Class ClusterSharding$
- java.lang.Object
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- org.apache.pekko.cluster.sharding.ClusterSharding$
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- All Implemented Interfaces:
ExtensionId<ClusterSharding>
,ExtensionIdProvider
public class ClusterSharding$ extends java.lang.Object implements ExtensionId<ClusterSharding>, ExtensionIdProvider
This extension provides sharding functionality of actors in a cluster. The typical use case is when you have many stateful actors that together consume more resources (e.g. memory) than fit on one machine. - Distribution: You need to distribute them across several nodes in the cluster - Location Transparency: You need to interact with them using their logical identifier, without having to care about their physical location in the cluster, which can change over time.'''Entities''': It could for example be actors representing Aggregate Roots in Domain-Driven Design terminology. Here we call these actors "entities" which typically have persistent (durable) state, but this feature is not limited to persistent state actors.
'''Sharding''': In this context sharding means that actors with an identifier, or entities, can be automatically distributed across multiple nodes in the cluster.
'''ShardRegion''': Each entity actor runs only at one place, and messages can be sent to the entity without requiring the sender to know the location of the destination actor. This is achieved by sending the messages via a
ShardRegion
actor, provided by this extension. TheShardRegion
knows the shard mappings and routes inbound messages to the entity with the entity id. Messages to the entities are always sent via the localShardRegion
. TheShardRegion
actor is started on each node in the cluster, or group of nodes tagged with a specific role. TheShardRegion
is created with two application specific functions to extract the entity identifier and the shard identifier from incoming messages.Typical usage of this extension: 1. At system startup on each cluster node by registering the supported entity types with the
ClusterSharding.start(java.lang.String, org.apache.pekko.actor.Props, org.apache.pekko.cluster.sharding.ClusterShardingSettings, scala.PartialFunction<java.lang.Object, scala.Tuple2<java.lang.String, java.lang.Object>>, scala.Function1<java.lang.Object, java.lang.String>, org.apache.pekko.cluster.sharding.ShardCoordinator.ShardAllocationStrategy, java.lang.Object)
method 1. Retrieve theShardRegion
actor for a named entity type withClusterSharding.shardRegion(java.lang.String)
Settings can be configured as described in thepekko.cluster.sharding
section of thereference.conf
.'''Shard and ShardCoordinator''': A shard is a group of entities that will be managed together. For the first message in a specific shard the
ShardRegion
requests the location of the shard from a centralShardCoordinator
. TheShardCoordinator
decides whichShardRegion
owns the shard. TheShardRegion
receives the decided home of the shard and if that is theShardRegion
instance itself it will create a local child actor representing the entity and direct all messages for that entity to it. If the shard home is anotherShardRegion
, instance messages will be forwarded to thatShardRegion
instance instead. While resolving the location of a shard, incoming messages for that shard are buffered and later delivered when the shard location is known. Subsequent messages to the resolved shard can be delivered to the target destination immediately without involving theShardCoordinator
. To make sure at-most-one instance of a specific entity actor is running somewhere in the cluster it is important that all nodes have the same view of where the shards are located. Therefore the shard allocation decisions are taken by the centralShardCoordinator
, a cluster singleton, i.e. one instance on the oldest member among all cluster nodes or a group of nodes tagged with a specific role. The oldest member can be determined bypekko.cluster.Member#isOlderThan
.'''Shard Rebalancing''': To be able to use newly added members in the cluster the coordinator facilitates rebalancing of shards, migrating entities from one node to another. In the rebalance process the coordinator first notifies all
ShardRegion
actors that a handoff for a shard has begun.ShardRegion
actors will start buffering incoming messages for that shard, as they do when shard location is unknown. During the rebalance process the coordinator will not answer any requests for the location of shards that are being rebalanced, i.e. local buffering will continue until the handoff is complete. TheShardRegion
responsible for the rebalanced shard will stop all entities in that shard by sending them aPoisonPill
. When all entities have been terminated theShardRegion
owning the entities will acknowledge to the coordinator that the handoff has completed. Thereafter the coordinator will reply to requests for the location of the shard, allocate a new home for the shard and then buffered messages in theShardRegion
actors are delivered to the new location. This means that the state of the entities are not transferred or migrated. If the state of the entities are of importance it should be persistent (durable), e.g. withpekko-persistence
so that it can be recovered at the new location.'''Shard Allocation''': The logic deciding which shards to rebalance is defined in a plugable shard allocation strategy. The default implementation
LeastShardAllocationStrategy
picks shards for handoff from theShardRegion
