Introduces the entity manager framework

[huntc]

A Rust implementation of Akka Persistence focused on edge-based scenarios. Broad target support, including wasm32-wasi and wasm32-unknown-unknown (browser).

Functionality:

• create, update and delete entities
• event source entities from a generic stream of records
• produce a stream of records of subsequent events
• Effects to emit events, reply data, run async code and an and combinator to compose them in any order
• adapter infrastructure with an implementation for Streambed's Logged file-based commit log
• a fully fledged IoT example that supports HTTP queries on the entity manager, and UDP posts to simulate some type of temperature sensor

Here's a snippet of code that shows how an entity manager can be assembled and used with Logged:

let commit_log = FileLog::new("/dev/null");

let marshaller = MyEventMarshaler;

let file_log_topic_adapter = FileLogTopicAdapter::new(
    commit_log,
    marshaller,
    "some-consumer".to_string(),
    "some-topic".to_string(),
);

let my_behavior = MyBehavior;

let (_, my_command_receiver) = mpsc::channel(10);

EntityManager::new(my_behavior, file_log_topic_adapter, my_command_receiver);

[huntc]

Another API approach could be more similar to Kalix, with separate annotated methods for each command or event handler, and the event sourced behaviour is derived from these. Not familiar with rust ecosystem, but seems that some rust libraries provide both kinds of approaches — direct APIs and derived APIs.

I’ve kept things in line with Akka Persistence as per Patrick’s prototype code. I think there could be subsequent opportunities for building a field based approach on top if required.

[huntc]

I'm quite pleased with this week. We now have a working entity manager. Please check out the unit tests to check out how things hang together.

One thing that I've not yet quite worked out is how a developer of Akka persistence on the JVM expresses that an entity is to be created... I'm thinking that this could be a user's entity command declared for the purposes of creating an entity. An event should still be emitted. Will think on it more, but it'd be great to get some insights re Akka persistence on the JVM. Thanks.

[huntc]

The PR is now functionally complete and ready for a thorough review.

[huntc]

Got together with @pvlugter . Here's what is going to change by tomorrow:

• [x] Return DeleteEntity as an effect and remove entity operations
• [x] Store a deletion flag with a record
• [x] Don’t spawn effect - perform inline - along with applying the event handler
• [x] Replace the sink of events with a function to sink events