10_Cluster_health.asciidoc

Cluster health

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

GET /_cluster/health

which, on an empty cluster with no indices, will return something like:

{
   "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 meaning 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 above colors.