When working with Elasticsearch, synonyms are crucial for enhancing search relevance. Specifically, contraction synonyms (or explicit mapping synonyms) are designed to narrow search terms, mapping broader terms to more specific ones. However, users sometimes encounter situations where these contraction synonyms behave unexpectedly, acting like equivalent synonyms instead. This article explores this issue, using a practical example to demonstrate the problem and discuss potential solutions for achieving true unidirectional synonym mapping in Elasticsearch.
The core problem arises when a defined contraction synonym rule, intended to map terms like “vitamin” or “supplement” to a more specific phrase like “peak performance,” instead results in bidirectional mapping. This means that searching for “peak performance” also incorrectly retrieves documents containing “vitamin” or “supplement,” which is not the desired behavior for a contraction synonym.
Consider the following Elasticsearch analyzer configuration designed to implement contraction synonyms:
{ "filter": { "english_keywords": { "keywords": [ "example" ], "type": "keyword_marker" }, "english_stemmer": { "type": "stemmer", "language": "english" }, "synonyms_en": { "type": "synonym", "expand": "false", "synonyms": [ "vitamin, supplement => peak performance", "soap => wash", "protein => access", "kids => koala" ] }, "english_possessive_stemmer": { "type": "stemmer", "language": "possessive_english" }, "english_stop": { "type": "stop", "stopwords": "_english_" } }, "analyzer": { "custom_en": { "filter": [ "english_possessive_stemmer", "lowercase", "synonyms_en", "english_stop", "english_keywords", "english_stemmer" ], "tokenizer": "standard" } } }In this setup, the synonyms_en filter is configured with "expand": "false", aiming to prevent synonym expansion and enforce contraction. The synonym rule "vitamin, supplement => peak performance" is intended to ensure that when a user searches for “vitamin” or “supplement,” they will find products related to “peak performance.”
However, when a query is executed for “peak performance” using this analyzer, unexpected results can occur:
{ "from": 0, "size": 10, "_source": [ "name.regular" ], "query": { "bool": { "must": { "multi_match": { "query": "peak performance", "fuzziness": "auto", "operator": "and", "type": "most_fields", "fields": [ "name.regular.en" ] } } } }, "post_filter": { "term": {"channel_id": 1} } }The query response reveals that, despite the intention of contraction, results containing “vitamin” and “supplement” are also returned alongside “peak performance” products:
{ "took": 7, "timed_out": false, "_shards": { "total": 5, "successful": 5, "skipped": 0, "failed": 0 }, "hits": { "total": 28, "max_score": 9.393171, "hits": [ { "_index": "en_us_primary", "_type": "_doc", "_id": "product_2799_1", "_score": 9.393171, "_source": { "name": { "regular": "Vitality Vitamin D3" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_3068_1", "_score": 8.375387, "_source": { "name": { "regular": "Calmicid Antacid Supplement" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_861_1", "_score": 8.023193, "_source": { "name": { "regular": "Peak Performance Men Save $47.95" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_862_1", "_score": 7.4778767, "_source": { "name": { "regular": "Peak Performance Longevity 50+ Save $47.95" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_980_1", "_score": 7.229685, "_source": { "name": { "regular": "Peak Performance Brain Women Save $90.92" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_7798_1", "_score": 7.229685, "_source": { "name": { "regular": "Peak Performance Heart Men Save $93.92" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_2708_1", "_score": 6.8760505, "_source": { "name": { "regular": "Sei Bella Fortifying Vitamin Lotion" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_967_1", "_score": 6.7668524, "_source": { "name": { "regular": "Peak Performance Metabolic Pack Men Save $78.93" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_860_1", "_score": 6.45929, "_source": { "name": { "regular": "Peak Performance Women Save $47.95" } } }, { "_index": "en_us_primary", "_type": "_doc", "_id": "product_592_1", "_score": 6.414261, "_source": { "name": { "regular": "Peak Performance Total Men Save $131.89" } } } ] } }This outcome suggests that the synonym filter is treating the rule as an equivalent synonym, causing bidirectional expansion rather than the intended unidirectional contraction.
Several factors could contribute to this unexpected behavior. One possibility is the version of Elasticsearch being used. Version 6.2, as mentioned in the original context, is quite outdated, and synonym handling might have evolved in later versions. It’s crucial to consult the Elasticsearch documentation specific to your version to ensure compatibility and understand any version-specific nuances in synonym behavior.
Another aspect to investigate is the interaction of the synonym filter with other filters in the custom analyzer. While "expand": "false" is set for the synonym filter, other filters like stemming or lowercase could potentially influence how terms are processed and matched against synonym rules.
To troubleshoot, consider these steps:
- Verify Analyzer Application: Ensure that the
custom_enanalyzer is correctly applied to the field being queried (name.regular.enin this case). Incorrect analyzer application can lead to synonyms not being processed as intended. - Simplify Synonym Rules: Test with a very basic synonym rule in isolation to rule out complexities arising from multiple rules or term interactions. For example, try just
"vitamin => peak performance"and see if the behavior changes. - Analyze Token Streams: Utilize the Elasticsearch Analyze API to examine the token stream generated by your analyzer for both the input terms (“vitamin,” “peak performance”) and the indexed content. This will provide insights into how terms are being tokenized and filtered, helping to pinpoint where the synonym expansion is occurring unexpectedly.
- Upgrade Elasticsearch Version: If possible, consider upgrading to a more recent Elasticsearch version. Newer versions often include bug fixes and improvements in text analysis and synonym handling.
In conclusion, achieving true contraction synonyms in Elasticsearch requires careful configuration and understanding of how the synonym filter interacts with the analyzer chain. By systematically investigating the analyzer settings, testing simplified scenarios, and leveraging Elasticsearch’s analysis tools, you can effectively diagnose and resolve issues preventing unidirectional synonym mapping and ensure your search results accurately reflect your intended synonym behavior. Always refer to the official Elasticsearch documentation for the most accurate and up-to-date information on synonym configuration and best practices.
