Memoria

Read Modes

When you load an aggregate with IDomainService.GetAggregate, you choose how it should be reconstructed. Memoria offers four read modes that trade off freshness against I/O and reconstruction cost.

The four modes

ReadMode Reads snapshot Applies new events after the snapshot Reconstructs from events if no snapshot Returns null when…
SnapshotOnly (default) yes no no no snapshot exists
SnapshotWithNewEvents yes yes no no snapshot exists
SnapshotOrCreate yes no yes no snapshot and no events
SnapshotWithNewEventsOrCreate yes yes yes no snapshot and no events

Which one should I pick?

Walk down the questions:

  1. Are you reading right after a write that already snapshotted? Use SnapshotOnly. Fastest path; one record fetched.
  2. Could other writers have appended events since your last snapshot? Use SnapshotWithNewEvents. Reads snapshot + the tail of events to apply.
  3. Are you bootstrapping or replaying after a schema change where the snapshot may not exist yet? Use SnapshotOrCreate. If a snapshot is missing, the aggregate is reconstructed from events and a fresh snapshot is stored.
  4. All of the above at once? Use SnapshotWithNewEventsOrCreate. Safe default for “I don’t care how it got here, just give me the latest aggregate state.”

Behaviour when no snapshot exists

SnapshotOrCreate and SnapshotWithNewEventsOrCreate will write a snapshot automatically the first time they reconstruct an aggregate from events. This is the supported way to introduce a new aggregate type, or to migrate to a changed aggregate structure — increase the aggregate version to force snapshot recreation.

SnapshotOnly and SnapshotWithNewEvents never write — they return null if the snapshot is missing, even when applicable events exist in the stream.

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