MVCC & Isolation Levels — Simple

Problem statement (interviewer prompt)

Explain MVCC and the ANSI isolation levels (Read Uncommitted → Serializable). For a banking app, pick the right isolation level for transfers, statement reads, and a daily reconciliation job, and explain the anomalies each level prevents.

flowchart LR
  T1[Txn 1<br/>reads snapshot]
  T2[Txn 2<br/>writes new version]
  R[(Row v1, v2, v3 ...<br/>versioned by tx id)]
  T1 --> R
  T2 --> R
  R -. GC old versions .-> GC[Vacuum / GC]

    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 T1,T2 service;
    class R datastore;

Each transaction sees a consistent snapshot; writers don't block readers.