Unexpected failure while processing shard started – How to solve this Elasticsearch error

Opster Team

Aug-23, Version: 1.3-1.3

Briefly, this error occurs when Elasticsearch encounters an unexpected issue while starting a shard. This could be due to a variety of reasons such as insufficient disk space, network connectivity issues, or corrupted data. To resolve this issue, you can try freeing up disk space, checking your network connection, or restoring the shard from a backup. If the problem persists, you may need to rebuild the index.

This guide will help you check for common problems that cause the log ” {} unexpected failure while processing shard started [{}] ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: cluster and shard.

Log Context

Log “{} unexpected failure while processing shard started [{}]” classname is ShardStateAction.java.
We extracted the following from Elasticsearch source code for those seeking an in-depth context :

                                    logger.debug("{} will apply shard started {}"; shardRouting.shardId(); shardRoutingEntry);

                            } catch (Throwable t) {
                                logger.error("{} unexpected failure while processing shard started [{}]"; t; shardRouting.shardId(); shardRouting);

                        if (shardRoutingToBeApplied.isEmpty()) {
                            return currentState;


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