System designFoundations
Data modeling and storage
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Storage engines (LSM & B-tree)The data structures databases use under the hood to store and retrieve bytes on disk.3 minData modelling & indexingDesigning your schema and indexes around how the data is actually queried.3 minBloom filtersA space-efficient way to say "definitely no" or "probably yes".3 minGraph DB storageWhy native graph databases are so much faster than SQL JOINs for highly connected data.3 minKey-value storesWhy you shouldn't use a heavy relational database just to remember user sessions.3 minSecondary indexesA table is sorted by its primary key. To find rows by any other column quickly, you keep a second little sorted book that points back to them.4 minTime-series storageHow the bytes actually sit on disk: a hot head chunk in memory, then a trail of immutable, time-ordered blocks that get merged and aged out.3 minBackup and restoreProtecting your state against human error and physical disasters.3 minBatch data processingChewing through terabytes of logs using Map and Reduce.3 minDesigning a parking lotA classic object-design question: model the lot, the spots, the cars, and the rules that decide who parks where and what they owe.4 minDesigning a pastebinTake a big blob of text, hand back a tiny shareable link, and trade reads against writes.3 minNested comment systemsHow Reddit and HackerNews structure infinitely deep conversation threads.3 minSharing a file peer to peerNobody downloads the whole file from one place. Everyone grabs a few pieces from each other and hands out what they already have.4 minTime-series databaseA database tuned for one shape of data: a metric, a timestamp, and a relentless stream of new points that only ever moves forward.3 minTo-do app backendA small service that remembers each person's tasks, and only ever shows them their own.4 minURL shortenerHand every long URL a short unique nickname, remember the mapping, and bounce visitors from the nickname back to the original.3 min