Promise to pause

Author: mohanr

_posts/2024-11-05-Raft.md

@@ -357,3 +357,60 @@ So the function that executes the election timer based on some conditions is thi
 
 {% endhighlight %}
 
+## Refactoring timer event code
+
+The key requirement that lead to this is a mechanism to pause and restart the timer at various stages. Further testing
+may disprove this hypothesis but for now we will try to reuse the timer instead of discarding and starting a new one.
+
+The OCaml eio library has just such a facility. It is cleaner than the original timer shown above.
+I have shown only that part of the code below.
+
+{% highlight ocaml %}
+
+  let unpaused = ref (Promise.create_resolved ())
+  let unpaused_resolver = ref None
+
+  let pause () =
+    let p, r  = Promise.create() in
+     unpaused := p;
+     unpaused_resolver := Some r
+
+ let unpause () =
+   match !unpaused_resolver with
+   | Some r ->
+    Promise.resolve r ()
+   | None -> ()
+
+ let periodic_timer env =
+   Eio.Switch.run @@ fun _ ->
+   let timeout_duration = election_timeout in
+   let term_started = current_term.currentterm in
+   let mutex = Eio.Mutex.create() in
+   let cond = Eio.Condition.create () in
+   let clock = Eio.Stdenv.clock env in
+   Fiber.both  (fun () ->
+     while true do
+        Promise.await !unpaused;
+        Eio.Condition.broadcast cond;
+        Eio.Time.sleep clock 0.5;
+      done
+  )
+
+  (fun () ->
+    let rec loop () =
+       traceln "Waiting for timer event ";
+       await_timeout cond;
+       checkelection_state mutex timeout_duration term_started;
+
+       traceln "timer event ";
+      Fiber.yield ();
+      loop ()
+    in
+    loop ()
+  );
+
+
+{% endhighlight %}
+
+This code facilitates a mechanism that allows use to _pause_ the timer when we don't need it and start
+it again.