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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.
Design resumable uploads, durable metadata and secure downloads for large files.
Reserve seats with expiring holds and reconcile payment without overselling.
Match riders with nearby drivers while handling stale locations and acceptance races.
Coordinate orders, stock and payment with idempotent steps and compensation.
Design durable enqueue, visibility leases and stale-ack protection.
Allocate compatible compartments and coordinate pickup credentials with hardware outcomes.
Coordinate leases and fencing tokens without pretending expired owners stop executing.
Protect scarce inventory with a waiting room, admission tokens and recoverable reservations.
Model cart mutations, version checks and reproducible price quotes.
Accept one logical vote per participant and publish recoverable live aggregates.
Append versioned domain events and rebuild read models without duplicating effects.
Implement immutable snapshots, compare-and-swap and tombstones in a small MVCC model.
Synchronize immutable file versions with cursors, tombstones and explicit conflicts.
Version event contracts with explicit compatibility direction and deterministic checks.
Bridge a consistent snapshot to a retained change log and replay downstream safely.
Validate staged data, report row errors and commit retryable batches without silent duplication.
Model authenticated sessions, withdrawal authorization and uncertain dispenser outcomes.
Traverse remote pages with bounded prefetch, opaque cursors and explicit resume semantics.
Recover long-running workflows with durable history, timers and idempotent activities.
Share document revisions with optimistic concurrency, resumable history and access revocation.
Apply a restricted JSON patch language with escaped paths and all-or-nothing behavior.
Post balanced immutable journal entries with exact arithmetic and idempotent references.
Reserve and settle long-lived resource quotas across concurrent services.
Serve versioned online features and reproduce training data without future-information leakage.
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