What Is Durability?

It's like a scene from The Walking Dead: you're just trying to get through the day, minding your own business, when all of a sudden, everything goes dark, and the world seems to be crumbling around you. You are still determining how long it will last and if you'll ever see the light again. Then, like a beacon of hope in the night, something happens—a flash of light! Suddenly, there is life again. That's what durability is like in databases: even if there are problems with your database system, your data will still be accessible and readable when things come back up .in order to replay them in case of a system failure. Systems that rely on durability are less likely to lose data and can serve their purpose even if they are offline. In databases, durability is usually ensured by using a write-ahead log of all changes made to the database. In the event of a system failure, the record can be "replayed" to restore the database to its previous state. In the event of a power outage, the system can be shut down and then restarted, at which point it will replay the log to restore the database to its previous state. When you're running a hospital, bank, or any other organization whose very existence depends on the information systems run on databases, the ability to recover 100% of all committed transactions is crucial. The recovery rate must be 100 percent, not 90 percent or even 99.6 percent. In addition, this recovery must be permanent, meaning all transactions must be reconstructed, even if the database server crashes due to OS failure or power loss. It's a little like when you're waiting in line at a bank, and the teller says, "I'm sorry, but your transaction will not be complete until I see it on my screen." That's what durable means: your data is visible, available, and ready to be acted upon.