In an effort to provide information for those of you who are newcomers to databases, especially with the wonderul new Sql Server Express, I have committed myself to about half a dozen posts or so targeted directly at you to help you understand and build better databases.  Stay tuned to this blog for weekly updates to this series, or subscribe to the rss feed.

The ACID model has been a cornerstone of database modeling for a very long time. Hopefully, its something that is still taught in CS courses. ACID lays out 4 goals that must be met before your database can even begin to be considered useful and reliable.

Atomicity – Every transaction that occurs within the scope of the database is a single piece of work: Atomic. The atomic nature of transactions is maintained by ensuring that if any one part of a transaction fails, then the entire transaction fails. This is easy to do because of the singular purpose served by the transaction. This is also referred to as the "all or nothing" approach.

Consistency - All data in the database must exist in a consistent state once a transaction completes. To help out with this, we use rules, keys, normalization, etc all help to maintain data integrity. All of these things combined helps to ensure our data always exists in a consistent state.

Isolation – Modifications to your data must be isolated from other concurrent transactions. No concurrent transaction (Transaction A) should see any data being modified by another concurrent transaction (Transaction B) while it is in an intermediate state. Transaction A should see the data as it existed prior to being modified by Transaction B, or after Transaction B has completed and committed its changes.

Durability – After a transaction has successfully completed and committed it changes, the data is permanently in place in the database, even in the event of hardware or software failures. Database backups and transaction logs help you out with this by facilitating a means of restoring committed transactions.

Next topic in this series: Normalization.