log.clj

(ns example.log)

(defprotocol Logger
  (log [this msg]))

(defn map* [f logger]
  (reify Logger
    (log [this msg] (log logger (f msg)))))


;; Convenience fns. You probably want trace, debug and failure as well. And add :timestamp, :host etc.

(defn info [logger service desc]
  (log logger {:service service :level :info :description desc}))

(defn warn [logger service desc]
  (log logger {:service service :level :warn :description desc}))

(defn error [logger service desc]
  (log logger {:service service :level :error :description desc}))


;; Examples

(defn update-profile [logger user]
  (if (some? user)
    (info logger ::profile.update user)
    (warn logger ::profile.update {:reason "Not found"})))


(defn handle-user-request [logger user-id]
  (let [logger* (map* #(assoc % :user-id user-id) logger)
        user (comment (get-user-from-db))]
    (update-profile logger* user)))


;; Some loggers. Comment them if you don't have the appropriate dependencies

(defrecord StdoutLogger [] Logger
  (log [this msg] (println msg)))


(require '[clj-http.client :as http]
         '[cheshire.core :as json])

(defrecord ElasticSearchLogger [] Logger
  (log [this msg] (http/post "elastic.domain" (json/encode msg))))


(require '[clojure.core.async :as async])

(defrecord AsyncLogger [channel] Logger
  (log [this msg] (async/go (async/>! channel msg))))

(defn try-async []
  (let [{:keys [channel] :as logger} (AsyncLogger. (async/chan))]
   (info logger ::some.data "Hello async")
   (async/go (println (async/<! channel)))
   42))


;; Test logs

(require '[clojure.test :refer :all])

(defrecord TestLogger [logs] Logger
  (log [this msg] (swap! logs conj msg)))

(deftest logs-correct-data
  (let [{:keys [logs] :as logger} (TestLogger. (atom []))]
    (handle-user-request logger 123)
    (is (= [[::profile.update 123]] (map (juxt :service :user-id) @logs)))))