Comments by "Daniel Sandberg" (@ddanielsandberg) on "What Are Reactive Systems?" video.

@michaelrstover Event Sourcing has the concept of snapshots. I.e. we occasionally inject snapshots in the sequence of events which represents the state up till that point in time. When we are replaying we only need to go back to the latest snapshot but if we want to we can ignore the snapshots and go all the way back to the beginning. To avoid the same book being ordered multiple times we make sure that all actions are idempotent, which is hard, but doable.
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