A deep dive into configuring synonyms in Elasticsearch and OpenSearch, covering the core trade-off between index-time and query-time synonym expansion, why the synonym_graph filter is preferred over the legacy synonym filter for multi-word terms, and how to hot-reload synonym sets without reindexing. The guide explains equivalent vs. explicit synonym rules, the updateable: true pattern with a dedicated search_analyzer, and how the two engines diverge: Elasticsearch 8.10+ offers a Synonyms API (_synonyms endpoint) that auto-reloads analyzers, while OpenSearch relies on POST /_plugins/_refresh_search_analyzers with the ISM plugin. Production concerns include managing large synonym sets on Amazon OpenSearch Service via S3-backed custom packages, avoiding transitive synonym chains and brand-name leaks, and measuring relevance impact before and after every change.
