Leader/Follower Replication — Simple

Problem statement (interviewer prompt)

Design the replication layer of a relational database for high availability + read scale. Cover synchronous vs asynchronous vs semi-synchronous modes, failover semantics, replica lag handling, and how to provide read-your-writes within a session.

flowchart LR
  C([Clients])
  L[(Leader<br/>writes)]
  F1[(Follower 1)]
  F2[(Follower 2)]
  C -->|writes| L
  L -. replicate log .-> F1
  L -. replicate log .-> F2
  C -->|reads| F1
  C -->|reads| F2

    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 client;
    class L,F1,F2 datastore;