Question

I’m trying to work out how to get sessions and flash working in Google App Engine. Could someone provide a clear example using either Ring or Sandbar? I think I have sandbar working, specifically it doesn’t tell me that Var sandbar.stateful-session/sandbar-flash is unbound and when I dump the handler I get :flash and :session though I’m not certain if that is a sandbar session or a ring one. For completeness I will mention that I am using the latest versions of appengine-magic, ring, hiccup and sandbar. There do not appear to be any incompatibilities or issues.

So a clear example preferably with use of flash-put!, flash-get, session-put! and session-get.

Answer 1

I don’t usually like answering my own questions, however in this case I’ll make an exception because:

a) There isn’t a lot of easy to understand examples out there.

b) It would be nice to have a quick working example for others to use.

Note: appengine-magic is not required here, this will also work with normal ring sessions

Code

;; Place in a file called session.clj in the example project
(ns example.session
    "Works with the normal ring sessions 
     allowing you to use side-effects to manage them")

(declare current-session)

(defn wrap-session! [handler]
  (fn [request]
    (binding [current-session (atom (or (:session request) {}))]
      (let [response (handler request)]
        (assoc response :session @current-session)))))

(defn session-get
  ([k] (session-get k nil))
  ([k default] (if (vector? k)
                 (get-in @current-session k)
                 (get @current-session k default))))

(defn session-put!
  ([m]
     (swap! current-session (fn [a b] (merge a m)) m))
  ([k v]
     (swap! current-session (fn [a b] (merge a {k b})) v)))

(defn session-pop! [k]
  (let [res (get @current-session k)]
    (swap! current-session (fn [a b] (dissoc a b)) k)
    res))

(defn session-delete-key! [k]
  (swap! current-session (fn [a b] (dissoc a b)) k))

(defn session-destroy! []
  (swap! current-session (constantly nil)))


;; Place in a file called core.clj in the example project
(ns example.core
  (:use compojure.core
        [ring.middleware.keyword-params :only [wrap-keyword-params]]
        [ring.middleware.session :only [wrap-session]]
        [ring.middleware.session.cookie :only [cookie-store]]
        [example session])
  (:require [appengine-magic.core :as ae]))

(declare current-session)

(defroutes example-app-routes
  (GET "/" _
       (fn [req]
         (let [counter (session-get :counter 0)]
           {:status 200
            :headers {"Content-Type" "text/plain"}
            :body (str "started: " counter)})))
  (GET "/inc" _
       (fn [req]
         (let [counter (do 
                        (session-put! :counter (inc (session-get :counter 0)))
                        (session-get :counter))]
           {:status 200
            :headers {"Content-Type" "text/plain"}
            :body (str "incremented: " counter)}))))

(def example-app-handler
  (-> #'example-app-routes
      wrap-keyword-params
      wrap-session!
      (wrap-session {:cookie-name "example-app-session"
                     :store (cookie-store)})))

(ae/def-appengine-app example-app #'example-app-handler)

How to use it

Navigating to http://127.0.0.1:8080/inc increments the counter in the session and http://127.0.0.1:8080/ will display the value of counter in the session.

wrap-session! is not required for sessions to work, just using

(wrap-session {:cookie-name "example-app-session"
                     :store (cookie-store)})

will give you working functional sessions. However I wanted to manage my sessions with side-effects and wrap-session! provides that functionality. To use flash like functionality, simply use session-put! to put a value into the session and then use session-pop! to remove it.

Hope that’s helpful.