Transactions & ACID

You are moving $50 from your checking account to your savings account. Under the hood that is two separate writes: subtract 50 from checking, add 50 to savings. Now imagine the server crashes after the first write and before the second. The money left checking and never arrived anywhere. It is not in either account. It is gone.

Nothing about a database makes this impossible by default. Two writes are two writes, and the world is allowed to end between them. A transaction is the tool that makes a group of writes behave like one indivisible action: either every write in the group happens, or none of them do. There is no in-between state that anyone can ever see.

This unit is about recognizing that shape. Interviewers rarely ask you to recite what ACID stands for. They describe a feature, like checkout or seat booking, and watch whether you notice the moment where two things must change together or not at all. That instinct is what you are building here.