harvest 1.3.0

Event store for Dart with multiple backends

Harvest

Event store for Dart with multiple backends for easly creating event sourced applications on both the browser and VM.

How does it work

Harvest stores and load events based on identifers

main() {
	// wire up event store
	var messageBus = new MessageBus();
	var eventStore = new MemoryEventStore();
	eventStore.stream
	
	// create some events
	var event1 = ...
	var event2 = ...
	
	// store them
	eventStore.saveEvents(id, [event1, event2], expectedVersion:0);
}	

Introduction

Event sourcing is the concept of saving and retriving domain objects by the events that occured on them rather than by their current state (as is done in CRUD style persistence layers). To understand this concept 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

Why is this a trick ?.

For certain applications the eventlog can be useful in itself such as a audit trail in a financial system. It can also be a valuable for complex applications since it makes each use case explicit by forcing programmers to make event types for every action that can occur.

Further it also makes debugging easy since you can replay the event log to recreate any former system state where an error occurred Finally it also makes implementing online/offline synchronization managable, since the offline application can just queue up events and replay them on the backend once it comes online.

For more information, see the provided example application.

Supported Storage Engines

Currently DartStore implements the following event stores.

• Memory engine: suitable for testing purposes
• IndexDB: suitable for web applications (work in progress)
• Cordova: suitable for mobile applications (work in progress)

Links

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