doc: reduce db connections with SliceRangeShardAllocationStrategy (#582)

core/src/test/scala/akka/persistence/r2dbc/PayloadSpec.scala

@@ -17,6 +17,7 @@ import akka.persistence.r2dbc.TestActors.DurableStatePersister
 import akka.persistence.r2dbc.TestActors.Persister
 import akka.persistence.r2dbc.internal.codec.PayloadCodec
 import akka.persistence.r2dbc.internal.codec.PayloadCodec.RichRow
+import akka.serialization.jackson.JsonSerializable
 
 /**
  * The purpose of this test is to verify JSONB payloads, but it can also be run with ordinary BYTEA payloads. To test

core/src/test/scala/akka/persistence/r2dbc/TestConfig.scala

@@ -44,12 +44,6 @@ object TestConfig {
         refresh-interval = 1s
       }
     }
-    akka.actor {
-      serialization-bindings {
-        "akka.persistence.r2dbc.CborSerializable" = jackson-cbor
-        "akka.persistence.r2dbc.JsonSerializable" = jackson-json
-      }
-    }
     akka.actor.testkit.typed.default-timeout = 10s
     """))
       .withFallback(defaultConfig)

docs/src/main/paradox/data-partition.md

@@ -79,6 +79,32 @@ Each data partition corresponds to a table. You can copy the DDL statements for
 data partition suffix. For example `event_journal_0`, `event_journal_0_slice_idx`, `event_journal_1`, `event_journal_1_slice_idx`.
 Note that the index must also reference the parent table with same data partition suffix.
 
+## Reducing number of database connections
+
+When using the @extref:[default allocation strategy for Akka Cluster Sharding](akka:typed/cluster-sharding.html#shard-allocation)
+the there is no correlation between the slice of the entity and to which node the entity will be allocated. That means
+that there will be database connections from an Akka node to each of the databases. With a large Akka cluster each
+database would have to handle many connections, maybe more than its connection limit. That would be an inefficient
+use of resources on both the Akka side and the databases.
+
+To reduce number of connections you can change the allocation strategy to @apidoc[SliceRangeShardAllocationStrategy].
+It will collocate entities with the same slice and contiguous range of slices to the same Akka node. Thereby
+the connections from one Akka node will go to one or a few databases since the database sharding is based on
+slice ranges.
+
+Java
+: @@snip [ShardingDocExample](/docs/src/test/java/jdocs/home/sharding/ShardingDocExample.java) { #sharding-init }
+
+Scala
+: @@snip [ShardingDocExample](/docs/src/test/scala/docs/home/sharding/ShardingDocExample.scala) { #sharding-init }
+
+
+Note that `SliceRangeShardAllocationStrategy` also requires change of the message extractor to
+@apidoc[SliceRangeShardAllocationStrategy.ShardBySliceMessageExtractor].
+
+Do not change shard allocation strategy in a rolling update. The cluster must be fully stopped and then started again
+when changing to a different allocation strategy.
+
 ## Changing data partitions
 
 The configuration of data partitions and databases **must not** be changed in a rolling update, since the data must

docs/src/test/java/jdocs/home/sharding/ShardingDocExample.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright (C) 2024 Lightbend Inc. <https://www.lightbend.com>
+ */
+package jdocs.home.sharding;
+
+//#sharding-init
+import akka.actor.typed.ActorSystem;
+import akka.actor.typed.Behavior;
+import akka.cluster.sharding.typed.SliceRangeShardAllocationStrategy;
+import akka.cluster.sharding.typed.javadsl.ClusterSharding;
+import akka.cluster.sharding.typed.javadsl.Entity;
+import akka.cluster.sharding.typed.javadsl.EntityTypeKey;
+import akka.persistence.Persistence;
+
+//#sharding-init
+
+public class ShardingDocExample {
+  public static class DeviceEntity {
+    public interface Command {
+    }
+
+    public static final EntityTypeKey<Command> ENTITY_TYPE_KEY =
+        EntityTypeKey.create(Command.class, "device");
+
+    public static Behavior<Command> create(String deviceId) {
+      return null;
+    }
+  }
+
+  public static void example() {
+    ActorSystem<?> system = null;
+
+    //#sharding-init
+    ClusterSharding.get(system).init(Entity.of(DeviceEntity.ENTITY_TYPE_KEY, entityContext ->
+            DeviceEntity.create(entityContext.getEntityId()))
+        .withMessageExtractor(new SliceRangeShardAllocationStrategy.ShardBySliceMessageExtractor<>(
+            DeviceEntity.ENTITY_TYPE_KEY.name(), Persistence.get(system)))
+        .withAllocationStrategy(new SliceRangeShardAllocationStrategy(10, 0.1)));
+   //#sharding-init
+  }
+}

docs/src/test/scala/docs/home/sharding/ShardingDocExample.scala

@@ -0,0 +1,38 @@
+/*
+ * Copyright (C) 2024 Lightbend Inc. <https://www.lightbend.com>
+ */
+package docs.home.sharding
+
+//#sharding-init
+import akka.actor.typed.ActorSystem
+import akka.actor.typed.Behavior
+import akka.cluster.sharding.typed.SliceRangeShardAllocationStrategy
+import akka.cluster.sharding.typed.SliceRangeShardAllocationStrategy.ShardBySliceMessageExtractor
+import akka.cluster.sharding.typed.scaladsl.ClusterSharding
+import akka.cluster.sharding.typed.scaladsl.Entity
+import akka.cluster.sharding.typed.scaladsl.EntityTypeKey
+import akka.persistence.Persistence
+
+//#sharding-init
+
+object ShardingDocExample {
+  object DeviceEntity {
+    sealed trait Command
+
+    val EntityKey: EntityTypeKey[Command] =
+      EntityTypeKey[Command]("DeviceEntity")
+
+    def apply(deviceId: String): Behavior[Command] = ???
+  }
+
+  val system: ActorSystem[_] = ???
+
+  //#sharding-init
+  ClusterSharding(system).init(
+    Entity(DeviceEntity.EntityKey)(entityContext => DeviceEntity(entityContext.entityId))
+      .withMessageExtractor(
+        new ShardBySliceMessageExtractor[DeviceEntity.Command](DeviceEntity.EntityKey.name, Persistence(system)))
+      .withAllocationStrategy(new SliceRangeShardAllocationStrategy(10, 0.1)))
+  //#sharding-init
+
+}

project/Dependencies.scala

@@ -88,5 +88,6 @@ object Dependencies {
       // r2dbcPostgres is already a transitive dependency from core, but
       // sometimes sbt doesn't understand that ¯\_(ツ)_/¯
       r2dbcPostgres,
-      TestDeps.akkaPersistenceTyped)
+      TestDeps.akkaPersistenceTyped,
+      "com.typesafe.akka" %% "akka-cluster-sharding-typed" % AkkaVersion)
 }