Non functional requirements: backups

This blog post is part of a series on non functional requirements, and how they take most of the effort.

The scenario

You want a third party to implement an application package to allow people to buy and sell widgets from their phone. Once the package has been developed, they will hand it over to you to sell, support, maintain and upgrade and you will be responsible for it,

At the back-end is a web server.

Requirements you have been given.

• We expect this application package to be used by all the major banks in the world.
• For the UK we expect the number of people who have an account to be about 10 million people
• We expect about 1 million trades a day.

See start here for additional topics.

Why backup?

You need to take backups, (and more importantly be able to restore them) for various reasons

• To recover from media failures.
• To recover from human failure. You may have mirrored disks, but if an operator deletes a file or table, it will be reliably deleted on the mirrored disks.
• You may be asked for historical information. 10 years ago, did this person have an account with you, and can you show the transactions on the account.

How to backup

For a simple file, it is easy to backup.

For a database, or file which is continually being updated, you need a more sophisticated approach. If a transaction is deleting funds from one account and incrementing the funds in a different account, you need to ensure that the backup has consistent data.

With databases you can back up an “inflight” database. If you need to restore it, it replays the transaction log and reapplies any transactions.

Other solutions is to have the main database read only, and do updates in a small database in front of the main database.

You could also partition the database, for example the A partition for surnames beginning with A, etc. These should be smaller than one large database, and so quicker to backup.

What do you backup?

You need to think about what you backup. For example people’s names and addresses do not change very much, but their current balance may change every day.

How long to keep the backup for?

You may have to keep backups for 10 years depending you your industry regulator.

How much does it cost ?

When you are specifying the project there will be many unknowns, so you need to make assumptions.

For example in the brief it says there will be 10 million users and 1 million trades a day.