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Problem library
High-level architecture and low-level object design, each with progressive hints, a written analysis, approach and worked solution.
Deliver email, SMS, and push notifications without losing work during provider outages.
Schedule durable jobs with leases, retries and safe recovery after worker crashes.
Coordinate orders, stock and payment with idempotent steps and compensation.
Deliver signed events with retries, endpoint isolation and replayable delivery history.
Design durable enqueue, visibility leases and stale-ack protection.
Model closed, open and half-open states with bounded recovery probes.
Manage bounded resources, acquisition deadlines and safe lease return under concurrency.
Create verifiable recovery points and prove they restore in an isolated environment.
Compose retry classification, backoff, cancellation and a total deadline.
Define write ownership, recovery objectives and split-brain prevention across regions.
Track device commands through reconnects, deduplication and ambiguous acknowledgements.
Produce resumable authorized exports with stable snapshots and bounded resources.
Validate staged data, report row errors and commit retryable batches without silent duplication.
Recover long-running workflows with durable history, timers and idempotent activities.
Schedule email sends with suppression, provider feedback and honest ambiguous-outcome handling.
Distribute immutable configuration revisions through canaries and observable rollout waves.
Migrate a live dataset through snapshot backfill, ordered change capture and fenced cutover.
Replicate immutable object generations with checksums, delete markers and fenced failover.
Synchronize external calendars with durable cursors, tombstones and conditional local edits.
Merge authorized results from heterogeneous sources under bounded latency and explicit partial coverage.
Coordinate atomic decisions across prepared participants with durable recovery and honest blocking semantics.
Reconcile desired resources with observed external state using generations, ownership and finalizers.
Recover committed key/value batches from framed journal records with a declared durability boundary.
Move key ranges between owners with snapshot catch-up, enforced epochs and directory cutover.
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