Time-Series Database — Simple

Problem statement (interviewer prompt)

Design a time-series database (Prometheus / InfluxDB / TimescaleDB). It must ingest 1M+ samples/s, compress to ~1 byte/sample, support range queries with aggregations, downsample for long-term storage, and gracefully reject high-cardinality writes.

flowchart LR
  W[Writers<br/>agents, services]
  ING([Ingest API])
  WAL[WAL]
  MEM[Mem buffer<br/>per-series]
  TSM[(Compressed TSM blocks)]
  Q[Query Engine]
  R([Reader / Dashboard])
  W --> ING --> WAL --> MEM
  MEM -. flush .-> TSM
  R --> Q --> TSM
  Q --> MEM

    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 W,WAL,MEM,Q service;
    class TSM datastore;
    class ING compute;
    class R obs;