Reliable delivery between a producer actor sending messages to sharded consumer actors receiving the messages.

The ShardingProducerController should be used together with ShardingConsumerController.

A producer can send messages via a ShardingProducerController to any ShardingConsumerController identified by an entityId. A single ShardingProducerController per ActorSystem (node) can be shared for sending to all entities of a certain entity type. No explicit registration is needed between the ShardingConsumerController and ShardingProducerController.

The producer actor will start the flow by sending a ShardingProducerController.Start message to the ShardingProducerController. The ActorRef in the Start message is typically constructed as a message adapter to map the ShardingProducerController.RequestNext to the protocol of the producer actor.

The ShardingProducerController sends RequestNext to the producer, which is then allowed to send one message to the ShardingProducerController via the sendNextTo in the RequestNext. Thereafter the producer will receive a new RequestNext when it's allowed to send one more message.

In the RequestNext message there is information about which entities that have demand. It is allowed to send to a new entityId that is not included in the RequestNext.entitiesWithDemand. If sending to an entity that doesn't have demand the message will be buffered. This support for buffering means that it is even allowed to send several messages in response to one RequestNext but it's recommended to only send one message and wait for next RequestNext before sending more messages.

The producer and ShardingProducerController actors are supposed to be local so that these messages are fast and not lost. This is enforced by a runtime check.

There will be one ShardingConsumerController for each entity. Many unconfirmed messages can be in flight between the ShardingProducerController and each ShardingConsumerController. The flow control is driven by the consumer side, which means that the ShardingProducerController will not send faster than the demand requested by the consumers.

Lost messages are detected, resent and deduplicated if needed. This is also driven by the consumer side, which means that the ShardingProducerController will not push resends unless requested by the ShardingConsumerController.

Until sent messages have been confirmed the ShardingProducerController keeps them in memory to be able to resend them. If the JVM of the ShardingProducerController crashes those unconfirmed messages are lost. To make sure the messages can be delivered also in that scenario the ShardingProducerController can be used with a DurableProducerQueue. Then the unconfirmed messages are stored in a durable way so that they can be redelivered when the producer is started again. An implementation of the DurableProducerQueue is provided by EventSourcedProducerQueue in akka-persistence-typed.

Instead of using tell with the sendNextTo in the RequestNext the producer can use context.ask with the askNextTo in the RequestNext. The difference is that a reply is sent back when the message has been handled. If a DurableProducerQueue is used then the reply is sent when the message has been stored successfully, but it might not have been processed by the consumer yet. Otherwise the reply is sent after the consumer has processed and confirmed the message.

It's also possible to use the ShardingProducerController and ShardingConsumerController without resending lost messages, but the flow control is still used. This can be more efficient since messages don't have to be kept in memory in the ProducerController until they have been confirmed, but the drawback is that lost messages will not be delivered. See configuration only-flow-control of the ShardingConsumerController.

The producerId is used in logging and included as MDC entry with key "producerId". It's propagated to the ConsumerController and is useful for correlating log messages. It can be any String but it's recommended to use a unique identifier of representing the producer.

If the DurableProducerQueue is defined it is created as a child actor of the ShardingProducerController actor. ProducerController actors are created for each destination entity. Those child actors use the same dispatcher as the parent ShardingProducerController.