Aggregate Snapshots

Snapshotting functionality involves the following methods:

• get snapshotVersion(): number - version of the latest snapshot
• get shouldTakeSnapshot(): boolean - defines whether a snapshot should be taken
• takeSnapshot(): void - adds state snapshot to the changes collection, being invoked automatically by the AggregateCommandHandler
• makeSnapshot(): object - protected method used to snapshot an aggregate state
• restoreSnapshot(snapshotEvent): void - protected method used to restore state from a snapshot

If you are going to use aggregate snapshots, you either need to keep the state structure simple (it should be possible to clone it using JSON.parse(JSON.stringify(state))) or override makeSnapshots and restoreSnapshot methods with your own serialization mechanisms.

In the following sample a state snapshot will be taken every 50 events and added to the aggregate changes queue:

class UserAggregate extends AbstractAggregate {
  get shouldTakeSnapshot() {
    return this.version - this.snapshotVersion > 50;
  }
}

If your state is too complex and cannot be restored with JSON.parse or you have data stored outside of aggregate state, you should define your own serialization and restoring functions:

class UserAggregate extends AbstractAggregate {
  makeSnapshot() {
    // return a field, stored outside of this.state
    return { trickyField: this.trickyField };
  }
  restoreSnapshot({ payload }) {
    this.trickyField = payload.trickyField;
  }
}