Why are rollbacks needed?

Why are rollbacks so important?

Is it to prevent data (like data in a SQL DB) from being in an inconsistent state?

If so, how comes the data "store" (the SQL DB or whatever) made it possible in the first place to become in a corrupt state?

Are there data stor开发者_运维百科age mechanisms that don't have a need for "rollbacks"?

Rollbacks are important in case of any kind of errors appearing during database operational. They can really save the day in case of database server crashes or a critical exception is thrown in an application that modifies contents of DB. When a significant DB operation is performed (i.e. updates, inserts, etc.) and the process is broken in the middle, it would be very hard to trace which operations were successful and usage of DB afterward would be very complicated.

The "store" itself does not generally have a built-in mechanism for consistency control - this is exactly why we use rollbacks and transactions. This can be perceived as a sort of 'live backup' mechanism.

There are cases, when you need insert/update data in many related tables - if you didn't have transactional logic, then any errors somewhere in middle of process could make data inconsistent.

Simple example. Say you need to insert both order header data into orders table and order lines into lines table. You insert order header, read identity, start inserting order lines - but this second insert fails on whatever reason. Only reliable way to recover from this situation is to rollback first insert - either explicitly (when your connection to db is alive) or implicitly (when link is gone down).