Why did you use two (or more) databases instead of one?

Question

Many database libraries are installed for multiple database connections, but I never knew about a scripting application that needed to connect to two databases at the time of its launch. (compiled, working with a daemon languages ​​is another matter).

I understand that you have subordinate databases, so you can distribute the load, but usually only one of them is selected at startup to deal with these scenarios.

So why should a PHP or Ruby application connect to multiple databases? Or rather, why did you split the data among several databases?

The only thing I can think of is a poor design from a slowly developing system that started in several separate parts.

Answer 1

Are you talking about different physical database servers or different databases in the sense of "schema"?

As for physical servers, if you use MySQL replication, you can write to the master and always read from the slave. This helps to share the load between each database.

Answer 2

The simple answer is scalability.

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