NOTE
Elasticsearch Overview
Distributed search and analytics built on Lucene: documents, indices, shards, replicas, inverted indexes, and near-real-time behavior.
This is a historical learning note and may contain outdated or incomplete understanding.
Elasticsearch is a distributed search/analytics engine built on Apache Lucene. Data is modeled as JSON-like documents stored in indices and distributed across shards with optional replicas.
Its strengths are full-text search, filtering, aggregations, and horizontally distributed query/index execution. It is not a drop-in relational database replacement: joins, transactions, uniqueness constraints, and consistency semantics differ.
Understand mappings/analyzers, shard design, refresh/durability, query execution, and lifecycle/retention before tuning low-level settings.