Event store — appends and retrieves domain events. Uses a sequenced item mapper with a record manager to map domain events to database records in ways that can be easily extended and replaced.
Layer base classes — suggest how to structure an event sourced application. The library has base classes for application objects, domain entities, entity repositories, domain events of various types, mapping strategies, snapshotting strategies, cipher strategies, etc. They are well factored, relatively simple, and can be easily extended for your own purposes. If you wanted to create a domain model that is entirely stand-alone (recommended by purists for maximum longevity), you might start by replicating the library classes.
Notifications and projections — reliable propagation of application events with pull-based notifications allows the application state to be projected accurately into replicas, indexes, view models, and other applications.
Process and systems — scalable event processing with application pipelines. Runnable with single thread, multiprocessing on a single machine, and in a cluster of machines using the actor model. Parallel pipelines are synchronised with causal dependencies.
Versioning - allows model changes to be introduced after an application has been deployed. Both domain events and domain entity classes can be versioned. The recorded state of an older version can be upcast to be compatible with a new version. Stored events and snapshots are upcast from older versions to new versions before the event or entity object is reconstructed.
Snapshotting — avoids replaying an entire event stream to obtain the state of an entity. A snapshot strategy is included which reuses the capabilities of this library by implementing snapshots as events.
Hash chaining — Sequences of events can be hash-chained, and the entire sequence of events checked for data integrity. Information lost in transit or on the disk from database corruption can be detected. If the last hash can be independently validated, then so can the entire sequence.
Correlation and causation IDs - Domain events can easily be given correlation and causation IDs, which allows a story to be traced through a system of applications.
Compression - reduces the size of stored domain events and snapshots, usually by around 25% to 50% of the original size. Compression reduces the size of data in the database and decreases transit time across a network.
Application-level encryption — encrypts and decrypts stored events and snapshots, using a cipher strategy passed as an option to the sequenced item mapper. Can be used to encrypt some events, or all events, or not applied at all (the default). This means data will be encrypted in transit across a network (“on the wire”) and at disk level including backups (“at rest”), which is a legal requirement in some jurisdictions when dealing with personally identifiable information (PII) for example the EU’s GDPR.
Optimistic concurrency control — ensures a distributed or horizontally scaled application doesn’t become inconsistent due to concurrent method execution. Leverages optimistic concurrency controls in adapted database management systems.
Worked examples — simple example application and systems, with an example entity class, example domain events, and an example database table. Plus lots of examples in the documentation.