As introduced in Chapter 2, Architectural Styles, event sourcing means that instead of always storing the whole state of your application, possibly dealing with conflicts during updates, you can just store the changes that happened to your application's state. Using event sourcing can boost your app's performance by eliminating concurrent updates and allowing all interested parties to perform gradual changes to their state. Saving the history of the operations done (for example, market transactions) can allow easier debugging (by replaying them later) and auditing. This also brings more flexibility and extensibility to the table. Some domain models can get much simpler when event sourcing is introduced.
One cost of event sourcing is being eventually consistent. Another one is slowing down the startup of your application – unless you make periodic snapshots of the state or can use the read-only store as in CQRS, discussed in the previous section...
