harvest 1.4.0

Event store for Dart with multiple backends

Harvest

Harvest is a event store for Dart with multiple backends. Harvest creates and persists streams of events, where each persistent event stream is identified by a Guid for future retrival.

Quick Guide

1. Add the folowing to your pubspec.yaml and run pub install

    dependencies:
      harvest: any

2. Add harvest to some code and run it

main() {
	var streamId = new Guid();
	var eventStore = new MemoryEventStore();
	// get a event stream for streamId 
	var eventStream = eventStore.openStream(streamId);
	
	// create some events
	var event1 = ...
	var event2 = ...
	
	// store them
	eventStream.addAll([event1, event2]);
	eventStrem.commitChanges();
}	

Why do this ?

Event sourcing is the concept of saving and retriving object state by the events that occured on them rather than by their current state. Consider the following bank use case:

1. User creates account
2. User deposits 10$
3. User withdraws 2$

In a CRUD application you would now have a BankAccount object with an amount property with the value 8. In a event sourced application you have a BankAccount object and 3 events for it

1. AccountCreated
2. AmountDeposited
3. AmountWithdrawn

Where is this useful?

• For certain applications the eventlog can be useful in itself such as a audit trail in a financial system.
• It can help manage complexity in large applications by forcing programmers to make event types for every action that can occur.
• It makes debugging easy since you can replay the event log to recreate any former system state where an error occurred.
• It makes mobile app synchronization a breeze, since the offline app can just queue up events and replay them on the backend once it comes online.
• In applications using the CQRS architecture pattern.

For more information, see the provided example application.

Supported Storage Engines

Harvest supports the following event stores.

• FileEventStore: Durable disk based event store
• IndexeddbEventStore: Durable IndexedDB based event store, suitable for web applications.
• MemoryEventStore: Non-durable memory based event store, suitable for testing purposes

Links

• https://github.com/joliver/EventStore