context.clj

(ns unilog.context
  "Provide a way to interact with Mapped Diagnostic Context")

(defn push-context
  "Add a key to the current Mapped Diagnostic Context"
  [k v]
  (org.slf4j.MDC/put (name k) (str v)))

(defn pull-context
  "Remove a key to the current Mapped Diagnostic Context"
  [k]
  (org.slf4j.MDC/remove (name k)))

(defn set-context
  "Sets the current Mapped Diagnostic Context"
  [ctx]
  (org.slf4j.MDC/setContextMap ctx))

(defmacro with-context
  "Execute body with the Mapped Diagnostic Context updated from
   keys found in the ctx map."
  [ctx & body]
  `(if-not (map? ~ctx)
     (throw (ex-info "with-context expects a map" {}))
     (let [copy# (org.slf4j.MDC/getCopyOfContextMap)]
       (try
         (doseq [[k# v#] ~ctx]
           (push-context k# v#))
         ~@body
         (finally
           (set-context (or copy# {})))))))

(defn mdc-fn*
  "Returns a function, which will install the same MDC context map in effect as in
  the thread at the time mdc-fn* was called and then call f with any given
  arguments. This may be used to define a helper function which runs on a
  different thread, but needs to preserve the MDC context."
  [f]
  (let [mdc (org.slf4j.MDC/getCopyOfContextMap)]
    (fn [& args]
      (when (some? mdc)
        (org.slf4j.MDC/setContextMap mdc))
      (apply f args))))

(defmacro mdc-fn
  "Returns a function defined by the given fntail, which will install the
  same MDC context map in effect as in the thread at the time mdc-fn was called.
  This may be used to define a helper function which runs on a different
  thread, but needs to preserve the MDC context."
  [& fntail]
  `(mdc-fn* (fn ~@fntail)))

(comment

  ;; Example usage, you'll need a pattern that shows MDC values
  (require '[clojure.tools.logging :refer [info]])

  (let [f (mdc-fn [] (with-context {:mdcval "bar"} (info "something")))]
    @(future
       (f))))