Replication, partitioning, and sharding
By the end of today you can say which of replication and sharding solves which problem, explain replication lag and the bug it causes, and name what you permanently give up by sharding.
YesterdayOn Day 60 you took read load off the database with a cache. Today the database itself is the limit.
TomorrowTomorrow, what having several copies of the truth does to consistency.
Why this matters
The database is the hardest thing to scale and the hardest to undo. Sharding in particular is close to irreversible, so understanding it before you need it is the only good time.
- Replication
- Replication lag
- Partitioning
- Sharding
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 minSet up a Postgres primary with one replica. Write to the primary, read from the replica, and measure the lag. Then construct the read-your-own-writes bug deliberately and observe it. Finally, write down three candidate shard keys for your library schema and the specific query each one would make painful.
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
Completion and mastery are tracked separately. Be honest, because an inflated rating only means the concept resurfaces sooner.
Mastery tracking
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Recap
- 01Replicas solve read load and do nothing for writes
- 02Replication lag breaks the expectation of seeing your own change
- 03Partitioning stays inside one database; sharding gives up joins
- 04Try everything else first, because sharding is close to irreversible
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.