Search Engine (Elasticsearch-like) — Simple

Problem statement (interviewer prompt)

Design a search engine like Elasticsearch: index billions of documents with an inverted index, support keyword + phrase + range + boolean queries, rank by BM25 + custom signals, return in <100ms p99, shard horizontally, and handle real-time updates.

flowchart LR
  D[Documents]
  IDX[Indexer]
  INV[(Inverted index<br/>shards / replicas)]
  Q[Query]
  RNK([Ranker])
  D --> IDX --> INV
  Q --> INV --> RNK --> Q

    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 D,IDX,Q service;
    class INV datastore;
    class RNK compute;