Elasticsearch ClusterManagerBlockException: Cluster manager not available

Brief Explanation

The "ClusterManagerBlockException: Cluster manager not available" error in Elasticsearch occurs when the cluster manager node is unreachable or not functioning properly. This error indicates that the cluster is unable to perform certain operations that require the cluster manager's involvement.

Impact

This error can have significant impact on the Elasticsearch cluster:

Cluster-wide operations may fail or be delayed
Index creation, deletion, or updates might be blocked
Shard allocation and relocation processes may be affected
Overall cluster health and stability can be compromised

Common Causes

Network issues preventing communication with the cluster manager node
Cluster manager node failure or crash
High load or resource constraints on the cluster manager node
Misconfiguration of cluster settings
Incompatible versions of Elasticsearch across nodes

Troubleshooting and Resolution Steps

Check the cluster health and status:
```
GET _cluster/health
GET _cat/nodes?v
```
Verify network connectivity to the cluster manager node.
Review Elasticsearch logs for any error messages or warnings related to the cluster manager.
Ensure the cluster manager node has sufficient resources (CPU, memory, disk space).
Check for any recent configuration changes that might have affected the cluster manager.
Restart the cluster manager node if it's unresponsive.
If the issue persists, consider promoting another node to become the cluster manager:
```
POST /_cluster/voting_config_exclusions?node_names=current_master_node_name
```
After resolving the issue, remove the voting config exclusions:
```
DELETE /_cluster/voting_config_exclusions
```

Best Practices

Implement proper monitoring and alerting for cluster health and node status.
Use a minimum of three master-eligible nodes for better cluster stability.
Regularly perform cluster health checks and maintenance.
Keep Elasticsearch versions consistent across all nodes in the cluster.
Implement proper backup and disaster recovery strategies.

Frequently Asked Questions

Q: Can I have multiple cluster manager nodes in an Elasticsearch cluster?
A: Yes, you can have multiple master-eligible nodes, but only one will be the active cluster manager at any given time. Having multiple master-eligible nodes provides redundancy and improves cluster stability.

Q: How does Elasticsearch choose a new cluster manager if the current one fails?
A: Elasticsearch uses a process called "master election" where eligible nodes vote to select a new cluster manager. This process is automatic and based on factors like node ID and cluster state version.

Q: What's the difference between a cluster manager node and a data node?
A: A cluster manager node is responsible for cluster-wide actions like creating/deleting indices and tracking node membership. Data nodes hold the shards containing indexed documents and handle data-related operations.

Q: How can I prevent split-brain scenarios in my Elasticsearch cluster?
A: To prevent split-brain scenarios, ensure you have an odd number of master-eligible nodes and set the discovery.zen.minimum_master_nodes (for versions before 7.0) or cluster.initial_master_nodes (for version 7.0+) setting appropriately.

Q: What should I do if all master-eligible nodes in my cluster fail simultaneously?
A: If all master-eligible nodes fail, you'll need to restore from backups or snapshots. To prevent this scenario, ensure you have at least three master-eligible nodes spread across different availability zones or data centers.