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Interviews: What is ACID?

By Steve Jones,

I once had a job interview with a large group of employees at a company. They were arranged around a table, with me at one end. The interview was a round robin affair, with someone asking me a question and me answering. They would take turns, going around the table for a couple hours. Not the easiest or hardest interview I've had, but one of the more interesting.

The first question from the gentleman to my left was, "What does ACID mean?". My answer: "Isolated, Consistent, ..."

I don't have a problem taking a few seconds to think, but I have that internal clock, much like an American football NFL quarterback. Mine is not the 5 second one many QBs have, but I know that after about 20 seconds, people are wondering if I'll answer at all. Or if I'm still focused on the interview. If I go 30 seconds, I might have blown the interview. What would you do here? Do you know the other terms? More importantly, can you answer at a level beyond the words of the acronym?

My response after 10 seconds or so was to admit I couldn't remember the word. I could google for the meaning, but I did know this applies to relational database transactions, intending to ensure that we always have a known state for our data. We know that each transaction must full complete or be completely undone. We can't have the classic issue of depositing money in one account and failing to remove it from another. We also cannot have other transactions interfere with other's work. This means a transaction on a piece of data must complete before the next transaction can modify it. We also must ensure that if the system were to crash, our database could not restart with data in an unknown state. Therefore, in SQL Server, we write to the log first, ensuring the transaction is complete before we can be sure the data changes are hardened. There are more details you could add to my answer, but this is a core foundational part of relational databases.

I have never been asked this question in another interview, but I do think this is one of those core concepts that helps me understand and explain other parts of SQL Server, and even of how to build software. I think understanding this will help you answer other interview questions with more depth and knowledge. Hopefully, you'll have a better answer than I did, with the actual words that make up the acronym.

 
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