Building an Event Store in Node.js

Jan 21, 2015

As I quite like the idea of event sourcing I decided to build a simple event store in Node.js.

We'll be able to store events, retrieve events and react (do something useful) when an event has been stored.

You can find the complete code in the GitHub repository.

Adding Events

Each event is associated with a stream. A stream can be thought of as a container for events related to a concrete object, for example a customer.

For simplicity, we'll store all events in memory, but it should be quite easy to keep them in a file or a database.

var events = {};

exports.add = function (stream, event, payload) {
  var e = {
    event: event,
    payload: payload,
    timestamp: Date.now()
  };

  storeEvent(stream, e);
};

function storeEvent(stream, event) {
  events[stream] = events[stream] || [];
  events[stream].push(event);
}

Retrieving Events

We can retrieve all events from a stream easily.

exports.all = function (stream) {
  return events[stream] || [];
};

Reacting to Events

To do something useful after an event has been stored, we first have to register a function that will handle the event.

Handlers are often used as projections in a CQRS system to update a query model.

var handlers = {};

exports.on = function (event, handler) {
  handlers[event] = handlers[event] || [];
  handlers[event].push(handler);
};

Next we'll have to modify the add function to also call all registered handlers.

exports.add = function (stream, event, payload) {
  // ...
  handleEvent(e);
};

function handleEvent(event) {
  var hs = handlers[event.event] || [];
  for (var i = 0; i < hs.length; i++)
    hs[i](event);
}

When a handler is actually called the stored event is passed as its only argument.

Testing

Here is a quick Mocha test. It should be stored under the test directory.

var es = require('../eventstore.js');
var assert = require('assert');

describe('eventstore', function () {
  describe('#add', function () {
    it('should add an event to the store', function (done) {
      es.add('customer1', 'created', {
        firstName: 'John',
        lastName: 'Smith'
      });

      assert.equal(1, es.all('customer1').length);
      assert.equal(0, es.all('customer2').length);
      done();
    });
  });

  describe('#on', function () {
    it('should react after an event has been stored', function (done) {
      es.on('created', function (event) {
        done();
      });
      es.on('deleted', function (event) {
        throw new Error("This shouldn't happen");
      });

      es.add('customer1', 'created', {
        firstName: 'John',
        lastName: 'Smith'
      });
    });
  });
});