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Browse 4 real-world technical and behavioral interview questions about Fan out. Review scenarios, edge cases, and architectural best practices.
Presence at 10 million users is a lease system, not a connection-state table. Clients renew short-lived keys, the dot expires when renewals stop, and the real cost is bounded fan-out to visible watchers rather than every contact.
SQL join duplicates cause double counting when two one-to-many joins multiply rows before aggregation. Aggregate each child table to one row per parent first, then join those results back to the parent. Use this sql queries answer to show the decision, trade-off, and evidence rather than a memorised definition.
Size it first: 200M daily actives opening the feed six times a day is 42k reads/s at peak, while 100M posts fanned out to 200 followers each is 231k feed writes/s, so writes dominate. Push to ordinary accounts, pull for high-follower ones, and store ids so deletes and blocks filter on read.
A hot partition from one account cannot be fixed by ordinary rebalancing because it is one key. Split the key into write-sharded sub-keys or aggregate writes before storage; both choices move cost to reads, ordering, rollups or approximation.