Transactions, and what ACID actually protects
By the end of today you can say which failures a transaction prevents and which it does not, and recognise the race condition that survives even inside one.
YesterdayOn Day 43 you made reads fast. Today you make writes correct when several people write at once.
TomorrowTomorrow, the ORM layer that sits above all this, and the ways it hides what you just learned.
Why this matters
Money moving twice, inventory going negative, and two users claiming the same seat are all transaction problems. They appear only under concurrency, which is why they reach production.
- Transactions
- ACID
- Isolation levels
- Race conditions
Learn it
80 minCopy this into Claude or ChatGPT. It quizzes you before it explains anything, which is deliberate. The resources under it are how you check what it told you.
Today's Master Prompt
Free · sign inA prompt written for this day alone: your level, the exact scope, what to leave out, and an instruction to quiz you before it explains anything. Paste it into Claude or ChatGPT and it teaches you today's material.
Check it against something that is not a model
An assistant can be fluent and wrong, and on a topic you met today you will not catch it. These cover the same ground and were made by people who do this for a living, so they are what you hold the explanation up against. They are other people's work and we only link to them, so judge them for yourself.
4 hand-picked resources
Free · sign inVideos, official docs and articles covering the same ground, each opened and annotated by hand. They are what you check the assistant against on a day you cannot yet catch it being wrong.
Build it
50 minOpen two psql sessions against the same table and reproduce a lost update: both read a value, both compute a new one, both write. Confirm the second overwrites the first. Then fix it three ways, with SELECT FOR UPDATE, with an atomic update expression, and with a version column, and write one sentence on when each fits.
Recall it
20 minAnswer out loud, reveal, then mark honestly whether you had it. That score is the only thing on this page you do not get to choose.
5 recall questions
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Rate it
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Mastery tracking
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Recap
- 01A transaction gives all-or-nothing, not mutual exclusion
- 02Read-then-write races survive inside a transaction at default isolation
- 03Each isolation level names exactly which anomaly it still permits
- 04Never hold a transaction open across a network call
Your progress
Free · sign inMark days complete, pick up where you left off across devices, and watch completion and mastery diverge. Free, and the account exists only so ninety days of work cannot vanish with a cleared browser.