test: HandoverIT-POC - FIX: `TestWriter` must use `Flux#concatMap()`

* `Flux#flatMap()` executes in parallel. Hence, the enqueued messages
  are _not_ send in the order, that they were enqueued.
* Therefore, the `TestWriter` was refactored to use `Flux#concatMap()`,
  which executes serially.

diff --git a/src/test/java/de/juplo/kafka/chat/backend/TestWriter.java b/src/test/java/de/juplo/kafka/chat/backend/TestWriter.java
index a6ac569..aa3c6c1 100644 (file)
--- a/src/test/java/de/juplo/kafka/chat/backend/TestWriter.java
+++ b/src/test/java/de/juplo/kafka/chat/backend/TestWriter.java
@@ -44,8 +44,9 @@ public class TestWriter
         })
         .delayElements(Duration.ofMillis(ThreadLocalRandom.current().nextLong(500, 1500)))
         .map(i -> "Message #" + i)
-        .flatMap(message -> sendMessage(chatRoom, message)
-            .retryWhen(Retry.fixedDelay(30, Duration.ofSeconds(1))))
+        .concatMap(message -> sendMessage(chatRoom, message)
+            .log(user.getName())
+            .retryWhen(Retry.fixedDelay(60, Duration.ofSeconds(1))))
         .doOnNext(message ->
         {
           numSentMessages++;