Added classic CQRS flow in .NET

[oskardudycz]

Added E-Commerce sample showing the classic Event Sourcing and CQRS flow with EvenStoreDB in .NET.

Overview

• typical Event Sourcing with CQRS flow.
• stores events to EventStoreDB.
• shows how to organise the write model and read model handling.
• Builds read models using Subscription to $all.
• Read models are stored as ElasticSearch documents.
• Shows how to unit and integration test solution.

Write Model

• Provides the basic boilerplate together with Core projects,
• EventStoreDBRepository repository to load and store aggregate state,
• IProjection interface to handle the same way stream aggregation and materialised projections,
• Thanks to that added dedicated AggregateStream method for stream aggregation
• See sample Aggregate and base class

Read Model

• Read models are rebuilt with eventual consistency using subscribe to all EventStoreDB feature,
• Added hosted service SubscribeToAllBackgroundWorker to handle subscribing to all. It handles checkpointing and simple retries if the connection was dropped.
• Added ISubscriptionCheckpointRepository for handling Subscription checkpointing.
• Added checkpointing to EventStoreDB stream with EventStoreDBSubscriptionCheckpointRepository and dummy in-memory checkpointer InMemorySubscriptionCheckpointRepository,
• Added ElasticSearchProjection as a sample how to project with left-fold into external storage. Another (e.g. ElasticSearch, EntityFramework) can be implemented the same way.

Tests

• Added sample of unit testing in Carts.Tests:
  • Aggregate unit tests
  • Command handler unit tests
• Added sample of integration testing in Carts.Api.Tests
  • API integration tests

Other

• EventTypeMapper class to allow both convention-based mapping (by the .NET type name) and custom to handle event versioning,
• StreamNameMapper class for convention-based id (and optional tenant) mapping based on the stream type and module,
• IoC registration helpers for EventStoreDB configuration,

Besides that:

• Added README.md with repository description
• Added CONTRIBUTING.md with contribution guidelines
• set MIT Licence
• Github Action with build, test and acceptance tests for the sample and test report

[oskardudycz]

@ylorph, I think that I applied or answered or your comments. Thanks for checking that :+1: Good feedback.