Projections allow us to project data from an event store to other types of stores for optimized querying (e.g. elasticsearch).

Defining a MetaStore

DDES needs somewhere to store projection progress. A MetaStore is a basic key/value store and can be used to store metadata for for multiple projections.

const metaStore = new AwsMetaStore({
tableName: 'ddes-meta',

Like with an EventStore, you need to run setup() before using the MetaStore.

await metaStore.setup()

Defining a projection

const forumsProjection = new Projection({
name: 'forums',
aggregateClasses: {Forum, ForumThread}
dependencies: {
ForumThread: {
Forum: (forumThread, forum) =>
forumThread.keyProps.forumId === forum.keyProps.forumId,

async processEvents(events) {
// order and dependencies are maintained by DDES,
// so the the provided events set is safe to process in parallel

Projection setup

Before projecting data, the projection needs to be configured.

await forumsProjection.setup({
startsAt: new Date('2018-01-01') // date of the first commit in the store to project

Running projector

const projector = new Projector([forumsProjection], {eventStore: mainStore}})