From Elasticsearch 8 to 9.x and OpenSearch 3: What's New, What Was Removed and Which to Choose
You upgrade from Elasticsearch 8 to 9.x by moving to the latest 8.19 first, clearing deprecation warnings and taking a snapshot; OpenSearch 3 shares the core but goes its own way and needs its own .NET client. With real output: parameters that warn on 8.19 returning 400 on 9.5, the deprecation API, opening 8.19 data on 9.5 with no way back, LOOKUP JOIN, the Elastic client getting a 406 from OpenSearch, the same search with OpenSearch.Client, and a decision table.
14 Nov 2026
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13 min
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