Consistency, availability, and CAP intuition
By the end of today you can state what CAP actually says rather than the popular misreading, decide what a given feature needs, and describe eventual consistency in terms a product owner would accept.
YesterdayOn Day 61 you created several copies of the data. Today you deal with the consequence: they can disagree.
TomorrowTomorrow, queues, which are how systems stay responsive while accepting that some work happens later.
Why this matters
CAP is the most misquoted idea in the industry. Understanding it properly turns an abstract debate into a concrete product decision about what a user should see during a network fault.
- CAP
- Eventual consistency
- Consistency per feature
- Partitions are not optional
- When not to use a relational database
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.
3 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
45 minTake a product you use daily and list eight features. For each, decide whether it needs strong consistency or tolerates eventual consistency, and write one sentence on what a user would see if it were briefly stale. Then pick the one you are least sure about and argue both sides.
Recall it
25 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
Free · sign inQuestions you answer from memory, then grade yourself against the real answer. The score is carried into the mastery rating below it, so an honest miss cannot quietly become a tick.
Rate it
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
Free · sign inRate yourself against five named criteria per concept. Completion and mastery are tracked separately, and anything you rate shakily comes back automatically on a spaced schedule.
Recap
- 01Partitions are not a choice; what you choose is how to behave during one
- 02Consistency in CAP means reads see the latest write, not that constraints hold
- 03The decision belongs to a feature, not to a system
- 04Last write wins means somebody's change disappeared silently
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.