From 11534551da0ba7853163cf4f2c11679ac2b25bc4 Mon Sep 17 00:00:00 2001
From: dgb23 <denis.baudinot@gmail.com>
Date: Tue, 7 Mar 2023 00:13:15 +0100
Subject: [PATCH] Update composing_routers.md

I wondered how one would wrap a ring handler that can be recreated at runtime without restarting the server. @ikitommi suggested to use the recently added `reloading-ring-handler` as a starting point.

I propose a small example here that illustrates the pattern.
---
 doc/advanced/composing_routers.md | 46 ++++++++++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/doc/advanced/composing_routers.md b/doc/advanced/composing_routers.md
index 4a0f37c9..87a14ea0 100644
--- a/doc/advanced/composing_routers.md
+++ b/doc/advanced/composing_routers.md
@@ -405,9 +405,53 @@ All the beer-routes now match in constant time.
 |-----------------|---------|-----------------------
 | `/beers/sahti`  | 40ns    | static
 
+### Wrapping a swappable ring handler
+
+In order for a ring handler to be recomposed, we can wrap it into a handler that dereferences it on request.
+
+```clj
+(defn deref-handler [rf]
+  (fn
+    ([request] (@rf request))
+    ([request respond raise] (@rf request respond raise))))
+```
+
+A simplified beer router version that creates a ring-handler.
+
+```clj
+(defn create-ring-handler [beers]
+  (ring/ring-handler
+   (ring/router
+    [["/beers"
+      (when (seq beers)
+        (for [beer beers]
+          [(str "/" beer)
+           {:get (fn [_] {:status 200 :body beer})}]))]])))
+
+(def ring-handler
+  (atom (create-ring-handler nil)))
+
+(defn reset-router! [beers]
+  (reset! ring-handler (create-ring-handler beers)))
+```
+
+We don't have any matching routes yet.
+
+```clj
+((deref-handler ring-handler) {:request-method :get :uri "/beers/lager"})
+; nil
+```
+
+But we can add them later.
+
+```clj
+(reset-router! ["lager"])
+((deref-handler ring-handler) {:request-method :get :uri "/beers/lager"})
+; {:status 200, :body "lager"}
+```
+
 ## TODO
 
-* add an example how to do dynamic routing with `reitit-ring`
 * maybe create a `recursive-router` into a separate ns with all `Router` functions implemented correctly? maybe not...
 * add `reitit.core/merge-routes` to effectively merge routes with route data