middleware.md

Middlewares in Ziggurat

Version 3.0.0 of Ziggurat introduces the support of Middleware. Old versions of Ziggurat (< 3.0) assumed that the messages read from kafka were serialized in proto-format and thus it deserialized them and passed a clojure map to the mapper-fn. We have now pulled the deserialization function into a middleware and users have the freedom to use this function to deserialize their messages or define their custom middlewares. This enables ziggurat to process messages serialized in any format.

Custom Middleware usage

The default middleware default/protobuf->hash assumes that the message is serialized in proto format.

(require '[ziggurat.init :as ziggurat])

(defn start-fn []
    ;; your logic that runs at startup goes here
)

(defn stop-fn []
    ;; your logic that runs at shutdown goes here
)

(defn main-fn
  [{:keys [message metadata] :as message-payload}]
    (println message)
    :success)

(defn wrap-middleware-fn
    [mapper-fn :stream-id]
    (fn [message]
      (println "processing message for stream: " :stream-id)
      (mapper-fn (deserialize-message message))))

(def handler-fn
    (-> main-fn
      (wrap-middleware-fn :stream-id)))

(ziggurat/main start-fn stop-fn {:stream-id {:handler-fn handler-fn}})

The handler-fn gets a serialized message from kafka and thus we need a deserialize-message function. We have provided default deserializers in Ziggurat

Deserializing JSON messages using JSON middleware

Ziggurat 3.1.0 provides a middleware to deserialize JSON messages, along with proto. It can be used like this.

(def message-handler-fn
  (-> actual-message-handler-function
      (parse-json :stream-route-config)))

Here, message-handler-fn calls parse-json with a message handler function actual-message-handler-function as the first argument and the key of a stream-route config (as defined in config.edn) as the second argument.