with highest number of previously allocated shards. They will then be allocated to theShardRegion
with lowest number of previously allocated shards, i.e. new members in the cluster. This strategy can be replaced by an application specific implementation.'''Recovery''': The state of shard locations in the
ShardCoordinator
is stored withpekko-distributed-data
orpekko-persistence
to survive failures. When a crashed or unreachable coordinator node has been removed (via down) from the cluster a newShardCoordinator
singleton actor will take over and the state is recovered. During such a failure period shards with known location are still available, while messages for new (unknown) shards are buffered until the newShardCoordinator
becomes available.'''Delivery Semantics''': As long as a sender uses the same
ShardRegion
actor to deliver messages to an entity actor the order of the messages is preserved. As long as the buffer limit is not reached messages are delivered on a best effort basis, with at-most once delivery semantics, in the same way as ordinary message sending. Reliable end-to-end messaging, with at-least-once semantics can be added by usingAtLeastOnceDelivery
inpekko-persistence
.Some additional latency is introduced for messages targeted to new or previously unused shards due to the round-trip to the coordinator. Rebalancing of shards may also add latency. This should be considered when designing the application specific shard resolution, e.g. to avoid too fine grained shards.
The
ShardRegion
actor can also be started in proxy only mode, i.e. it will not host any entities itself, but knows how to delegate messages to the right location.If the state of the entities are persistent you may stop entities that are not used to reduce memory consumption. This is done by the application specific implementation of the entity actors for example by defining receive timeout (
context.setReceiveTimeout
). If a message is already enqueued to the entity when it stops itself the enqueued message in the mailbox will be dropped. To support graceful passivation without losing such messages the entity actor can sendShardRegion.Passivate
to its parentShardRegion
. The specified wrapped message inPassivate
will be sent back to the entity, which is then supposed to stop itself. Incoming messages will be buffered by theShardRegion
between reception ofPassivate
and termination of the entity. Such buffered messages are thereafter delivered to a new incarnation of the entity.
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Field Summary
Fields Modifier and Type Field Description static ClusterSharding$
MODULE$
Static reference to the singleton instance of this Scala object.
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Constructor Summary
Constructors Constructor Description ClusterSharding$()
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description ClusterSharding
createExtension(ExtendedActorSystem system)
Is used by Pekko to instantiate the Extension identified by this ExtensionId, internal use only.ClusterSharding
get(ActorSystem system)
Returns an instance of the extension identified by this ExtensionId instance.ClusterSharding
get(ClassicActorSystemProvider system)
Returns an instance of the extension identified by this ExtensionId instance.ClusterSharding$
lookup()
Returns the canonical ExtensionId for this Extension-
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
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Methods inherited from interface org.apache.pekko.actor.ExtensionId
apply, apply, equals, hashCode
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Field Detail
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MODULE$
public static final ClusterSharding$ MODULE$
Static reference to the singleton instance of this Scala object.
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Method Detail
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get
public ClusterSharding get(ActorSystem system)
Description copied from interface:ExtensionId
Returns an instance of the extension identified by this ExtensionId instance. Java API For extensions written in Scala that are to be used from Java also, this method should be overridden to get correct return type.override def get(system: ActorSystem): TheExtension = super.get(system)
- Specified by:
get
in interfaceExtensionId<ClusterSharding>
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get
public ClusterSharding get(ClassicActorSystemProvider system)
Description copied from interface:ExtensionId
Returns an instance of the extension identified by this ExtensionId instance. Java API For extensions written in Scala that are to be used from Java also, this method should be overridden to get correct return type.override def get(system: ClassicActorSystemProvider): TheExtension = super.get(system)
- Specified by:
get
in interfaceExtensionId<ClusterSharding>
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lookup
public ClusterSharding$ lookup()
Description copied from interface:ExtensionIdProvider
Returns the canonical ExtensionId for this Extension- Specified by:
lookup
in interfaceExtensionIdProvider
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createExtension
public ClusterSharding createExtension(ExtendedActorSystem system)
Description copied from interface:ExtensionId
Is used by Pekko to instantiate the Extension identified by this ExtensionId, internal use only.- Specified by:
createExtension
in interfaceExtensionId<ClusterSharding>
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