Projections

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({
  metaStore,
  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}})

projector.start()