Failed to prepare/warm – How to solve related issues

Opster Team

Jan-20, Version: 1.7-8.0

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This guide will help you check for common problems that cause the log ” Failed to prepare/warm ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: index.

Log Context

Log “Failed to prepare/warm” classname is
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

                     assert searcher.getIndexReader() instanceof ElasticsearchDirectoryReader :
                        "this class needs an ElasticsearchDirectoryReader but got: " + searcher.getIndexReader().getClass();
                    warmer.warm(new Searcher("top_reader_warming"; searcher; () -> {}));
                } catch (Exception e) {
                    if (isEngineClosed.get() == false) {
                        logger.warn("failed to prepare/warm"; e);
            return searcher;


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