Response from a Callback Function

When you are coding up a handler function, you may return several values. For all handlers you may return either a true or false or a map containing a response that will be merged with the response map built-up at callback time. For instance, the :malformed-request? handler, in the simplest case, would look like the following.

(fn [r] true)

That is, for all request a 400 response would be returned. If you'd like to provide body as well, you could provide this data in a response map.

(fn [r] {:headers {"content-type" "text/plain"}
         :body "The provided request was not data I like."})

Several callbacks are more complicated. For instance, the :is-authorized? callback expects to recieve a true if the client is authorized, a false if they or not or a String that will be used in the "WWW-Authenticate" header for the unauthorized client.

In this case if you need to return a body you may return a sequence. The first item in that sequence should be true or false, the second item should be a response map that will be merged into the response built-up at the time of callback. For instance, to provide a body for the :is-authorized? callback (this would be for unauthorized clients), you might return the following sequence:

[false {:headers {"www-authenticate" "Basic realm=\"Secure\""
                  "content-type" "text/plain"}
        :body "Accounts will lock after three failed attempts"}]

Note that since you are returning a response map instead of the String that the calling functions expect, it's assumed that you are handling the necessary header for the callback. These headers are noted in the comments for the resources. If ignored, your application will demonstrate unpredictable behavior.

Additionally, not all callbacks result in a response being sent to the client. For instance, the :allow-missing-post? callback is used to decide how an incoming request is handled. Returning a body from this function would make no sense. The callback functions that may use the above mechanism to return a response body are listed below.

• :is-conflict?
• :process-post
• :allow-missing-post?
• :delete-completed?
• :valid-entity-length?
• :known-content-type?
• :is-authorized?
• :malformed-request?
• :uri-too-long?
• :service-available? (false or a map if unavailable)