Analyze Cluster Reroute

My test cluster health was yellow. The X-Pack Monitoring pointed to some indices, which were yellow.

Monitoring Shard Allocation

If I check the shards on a specific index

GET _cat/shards/ep2-2017.10.15?V


index          shard prirep state        docs   store ip           node
ep2-2017.10.15 1     p      STARTED    453095 236.6mb etu
ep2-2017.10.15 1     r      UNASSIGNED                             
ep2-2017.10.15 0     p      STARTED    454530 237.2mb itu-dc2
ep2-2017.10.15 0     r      STARTED    454530 237.2mb itu

If you try to allocate the unassigned replica:

POST /_cluster/reroute
  "commands": [
      "allocate_replica": {
        "index": "ep2-2017.10.15",
        "shard": 1,
        "node": "etu-dc2"

We get an extended error reason.

  "error": {
    "root_cause": [
        "type": "remote_transport_exception",
        "reason": "[itu-dc2][][cluster:admin/reroute]"
    "type": "illegal_argument_exception",
    "reason": "[allocate_replica] allocation of [ep2-2017.10.15][1] on node {etu-dc2}{aeT1BPu2SjW1g3A18RnuTA}{QxY1ImcVQuuFFGjE31bOzw}{mtlplfohap05}{}{ml.max_open_jobs=10, rack=with-nas, box_type=hot, ml.enabled=true} is not allowed, reason: [YES(shard has no previous failures)][YES(primary shard for this replica is already active)][YES(explicitly ignoring any disabling of allocation due to manual allocation commands via the reroute API)][NO(target node version [5.6.1] is older than the source node version [5.6.3])][YES(the shard is not being snapshotted)][YES(node passes include/exclude/require filters)][YES(the shard does not exist on the same node)][YES(enough disk for shard on node, free: [183.5gb], shard size: [0b], free after allocating shard: [183.5gb])][YES(below shard recovery limit of outgoing: [0 < 2] incoming: [0 < 2])][YES(total shard limits are disabled: [index: -1, cluster: -1] <= 0)][YES(allocation awareness is not enabled, set cluster setting [cluster.routing.allocation.awareness.attributes] to enable it)]"
  "status": 400

Check for the NO condition:

[NO(target node version [5.6.1] is older than the source node version [5.6.3])]

After I checked my nodes, I saw there was a partial cluster upgrade.

GET /_cat/nodes?v&h=version,name,jdk

version name    jdk
5.6.1   itu     1.8.0_141
5.6.1   etu-dc2 1.8.0_141
5.6.3   etu     1.8.0_141
5.6.3   dev     1.8.0_141
5.6.1   itu-dc2 1.8.0_141

After the upgrade everything worked fine and the cluster health was back to green.