merge: #11854

11854: [Backport 8.0]:  Add warning for even replication factor r=oleschoenburg a=Zelldon

## Description

Backports #11831 
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Had to resolve some smaller merge conflicts, in the system context.


Co-authored-by: Christopher Zell <zelldon91@googlemail.com>

broker/src/main/java/io/camunda/zeebe/broker/system/SystemContext.java

@@ -35,6 +35,12 @@ public final class SystemContext {
       "Node id %s needs to be non negative and smaller then cluster size %s.";
   private static final String REPLICATION_FACTOR_ERROR_MSG =
       "Replication factor %s needs to be larger then zero and not larger then cluster size %s.";
+  private static final String REPLICATION_FACTOR_WARN_MSG =
+      "Expected to have odd replication factor, but was even ({}). Even replication factor has no benefit over "
+          + "the previous odd value and is weaker than next odd. Quorum is calculated as:"
+          + " quorum = floor(replication factor / 2) + 1. In this current case the quorum will be"
+          + " quorum = {}. If you want to ensure high fault-tolerance and availability,"
+          + " make sure to use an odd replication factor.";
   private static final String SNAPSHOT_PERIOD_ERROR_MSG =
       "Snapshot period %s needs to be larger then or equals to one minute.";
   private static final String MAX_BATCH_SIZE_ERROR_MSG =
@@ -96,6 +102,10 @@ private void validateConfiguration() {
 
     final var dataCfg = brokerCfg.getData();
 
+    if (replicationFactor % 2 == 0) {
+      LOG.warn(REPLICATION_FACTOR_WARN_MSG, replicationFactor, (replicationFactor / 2) + 1);
+    }
+
     final var snapshotPeriod = dataCfg.getSnapshotPeriod();
     if (snapshotPeriod.isNegative() || snapshotPeriod.minus(MINIMUM_SNAPSHOT_PERIOD).isNegative()) {
       throw new IllegalArgumentException(String.format(SNAPSHOT_PERIOD_ERROR_MSG, snapshotPeriod));