Document Database (MongoDB-style) — Simple

Problem statement (interviewer prompt)

Design a MongoDB-style document database: BSON documents grouped into collections, rich query language with secondary indexes, multi-document transactions, replica sets, sharding by a chosen shard key, and change streams (CDC).

flowchart LR
  C[App]
  R[mongos / Router]
  S1[(Shard 1<br/>replica set)]
  S2[(Shard 2<br/>replica set)]
  S3[(Shard 3<br/>replica set)]
  CS[(Config servers<br/>shard map)]
  C --> R
  R --> S1
  R --> S2
  R --> S3
  R --- CS

    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 C service;
    class R,S1,S2,S3,CS datastore;