Web / Apache Lucene Interview questions
Explain the execution flow of a search request in Lucene?
From the moment a query object is submitted to the moment ranked results come back, Lucene runs through a consistent per-segment pipeline:
flowchart LR
A[Query object] --> B[IndexSearcher.search]
B --> C[Weight created for query]
C --> D[Per-segment Scorer]
D --> E[Collector gathers matches]
E --> F[Per-segment results merged]
F --> G[TopDocs returned]
- The Query is turned into a Weight, which is query-wide but not tied to any single segment.
- For each segment, the Weight produces a Scorer that can iterate matching documents in that segment and compute a score for each.
- A Collector (commonly a top-N collector) consumes the Scorer's output, keeping only the best-scoring documents it's seen so far.
- Once every segment has been processed, per-segment top results are merged into a single ranked TopDocs, which IndexSearcher returns to the caller.
This per-segment design is exactly what lets Lucene search segments in parallel and lets Elasticsearch/Solr distribute the equivalent work across shards.
A Query is first converted into which object before scoring can begin?
What component keeps only the top-scoring documents as results are produced?
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