanent.". A client that gets a 410 can cache that result, never needing to reach the server again. It means it's gone forever. That client can decide to never look up that URI again.

Very often, "400 Bad Request" is the best HTTP you can use if you are not sure what to use. Then, describe what the error means using other HTTP components and/or the response body.

HTTP can be very simple. GET (ask for a representation) and POST (send a representation) as methods only. 200 (success), 400 (client error) and 500 (server error) as response codes only. It's the best way to start, then move to more elaborate protocol features as you learn.