Overview

Playground app modelling candidate simple path in a recruitment process implementing Event Sourcing(ES) + Command Query Resposibility Segregation (CQRS) concepts.

Command path is represented by Candidate aggregate. The aggregate is build on streams of sequential events, that are persisted (instead of persisting state itself). Query path is represented by CandidateProjection read model and is build by handling Candidate dispatched events.

Persistance

Aggregate state is reconstructed from stream of persisted StorageEvent (Event Sourcing). Each aggregate modification is manifested by specific type of DomainEvent dispatch. However all DomainEvent types are persisted in the same StorageEvent structure. Transformation between DomainEvent and StorageEvent is bidirectional and implemented by EventMapper class.

Read model synchronization options

• LocalEventPublisher that is a simplified implementation of event bus. There is no message broker, published events are directly passed to event handlers.
• CandidateProjection handler method called in use case method (in the same transaction as Candidate aggregate modification). However it increases coupling between Candidate aggreagte and CandidateProjection read model.
• any implementation of real event bus / message broker (RabbitMQ, Kafka). Thanks to it, read model can be placed in other module / application / system, and update is eventually consistent

Solution architecture

Application Architecture is port and adapters.

• secondary ports (interfaces) in application and domain layers
• secondary adapters (implementations) in infrastructure layer
• primary ports (application service and handlers) in application layer
• primary adapter (API for incoming request) is not implemented

Development

The implementation is simplified and omits some important aspects like:

• events versioning
• concurrency version control
• snapshoting of an aggregate
• genuine event bus implementation
• API to transport incoming requests

Huge part of this code was inspired by: https://breadcrumbscollector.tech/category/event-sourcing/

Install packages

>> pip install -r dev_requirements.txt

Run tests

>> PYTHONPATH=src pytest -x