Snapshots

In EventSourcing a Snapshot is used to memorize an object at a certain point in the EventStream, so that when later-on this object has to be retrieved again, rather than creating a fresh one and use it to process all relevant events, we can start with the snapshot (that already has the state of the object from before) and just process all the facts that happened since.

It is easy to see that storing and retrieving snapshots involves some kind of marshalling and unmarshalling, as well as some sort of Key/Value store to keep the snapshots.

Snapshot Serialization

Serialization is done using a SnapshotSerializer.


public interface SnapshotSerializer {
  byte[] serialize(SnapshotProjection a);

  <A extends SnapshotProjection> A deserialize(Class<A> type, byte[] bytes);

  boolean includesCompression();

  /**
   * In order to catch changes when a {@link SnapshotProjection} got changed, calculate a hash that
   * changes when the schema of the serialised class changes.
   *
   * <p>Note that in some cases, it is possible to add fields and use serializer-specific means to
   * ignore them for serialization (e.g. by using @JsonIgnore with Jackson).
   *
   * <p>Hence, every serializer is asked to calculate it's own hash, that should only change in case
   * changes to the projection where made that were relevant for deserialization.
   *
   * <p>This method is only used if no other means of providing a hash is used. Alternatives are
   * using the ProjectionMetaData annotation or defining a final static long field called
   * serialVersionUID.
   *
   * <p>Note, that the serial will be cached per class
   *
   * @param projectionClass the snapshot projection class to calculate the hash for
   * @return the calculated hash or null, if no hash could be calculated (makes snapshotting fail if
   *     no other means of providing a hash is used)
   */
  Long calculateProjectionSerial(Class<? extends SnapshotProjection> projectionClass);
}

As you can see, there is no assumption whether it produces JSON or anything, it just has to be symmetric. In order to be able to optimize the transport of the snapshot to/from the SnapshotCache, each SnapshotSerializer should indicate if it already includes compression, or if compression in transit might be a good idea. Factus ships with a default SnapshotSerializer, that - you can guess by now - uses Jackson. Neither the most performant, nor the most compact choice. Feel free to create one on your own.

Snapshot caching

The Key/Value store that keeps and maintains the snapshots is called a SnapshotCache.

Factus comes with a default SnapshotCache that uses FactCast to store/retrieve and maintain those cached snapshots. While this works reasonably well and is easy to use, as it does not involve any other piece of infrastructure, you might want to keep an eye on the load- and storage-requirements imposed by this. It is very easy to provide an implementation of SnapshotCache that uses for instance Redis or memcached instead, so that you keep this load away from FactCast for performance, scalability and in the end also cost efficiency reasons. Also it has an effect on the availability and maybe responsiveness of your application, but this is obviously outside of the scope of this document.

The SnapshotCache by default only keeps the last version of a particular snapshot, and deletes it after 90 days of being unused. See Properties

Serials

When a projection class is changed (e.g. a field is renamed or its type is changed), depending on the Serializer, there will be a problem with deserialization. In order to rebuild a snapshot in this case a “serial” is to be provided for the Projection. Only snapshots that have the same “serial” than the class in its current state will be used.

There are 3 ways to provide a serial:

  1. adding @ProjectionMetaData(serial = 1L) to the projection class will set the serial to 1
  2. adding private static final serialVersionUID = 1L to the projection class will set the serial to 1
  3. let the serial be calculated by the SnapshotSerializer.

If no serial is provided through any means, you will get a warning and snapshots will not be used beyond the lifecycle of the JVM. (Which will make your old snapshots pile up in the cache, so please don't rely on this)