Issue #11/clearing default registry

README.md

@@ -322,6 +322,33 @@ This is particularly useful for applications that do not expose an HTTP port
 themselves but shall still be scraped by Prometheus. By default, metrics will
 be exposed at `/metrics`.
 
+### Caveats
+
+#### Using the default registry and reload/refresh
+
+Many prometheus integration prefer the default registry
+(`CollectorRegistry/defaultRegistry`) and cannot be provoked to do
+otherwise. Using these and also at the same time using a workflow like
+[reloaded](http://thinkrelevance.com/blog/2013/06/04/clojure-workflow-reloaded)
+may cause the _underlying_ default registry to be initialized multiple
+times. A workaround is to use prometheus own `CollectorRegistry.clear`
+as a first step in initialization to make sure the default registry is
+clean and reusable. This will (of course) completely wipe the current
+state of the underlying `CollectorRegistry/defaultRegistry`.
+
+An example:
+```
+(defonce my-default-registry []
+  (do
+    (-> registry/default
+        ^CollectorRegistry (registry/raw)
+        (.clear))
+    (-> registry/default
+        ;; initializers or registrations
+	;; like...
+        (fn/initialize))))
+```
+
 ## License
 
 ```

test/iapetos/registry/default_registry_test.clj

@@ -0,0 +1,36 @@
+(ns iapetos.registry.default_registry_test
+  (:require [iapetos.registry :as registry]
+            [iapetos.core :as prometheus]
+            [iapetos.collector.fn :as fn]
+            [clojure.test :refer [deftest is testing]])
+  (:import [io.prometheus.client CollectorRegistry]))
+
+(defn init-default-registry []
+  (do
+    (-> registry/default
+        ^CollectorRegistry (registry/raw)
+        (.clear))
+    (-> registry/default
+        ;; ...other initializers or registrations
+        (fn/initialize))))
+
+(defn test-fn []
+  "EHLO")
+
+(deftest default-registry
+  (is (some? (registry/raw registry/default)))
+
+  (testing "default registry can be initialized a first time"
+    (let [default-registry (init-default-registry)]
+      (is (some? default-registry))
+      (fn/instrument! default-registry #'test-fn)
+      (is (string? (test-fn)))))
+
+  (testing "default registry can be initialized a second time"
+    (let [default-registry (init-default-registry)]
+      (is (some? default-registry))
+      (is (some? (prometheus/value default-registry :fn/duration-seconds)))
+      (fn/instrument! default-registry #'test-fn {:fn-name "test-fn"})
+      (is (string? (test-fn)))
+      (is (= 1.0 (:count (prometheus/value default-registry :fn/duration-seconds {:fn "test-fn"}))))))
+  )