Snapshotting is an optimization where the AggregateRoot’s state is saved in the event stream. With snapshotting the state of an aggregate can be restored from a snapshot rather than by replaying all of its events. In general there is no need for snapshotting when you have less than 100 Events in an AggregateRoot. By default snapshotting is turned off in Sequent.

Sequent supports snapshots on Aggregates that call enable_snapshots with a default threshold.

class UserNames < Sequent::AggregateRoot
  enable_snapshots default_threshold: 100, version: 1
end

Whenever more events are stored for an aggregate than its snapshot threshold a record is stored in the aggregates_that_need_snapshots table. You can use the rake sequent:snapshots:take_snapshots[limit] task to snapshot up to limit highest priority aggregates.

You can schedule this task to run in the background regularly as it will simply do nothing if there are no aggregates that need a new snapshot.

Important: The snapshot format is versioned, with the default version being 1. If the implementation of an aggregate changes (e.g. a new field is added, or a hash changed into an array, etc) the snapshot format becomes incompatible and the version must be increased. This ensures the new implementation will not try to use the old snapshot format.

Alternatively, you can delete all snapshots whenever a new version of your application is deployed. This will cause a temporary drop in performance when loading previously snapshotted aggregates, since all events will have to be reloaded and replayed when an aggregate is accessed.

To delete all snapshots, you can execute bundle exec rake sequent:snapshotting:delete_all.