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Cluster Health

Many statistics can be monitored in an Elasticsearch cluster, but the single most important one is cluster health, which reports a status of either green, yellow, or red:

GET /_cluster/health

On an empty cluster with no indices, this will return something like the following:

{
   "cluster_name":          "elasticsearch",
   "status":                "green", (1)
   "timed_out":             false,
   "number_of_nodes":       1,
   "number_of_data_nodes":  1,
   "active_primary_shards": 0,
   "active_shards":         0,
   "relocating_shards":     0,
   "initializing_shards":   0,
   "unassigned_shards":     0
}

  1. The status field is the one we’re most interested in.

The status field provides an overall indication of how the cluster is functioning. The meanings of the three colors are provided here for reference:

green

All primary and replica shards are active.

yellow

All primary shards are active, but not all replica shards are active.

red

Not all primary shards are active.

In the rest of this chapter, we explain what primary and replica shards are and explain the practical implications of each of the preceding colors.