An Elasticsearch cluster in Yellow status means all primary shards are allocated, but some replicas are not. The cluster works but without complete redundancy.
Symptoms
- Cluster health: yellow in _cluster/health
- Unassigned shards reported
- Correct performance but warnings
- No redundancy for certain indexes
Common Causes
- Single node: With a single node, replicas cannot be allocated.
- Insufficient disk space: ES refuses to allocate shards if disk is > 85% full.
- Node down: A cluster node is down and replicas are waiting.
Diagnostic Steps
- Check _cluster/health for details
- List unassigned shards with _cat/shards
- Examine disk space on nodes
- Check node state (_cat/nodes)
Automate with MoniTao
MoniTao can monitor your Elasticsearch:
- Monitoring of _cluster/health endpoint
- JSON content validation for status
- Alerts if status is not green
Best Practices
- Use at least 3 nodes for a production cluster
- Configure index.number_of_replicas according to your nodes
- Monitor disk space (< 80%)
- Set up alerts on cluster health
FAQ
Is Yellow critical?
No, the cluster works. But you don't have complete redundancy.
How to force replica allocation?
Add nodes or reduce number_of_replicas to 0 (not recommended).
What minimum disk space?
ES blocks allocations > 85% and goes read-only > 95%.
Does MoniTao monitor ES natively?
Via HTTP monitoring of _cluster/health with JSON validation.
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