From 81f93f53fa722a8f935367c6f705c198c3e4ee65 Mon Sep 17 00:00:00 2001
From: Jonas Chapuis <jonas.chapuis@zf.com>
Date: Sun, 12 Dec 2021 21:42:40 +0100
Subject: [PATCH] Avoid triggering side-effects for read-only commands

---
 documentation/src/main/paradox/effector.md                | 6 +++++-
 .../src/main/scala/endless/runtime/akka/Deployer.scala    | 8 +++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/documentation/src/main/paradox/effector.md b/documentation/src/main/paradox/effector.md
index 9df0249e..39d0cd46 100644
--- a/documentation/src/main/paradox/effector.md
+++ b/documentation/src/main/paradox/effector.md
@@ -14,10 +14,14 @@ trait Self[F[_], Alg[_[_]]] {
 trait Effector[F[_], S] extends StateReader[F, S] with Passivator[F] with Self[F]
 ```
 
-@scaladoc[Effector](endless.core.entity.Effector) is a typeclass used to describe side effects occurring **after** event persistence and entity recovery.
+@scaladoc[Effector](endless.core.entity.Effector) is a typeclass used to describe side effects occurring **after** event persistence and entity recovery. 
 
 Side-effects are typically asynchronous operations such as kafka writes, outgoing REST requests, and [entity passivation](https://doc.akka.io/docs/akka/current/typed/cluster-sharding.html#passivation) (flushing out of memory). `Effector` is used in a `Effector => F[Unit]` function provided upon entity deployment (e.g. @github[BookingEffector](/example/src/main/scala/endless/example/logic/BookingEffector.scala)). In the provided Akka runtime, the resulting `F[Unit]` is executed in *run & forget* mode so that command reply is not delayed by any lengthy side-effect (`Self` can be used to notify success or failure of asynchronous operations back to the entity).
 
+@@@ warning
+In the provided Akka runtime, read-only commands (commands that do not generate events) do not trigger side-effects, which corresponds to sound practice. 
+@@@
+
 @@@ note 
 Defining an effector is entirely optional with the Akka runtime, pass-in `(_, _) => EffectorT.unit` in @scaladoc[deployEntity](endless.runtime.akka.Deployer) to disable effector.
 @@@
diff --git a/runtime/src/main/scala/endless/runtime/akka/Deployer.scala b/runtime/src/main/scala/endless/runtime/akka/Deployer.scala
index f4667742..b5bb7a10 100644
--- a/runtime/src/main/scala/endless/runtime/akka/Deployer.scala
+++ b/runtime/src/main/scala/endless/runtime/akka/Deployer.scala
@@ -231,7 +231,7 @@ trait Deployer {
         .flatMap {
           case Left(error) =>
             Logger[F].warn(error) >> Effect.unhandled[E, Option[S]].thenNoReply().pure
-          case Right((events, reply)) =>
+          case Right((events, reply)) if events.nonEmpty =>
             Effect
               .persist(events.toList)
               .thenRun((state: Option[S]) =>
@@ -250,6 +250,12 @@ trait Deployer {
                 Reply(incomingCommand.replyEncoder.encode(reply))
               }
               .pure
+          case Right((_, reply)) =>
+            Effect
+              .reply[Reply, E, Option[S]](command.replyTo)(
+                Reply(incomingCommand.replyEncoder.encode(reply))
+              )
+              .pure
         }
       dispatcher.unsafeRunSync(effect)
     }