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Event stores are database management systems implementing the concept of event sourcing. They persists all state changing events for an object together with a timestamp, thereby creating time series for individual objects. The current state of an object can be inferred by replaying all events for that object from time 0 till the current time.
In contrast, the other types of DBMS store the current state of an object (and loose the history, if not explicitly modelled).
As an example, for a shopping cart object each insertion of a product (product name, quantity, price) would be persisted as a new event.
Supported operations on Event Stores are the addition of new events and querying event time series for objects. Not supported are modifications or deletions of already persisted events. That simplifies maintaining consistency in distributed systems.
Due to performance reasons many Event Stores realize concepts for holding snapshots (object states at a specific point in time).
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