Akka.Cluster.Sharding: enable prefer-oldest by default on `Replicat…

…or` (akkadotnet#7290)

* Akka.Cluster.Sharding: added tests for DData consistency settings

* Cluster.Sharding: enable `prefer-oldest` by default on `Replicator`

partially resolves akkadotnet#6973

* rolled back changes to sharding sample

src/contrib/cluster/Akka.Cluster.Sharding.Tests/ClusterShardingConfigSpec.cs

@@ -74,5 +74,21 @@ public void Should_cluster_sharding_settings_have_default_config()
             clusterShardingSettings.TuningParameters.CoordinatorStateReadMajorityPlus.Should().Be(5);
             clusterShardingSettings.TuningParameters.CoordinatorStateWriteMajorityPlus.Should().Be(3);
         }
+
+        [Fact]
+        public void ClusterSharding_replicator_settings_should_have_default_values()
+        {
+            ClusterSharding.Get(Sys);
+            var clusterShardingSettings = ClusterShardingSettings.Create(Sys);
+            var replicatorSettings = ClusterShardingGuardian.GetReplicatorSettings(clusterShardingSettings);
+
+            replicatorSettings.Should().NotBeNull();
+            replicatorSettings.Role.Should().BeNullOrEmpty();
+
+            // only populated when remember-entities is enabled
+            replicatorSettings.DurableKeys.Should().BeEmpty();
+            replicatorSettings.MaxDeltaElements.Should().Be(5);
+            replicatorSettings.PreferOldest.Should().BeTrue();
+        }
     }
 }

src/contrib/cluster/Akka.Cluster.Sharding/ClusterShardingGuardian.cs

@@ -297,7 +297,7 @@ public ClusterShardingGuardian(
             });
         }
 
-        private ReplicatorSettings GetReplicatorSettings(ClusterShardingSettings shardingSettings)
+        internal static ReplicatorSettings GetReplicatorSettings(ClusterShardingSettings shardingSettings)
         {
             var config = Context.System.Settings.Config.GetConfig("akka.cluster.sharding.distributed-data")
                 .WithFallback(Context.System.Settings.Config.GetConfig("akka.cluster.distributed-data"));

src/contrib/cluster/Akka.Cluster.Sharding/reference.conf

@@ -207,6 +207,9 @@ akka.cluster.sharding {
   distributed-data {
     # minCap parameter to MajorityWrite and MajorityRead consistency level.
     majority-min-cap = 5
+
+    # ShardCoordinator is singleton running on oldest
+    prefer-oldest = on
 
     durable.keys = ["shard-*"]