Missing shard – How to solve this Elasticsearch exception

Opster Team

August-23, Version: 7.12-8.9

Briefly, this error occurs when Elasticsearch cannot find a specific shard. This could be due to a node failure, network issues, or disk corruption. To resolve this issue, you can try to restart the Elasticsearch node, check the network connectivity between nodes, or restore the shard from a snapshot. If the shard is permanently lost and no snapshot is available, you may need to reindex your data.

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Briefly, this error occurs when Elasticsearch cannot find the specified shard. The solution is to check the shard name or ID and ensure that it exists and is spelled correctly.

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This guide will help you check for common problems that cause the log ” missing shard ” to appear. To understand the issues related to this log, read the explanation below about the following Elasticsearch concepts: shard, plugin.

Missing Shard

A missing shard error indicates that one or more indices does not have a primary shard available, and furthermore cannot promote a replica shard to replace the missing one. It is usually associated with a cluster red status.

Why this error occurs

There can be several reasons why this error occurs:

1. There are no replicas available to promote

A missing shard error will only happen if there are no available replicas. If there were an available replica, then it would be promoted to primary shard. This may happen because users only have one node, or, if by design users intentionally specified number_of_replicas:0.   

2. Node crashes

If more than one node becomes overwhelmed or stops operating for any reason, for example, due to “out of memory” errors, then users will end up with missing shards.  

3. Networking issues

If nodes are not able to reach each other reliably, then the nodes will lose contact with one another and the cluster will consider all of the shards on the lost node to be missing. Users may be able to detect this situation by repeated messages in the logs about nodes leaving or rejoining the cluster.

Finding the cause of missing shards

Users can use the cluster allocation API:

GET /_cluster/allocation/explain

By running the command above, users will get an explanation of the allocation status of the first unallocated shard found.

  "index" : "my_index",
  "shard" : 0,
  "primary" : false,
  "current_state" : "unassigned",
  "unassigned_info" : {
    "reason" : "NODE_LEFT",
    "at" : "2017-01-04T18:53:59.498Z",
    "details" : "node_left[G92ZwuuaRY-9n8_tc-IzEg]",
    "last_allocation_status" : "no_attempt"
  "can_allocate" : "allocation_delayed",
  "allocate_explanation" : "cannot allocate because the cluster is still waiting 59.8s for the departed node holding a replica to rejoin, despite being allowed to allocate the shard to at least one other node",
  "configured_delay" : "1m",                      
  "configured_delay_in_millis" : 60000,
  "remaining_delay" : "59.8s",                    
  "remaining_delay_in_millis" : 59824,
  "node_allocation_decisions" : [
      "node_id" : "pmnHu_ooQWCPEFobZGbpWw",
      "node_name" : "node_t2",
      "transport_address" : "",
      "node_decision" : "yes"
      "node_id" : "3sULLVJrRneSg0EfBB-2Ew",
      "node_name" : "node_t0",
      "transport_address" : "",
      "node_decision" : "no",
      "store" : {                                 
        "matching_size" : "4.2kb",
        "matching_size_in_bytes" : 4325
      "deciders" : [
          "decider" : "same_shard",
          "decision" : "NO",
          "explanation" : "the shard cannot be allocated to the same node on which a copy of the shard already exists [[my_index][0], node[3sULLVJrRneSg0EfBB-2Ew], [P], s[STARTED], a[id=eV9P8BN1QPqRc3B4PLx6cg]]"

The above api returns:

“unassigned_info”  => The reason why the shard became unassigned.

“node_allocation_decision” => A list of explanations for each node, which explain whether it could potentially receive the shard.

“deciders” => The decision and explanation.

How to recover a missing primary shard

A lost primary shard is usually recovered automatically by promoting its replica. However, the cluster allocation explain API may indicate that this is not possible.

  "index" : "test",
  "shard" : 0,
  "primary" : true,
  "current_state" : "unassigned",
  "unassigned_info" : {
    "reason" : "NODE_LEFT",
    "at" : "2017-01-04T18:03:28.464Z",
    "details" : "node_left[OIWe8UhhThCK0V5XfmdrmQ]",
    "last_allocation_status" : "no_valid_shard_copy"
  "can_allocate" : "no_valid_shard_copy",
  "allocate_explanation" : "cannot allocate because a previous copy of the primary shard existed but can no longer be found on the nodes in the cluster"

In this case, the following options are available:

Wait for the node to come back online

If the lost node went down or restarted, it may be a matter of time before the node is restarted and the shard becomes available again.  

Restore a snapshot

It is generally preferable to restore a snapshot in a known state (eg. 30 minutes ago) than to try to recover corrupted data in an unknown state.

Restore from corrupted shard

As a last resort, if there is no way of recovering the node and if no snapshot is available, it might be possible to promote a stale shard. However, this means that data will be lost, and in the event that the lost node recovers, data will be overwritten with the stale data. 

The command to restore is:

POST /_cluster/reroute
        "commands" : [
              "allocate_stale_primary" : {
                    "index" : "test", "shard" : 0,
                    "node" : "es01",

Log Context

Log “missing shard” class name is TransportDownsampleIndexerAction.java. We extracted the following from Elasticsearch source code for those seeking an in-depth context :

 long numIndexed = 0;
 int successfulShards = 0;
 for (int i = 0; i < shardsResponses.length(); i++) {
 Object shardResponse = shardsResponses.get(i);
 if (shardResponse == null) {
 throw new ElasticsearchException("missing shard");
 } else if (shardResponse instanceof DownsampleIndexerAction.ShardDownsampleResponse r) {
 numIndexed += r.getNumIndexed();
 } else if (shardResponse instanceof Exception e) {
 throw new ElasticsearchException(e);


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