CQRS Documents by Greg Young
Figure 18 Event Stream with Snapshot
Figure 6 shows an Event Stream with a Rolling Snapshot placed within it. The process for rebuilding an Aggregate changes when using Rolling Snapshots. Instead of reading from the beginning of time forward, it is read backwards putting the events on to a stack until either there were no more events left or a snapshot was found. The snapshot would then if found be applied and the events would be popped off the stack and applied until the stack was empty. It is important to note that although this is an easy way to conceptualize how Rolling Snapshots work, that this is a less than ideal solution in a production system for various reasons. Further discussion on the implementation of Rolling Snapshots can be found in “Building an Event Storage�. The snapshot itself is nothing more than a serialized form of the graph at that given point in time. By having the state of that graph at that point in time replaying all the events prior to that snapshot can be avoided. Snapshots can be taken asynchronously by a process monitoring the Event Store. Introducing Rolling Snapshots allows control of the worst case when loading from events. The maximum number of events that would be processed can be tuned to optimize performance for the system in question. With the introduction of Rolling Snapshots it is a relatively trivial process to achieve one to two orders of magnitude of performance gain on the two operations that the Event Storage supports. It is important though to remember that Rolling Snapshots are just a heuristic and that conceptually the event stream is still viewed in its entirety.
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