Unexpected failure in this – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 8.9-8.9

Briefly, this error occurs when Elasticsearch encounters an unexpected issue during its operation. This could be due to a variety of reasons such as insufficient memory, corrupted indices, or network connectivity issues. To resolve this, you can try increasing the heap size if memory is the issue, check the health of your indices and repair if necessary, or ensure your network connection is stable. Additionally, check the Elasticsearch logs for more specific details about the error.

This guide will help you check for common problems that cause the log ” unexpected failure in [” + this + “] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: .

Log Context

Log “unexpected failure in [” + this + “]” classname is CancellableFanOut.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

                sendItemRequest(item; ActionListener.releaseAfter(itemResponseListener; refs.acquire()));
        } catch (Exception e) {
            // NB the listener may have been completed already (by exiting this try block) so this exception may not be sent to the caller;
            // but we cannot do anything else with it; an exception here is a bug anyway.
            logger.error("unexpected failure in [" + this + "]"; e);
            assert false : e;
            throw e;


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