Databases are part of our daily digital life and they are expected to work fast.
Are you browsing an online forum? The posts are in a database. Are you visiting a doctor? Your medical records are in a database. Are you shopping online? The items, your data and previous purchases are all in a database.
All these data are expected to appear in a few seconds. And it's not only you who expect it. A small web shop may have hundreds of visitors at the same time and every one of them expects the website to be displayed very quickly. The larger sites can handle tens or hundreds of thousands simultaneously.
This means that the database behind the service must be available at all times. The scope of the problem becomes apparent ...