Log Information

There is a long and storied history behind logging on the JVM, and by extension Clojure.

The point of logging is to have some visibility into your running program. For things deployed on a thousand machines this is a harder problem than a handful of servers. All we need is a structured way of outputting text.

• Add tools.logging to your deps.edn

  This is your "logging facade" - what you will use to emit logs.

  {:paths ["src"]
   :deps {org.clojure/clojure {:mvn/version "1.12.0"}
          ring/ring {:mvn/version "1.13.0"}
          metosin/reitit-ring {:mvn/version "0.5.5"}
          org.clojure/tools.logging {:mvn/version "1.3.0"}}
   :aliases {:nREPL
              {:extra-paths ["dev"]
               :extra-deps
                {nrepl/nrepl {:mvn/version "1.2.0"}}}
             :dev {:extra-paths ["dev"]}}}

• Add slf4j-simple to your deps.edn

  This is your "logging implementation" - the thing that determines how logs are actually emitted.

  slf4j-simple is, as the name suggests, the simplest possible implementation. It outputs all logs to standard error. If that is not enough for you then you can pick something else.

  Once you add this you should notice that you no longer get warnings when you start up. In addition, you will start to see logs from the underlying server libraries.

  {:paths ["src"]
   :deps {org.clojure/clojure {:mvn/version "1.12.0"}
          ring/ring {:mvn/version "1.13.0"}
          metosin/reitit-ring {:mvn/version "0.5.5"}
          org.clojure/tools.logging {:mvn/version "1.3.0"}
          org.slf4j/slf4j-simple {:mvn/version "2.0.16"}}
   :aliases {:nREPL
              {:extra-paths ["dev"]
               :extra-deps
                {nrepl/nrepl {:mvn/version "1.2.0"}}}
             :dev {:extra-paths ["dev"]}}}

• Make a log

  Since the server doesn't do much yet, emitting a log for each request that comes in is a decent enough start.

  (ns example.routes
    (:require [reitit.ring :as reitit-ring]
              [clojure.tools.logging :as log]))
  
  ...
  
  (defn root-handler
    [system request]
    (log/info (str (:request-method request) " - " (:uri request)))
    (let [handler (reitit-ring/ring-handler
                    (reitit-ring/router
                      (routes system))
                    #'not-found-handler)]
      (handler request)))