expand-headers

Expand each ring response header with seq-of-String values into separate map entries with string values and keys differentiated by case.

Why

The ring spec defines the format of Clojure response headers as (emphasis mine)

A Clojure map of HTTP header names to header values. These values may be either Strings, in which case one name/value header will be sent in the HTTP response, or a seq of Strings, in which case a name/value header will be sent for each such String value.

This may cause problems for ring apps exposed as AWS API Gateway-proxied Lambdas in datomic ions. The Lambda Proxy Integration does not accept a list of strings as the value for a header. For example, the ring session middleware breaks because of this if you use the cookie session store.

Since browsers treat HTTP headers as case-insensitive, a workaround is to separate each entry in the original map into a different key/value pair in the headers map, with keys differentiated by case. For example,

(require '[net.icbink.expand-headers.core :refer (expand-headers)])

(expand-headers {"a-bcd-efgh-ijkl-mnop-qrst" ["1" "2" "3" "4" "5"]
                 "cAse-lEfT-ALONE-wHeN" "value is a string"})

;=>

{"a-bcd-efgh-ijkl-mnop-qrst" "1",
 "A-bcd-efgh-ijkl-mnop-qrst" "2",
 "a-Bcd-efgh-ijkl-mnop-qrst" "3",
 "A-Bcd-efgh-ijkl-mnop-qrst" "4",
 "a-bCd-efgh-ijkl-mnop-qrst" "5",
 "cAse-lEfT-ALONE-wHeN" "value is a string"}

This library implements this workaround as a ring wrapper.

Usage

Add this repo to your deps.edn as a git dependency, then wrap your ring app with wrap-expand-headers. I think it's safe to add it as the outermost wrapper (i.e., last if you use the -> threading macro), but in any case it must contain all other wrappers that may introduce headers with seq-of-String values.

(require '[net.icbink.expand-headers.core :refer (wrap-expand-headers)])

(defn ring-handler [_]
  {:status 200
   :headers {"a-bcd-efgh-ijkl-mnop-qrst" ["1" "2" "3" "4" "5"]
             "cAse-lEfT-ALONE-wHeN" "value is string"}}
   :body "This better work...")

(def ring-app
  (-> ring-handler
      ;; other ring wrappers here
      wrap-expand-headers))

Status

Beta; API is unlikely to change in breaking ways but this has only been manually tested so far.

Known limitation: an exception will be raised if you have a header with more values to expand than 2^n where n is the number of letters in the header key (i.e., not including numbers, punctuation, or other characters that don't have a case).

HTH!

License

This is distributed under the Eclipse Public License 1.0, the same as Clojure.