Stage 1 · Lesson 4 of 6
How correct must the data be, and how fast
Some data must never be wrong, even for a moment: a wallet balance, seats left on a bus, stock in a shop. For these you want , where a group of changes either all happen or none do, and two people cannot both buy the last seat.
Other data can be slightly stale without harm: a like counter, a "people also viewed" list, a dashboard refreshed every minute. That data can live in a cache or a store with , where copies catch up after a short delay. Mark each piece of your data as "must be exact" or "can be a little behind".
The answer often decides between a , which gives strong guarantees by default, and faster tools that relax them.
Common trapputting money or inventory in a store chosen only for speed.
A question to check yourself, an AI practice session and 2 cheat sheet entries come with the full roadmap. Unlock
Resources for this stage
Start with· follow one of these
- CoursePaidGrokking Database Fundamentals for Tech Interviews
Design Gurus · Beginner · Subscription
A structured tour of database types, SQL versus NoSQL, ACID and indexes before you compare products
- CourseFreeCS50's Introduction to Databases with SQL
Harvard University · Beginner
Free Harvard course: the first lectures show how real data is modelled and queried