If graylog stops showing message streams it could be an issue with indexes.

Health status will report red in the web UI or with API:

# curl -X GET "localhost:9200/_cluster/health?pretty"

{
  "cluster_name" : "elasticsearch",
  "status" : "red",
  "timed_out" : false,
  "number_of_nodes" : 1,
  "number_of_data_nodes" : 1,
  "active_primary_shards" : 97,
  "active_shards" : 97,
  "relocating_shards" : 0,
  "initializing_shards" : 0,
  "unassigned_shards" : 1,
  "delayed_unassigned_shards" : 0,
  "number_of_pending_tasks" : 0,
  "number_of_in_flight_fetch" : 0,
  "task_max_waiting_in_queue_millis" : 0,
  "active_shards_percent_as_number" : 98.9795918367347
}

Check the shards status on the server with:

# curl -X GET "localhost:9200/_cat/shards?v"

index              shard prirep state         docs   store ip              node
...
myindex_154        2     p      UNASSIGNED                                 
...

Check the reason for the unassigned index:

# curl -XGET localhost:9200/_cluster/allocation/explain?pretty

{
  "index" : "myindex_154",
  "shard" : 2,
  "primary" : true,
  "current_state" : "unassigned",
  "unassigned_info" : {
    "reason" : "CLUSTER_RECOVERED",
    "at" : "2022-01-12T13:15:29.713Z",
    "last_allocation_status" : "no_valid_shard_copy"
  },
  "can_allocate" : "no_valid_shard_copy",
  "allocate_explanation" : "cannot allocate because all found copies of the shard are either stale or corrupt",
  "node_allocation_decisions" : [
    {
      "node_id" : "yusko5YtSRSs9QOtVjHutg",
      "node_name" : "yusko5Y",
      "transport_address" : "<some_public_ip>:9300",
      "node_decision" : "no",
      "store" : {
        "in_sync" : true,
        "allocation_id" : "Mr0BOW0RQqKdh-iTqDkjBw",
        "store_exception" : {
          "type" : "corrupt_index_exception",
          "reason" : "failed engine (reason: [merge failed]) (resource=preexisting_corruption)",
          "caused_by" : {
            "type" : "i_o_exception",
            "reason" : "failed engine (reason: [merge failed])",
            "caused_by" : {
              "type" : "corrupt_index_exception",
              "reason" : "checksum failed (hardware problem?) : expected=6fb91e47 actual=9406d419 (resource=BufferedChecksumIndexInput(MMapIndexInput(path=\"/var/lib/elasticsearch/nodes/0/indices/Qlh2XztFSIWu65B7nhmktQ/2/index/_3nhz.cfs\") [slice=_3nhz.fdt]))"
            }
          }
        }
      }
    }
  ]
}

You could delete the index but some messages will be lost!

curl -XDELETE 'localhost:9200/myindex_154'

Then you might have to recalculate the index ranges (System > Indices > index set > Maintenance > Recalculate index ranges) and/or manually rotate the write index (System > Indices > index set > Maintenance > Rotate active write index)

• Graylog 3.3.16
• Debian 9.13 Stretch

• https://www.datadoghq.com/blog/elasticsearch-unassigned-shards/
• https://www.elastic.co/guide/en/elasticsearch/reference/current/cat-shards.html#reason-unassigned
• https://discuss.elastic.co/t/restore-of-elasticsearch-data-fails-with-corruptindexexception-checksum-failed-hardware-problem/261619/3