Elasticsearch has many metrics that can be used to determine if a cluster is healthy. Listed below are the metrics that are currently a good idea to monitor with the reason(s) why they should be monitored and any possible recourse for issues.
Unless otherwise noted, all of the API requests work starting with 1.0.0.
Metrics are an easy way to monitor the health of a cluster and they can be easily accessed from the HTTP API. Each Metrics table is broken down by their source; the version of Elasticsearch that is required to retrieve the metric is noted by its name if it is not present in 1.0.0.
Each metric has an associated warning level and error level. These levels are meant to indicate when the associated Metric should be monitored versus acted upon as soon as possible.