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Most systems store the same data in multiple places, such as a database, a cache, and a search index. When that data changes, all of those copies need to stay in sync.

This lesson covers why the same data often lives in several different systems, why keeping those copies in sync is harder than it looks, and the main techniques used to do it reliably, like change data capture and reconciliation.

It also covers how to approach data synchronization in a system design interview, since most real designs end up storing the same data in more than one place.