Systems

A sharded, replicated in-memory cache: consistent-hashing routing, LRU eviction, replica failover, single-flight stampede control, and hot-key fan-out.

Local edits that apply instantly and reconcile when online: every device a leader, causality tracked with vector clocks, conflicts merged deterministically by CRDTs, and convergence guaranteed by eventual consistency.

Assign users to variants and measure the effect: stateless deterministic bucketing, feature-flag config for ramp and kill, an event pipeline that joins exposures to outcomes, and statistics that survive peeking and sample-ratio bugs.

Real-time ranked scores: a sorted set for O(log n) rank and range queries, tie-breaking and best-score semantics, score-bucketed sharding for cheap global rank, multiple materialized boards, and a cached hot top-N.

Aggregate and rank posts per user: fan-out on read vs write, the celebrity hot-partition, a hybrid delivery path, denormalized ranking signals, and a bounded materialized timeline.

Show a courier's live position and a fresh ETA: high-volume location pings absorbed by a stream, a geo-index for spatial queries, a smoothed derived view, and server push so the read path never touches the write path.

Send, receive, and store mail reliably: a durable queue, exponential backoff retries, a dead-letter path, idempotent accept and delivery, and hot/cold storage tiering.

Reliably call customer endpoints: at-least-once delivery over a durable queue, backoff-and-jitter retries, a dead-letter path with replay, signed payloads for receiver idempotency, and per-endpoint circuit breakers.

Sell limited stock under heavy contention without overselling: an atomic conditional decrement, idempotency keys for safe retries, time-bounded leases for checkout holds, and a reconciler that reclaims stranded inventory.

Resolve names to addresses: a recursive resolver built as a TTL-bound cache over the delegation walk, with negative caching, anycast PoPs, and serve-stale to keep the cold path off the network.

Accept, route, and serve requests: a few event-loop workers multiplexing tens of thousands of connections, pooled upstream connections, graceful drain on reload, and backpressure so the loop never outruns its memory.

Scrape, store, query, and alert on operational metrics: a pull-based scraper with discovery, a time-series store, cardinality limits at ingest, RED/USE aggregation, and alerting that debounces flaps and missing data.

Chunked, replicated storage with a metadata master: range-partitioned chunks, a master that stays out of the data path, chunk replication and heartbeat-driven re-replication, lease-based write ordering, and master high-availability over an operation log.

A schema-flexible document store with secondary indexes, walked from a single in-memory collection up through leader-follower and leaderless quorum replication.

Ingest and query high-volume metrics: append-only columnar chunks, delta-of-delta + XOR compression, time-based block pruning, a label inverted index, and cardinality control.

An embedded, crash-safe KV store built on an LSM tree: WAL durability, memtable flushes, Bloom-filtered reads, compaction, and MVCC snapshots.