Large websites - multiple databases or multiple tables?

Question

I am wondering how websites like youtube / facebook / stackoverflow etc. work. Do they use a single database with many tables or multiple databases? If so (several), why? Is this a faster option?

Answer 1

There is nothing to do with performance. Databases share unrelated data domains and member deployments.

If two tables can be associated with the same application, they must be in the same database.

The one exception is when the table is reused in many unrelated applications. It can then have its own database, and then applications can use multiple databases.

Answer 2

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: Youtube Architecture
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