Event Sourcing: Towards Clarity

Event Sourcing

• Domain Events as state medium
• Capturing intent
• Append-only DB

Building applications

application = state + behavior

State mutation

• Only a result of events
• Deterministic
• In-memory

onAuctionOfferPlaced(event) {
	var placedOffer = Offer.unflatten(event.eventPayload.offer);
	this._offers.push(placedOffer);
}

Implementing behavior

• Domain-Driven Design
• Plain old OOP
• Logic in methods

Implementing behavior: code

placeOffer(buyer, amount, date) {
	var offer = new Offer(buyer, amount, date);
	if (!this._opened) {
		throw new Error('The auction is not open for offers at this time');
	}
	
	this._stageEvent(new Event('AuctionOfferPlaced', { offer }));
}

Putting it together

• Single event repository
• Service layer
• Read side

var Auction = require('../Entities/Auction');
module.exports = function openAuction(params) {
	return repository.invoke(Auction, params.ID, function(instance) {
		instance.placeOffer(params.buyer, params.amount, new Date());
	});
};

Read side: CQRS

• Denormalized
• Replicated
• Asynchronous

Pub-Sub

• Automation
• Integration by default
• Messaging format

event: {
	eventType: 'AuctionOfferPlaced',
	eventPayload: { offer: { ... } }
}, commit: {
	sequenceID: '75525f22-381e-421b-8b9c-1f4623a32d56',
	sequenceSlot: 4,
	aggregateType: 'Auction'
}

Advantages

• Isomorphism
• Logic in plain sight
• Traceability
• Scalability

Sources

• https://github.com/rkaw92/esdf
• https://github.com/rkaw92/meet-reliability-auctions
• ...and anything you create!

Thanks!