The ``ShardRegion`` actor is started on each node in the cluster, or group of nodes
tagged with a specific role. The ``ShardRegion`` is created with two application specific
functions to extract the entity identifier and the shard identifier from incoming messages.
A shard is a group of entities that will be managed together. For the first message in a
specific shard the ``ShardRegion`` request the location of the shard from a central coordinator,
the ``ShardCoordinator``.
The ``ShardCoordinator`` decides which ``ShardRegion`` shall own the ``Shard`` and informs
that ``ShardRegion``. The region will confirm this request and create the ``Shard`` supervisor
as a child actor. The individual ``Entities`` will then be created when needed by the ``Shard``
actor. Incoming messages thus travel via the ``ShardRegion`` and the ``Shard`` to the target
``Entity``.
If the shard home is another ``ShardRegion`` instance messages will be forwarded
to that ``ShardRegion`` instance instead. While resolving the location of a
shard incoming messages for that shard are buffered and later delivered when the
shard home is known. Subsequent messages to the resolved shard can be delivered
to the target destination immediately without involving the ``ShardCoordinator``.
Scenario 1:
#. Incoming message M1 to ``ShardRegion`` instance R1.
#. M1 is mapped to shard S1. R1 doesn't know about S1, so it asks the coordinator C for the location of S1.
#. C answers that the home of S1 is R1.
#. R1 creates child actor for the entity E1 and sends buffered messages for S1 to E1 child
#. All incoming messages for S1 which arrive at R1 can be handled by R1 without C. It creates entity children as needed, and forwards messages to them.
Scenario 2:
#. Incoming message M2 to R1.
#. M2 is mapped to S2. R1 doesn't know about S2, so it asks C for the location of S2.
#. C answers that the home of S2 is R2.
#. R1 sends buffered messages for S2 to R2
#. All incoming messages for S2 which arrive at R1 can be handled by R1 without C. It forwards messages to R2.
#. R2 receives message for S2, ask C, which answers that the home of S2 is R2, and we are in Scenario 1 (but for R2).
To make sure that 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 central
``ShardCoordinator``, which is running as 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 logic that decides where a shard is to be located is defined in a pluggable shard
allocation strategy. The default implementation ``ShardCoordinator.LeastShardAllocationStrategy``
allocates new shards to the ``ShardRegion`` with least number of previously allocated shards.
This strategy can be replaced by an application specific implementation.
To be able to use newly added members in the cluster the coordinator facilitates rebalancing
of shards, i.e. migrate entities from one node to another. In the rebalance process the
coordinator first notifies all ``ShardRegion`` actors that a handoff for a shard has started.
That means they will start buffering incoming messages for that shard, in the same way as if the
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 completed. The ``ShardRegion`` responsible for the rebalanced shard
will stop all entities in that shard by sending the specified ``handOffStopMessage``
(default ``PoisonPill``) to them. When all entities have been terminated the ``ShardRegion``
owning the entities will acknowledge the handoff as completed to the coordinator.
Thereafter the coordinator will reply to requests for the location of
the shard and thereby allocate a new home for the shard and then buffered messages in the
``ShardRegion`` 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. with :ref:`persistence-java`, so that it can be recovered at the new
location.
The logic that decides which shards to rebalance is defined in a pluggable shard
allocation strategy. The default implementation ``ShardCoordinator.LeastShardAllocationStrategy``
picks shards for handoff from the ``ShardRegion`` with most number of previously allocated shards.
They will then be allocated to the ``ShardRegion`` with least number of previously allocated shards,
i.e. new members in the cluster. There is a configurable threshold of how large the difference
must be to begin the rebalancing. This strategy can be replaced by an application specific
implementation.
The state of shard locations in the ``ShardCoordinator`` is persistent (durable) with
:ref:`persistence-java` to survive failures. Since it is running in a cluster :ref:`persistence-java`
must be configured with a distributed journal. When a crashed or unreachable coordinator
node has been removed (via down) from the cluster a new ``ShardCoordinator`` 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 new ``ShardCoordinator`` becomes available.
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 using ``AtLeastOnceDelivery`` in :ref:`persistence-java`.
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.
