Job / Task Scheduler — Simple

Problem statement (interviewer prompt)

Design a distributed cron / job scheduler (Airflow / Quartz-cluster / a giant cron). Run tens of thousands of scheduled jobs across many workers with exactly-once-per-schedule semantics, retries, DAG dependencies, and resumability on worker failure.

flowchart LR
  USR([User / Cron entry])
  SCH([Scheduler])
  Q[[(Job Queue)]]
  W([Workers])
  DB[(Job state DB)]
  USR --> SCH --> Q --> W
  W --> DB

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class Q,DB datastore;
    class USR,SCH,W compute;