[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.