Elasticsearch Split Brain Scenario (multiple master nodes) - Common Causes & Fixes

Brief Explanation

A split brain scenario in Elasticsearch occurs when multiple nodes in a cluster believe they are the master node. This situation can lead to data inconsistencies and cluster instability.

Impact

The split brain scenario has a severe impact on cluster health and data integrity:

• Data inconsistency across the cluster
• Potential data loss
• Degraded cluster performance
• Unpredictable cluster behavior

Common Causes

1. Network issues causing node communication failures
2. Incorrect configuration of discovery settings
3. Insufficient master-eligible nodes
4. Misconfigured minimum_master_nodes setting (in older versions)
5. Hardware failures affecting node connectivity

Troubleshooting and Resolution Steps

1. Identify the affected nodes:

   • Use the GET /_cat/nodes?v API to list all nodes and their roles

2. Verify network connectivity:

   • Check network settings and firewall rules
   • Ensure all nodes can communicate with each other

3. Review discovery and cluster formation settings:

   • Check discovery.seed_hosts and `cluster.initial_master_nodes` settings
   • Ensure discovery.zen.minimum_master_nodes is set correctly (for versions before 7.0)

4. Adjust cluster settings:

   • Set cluster.no_master_block: all to prevent writes during split brain scenarios

5. Restart the cluster:

   • Stop all nodes
   • Start master-eligible nodes first, then data nodes

6. Monitor cluster health:

   • Use GET /_cluster/health to verify cluster status

7. Consider implementing a quorum-based solution:

   • Use an odd number of master-eligible nodes (3 or more)

Best Practices

• Always use an odd number of master-eligible nodes (3 or 5)
• Implement proper network segmentation and redundancy
• Regularly monitor cluster health and node status
• Use Elasticsearch Service or a managed solution for automatic split brain prevention
• Keep Elasticsearch updated to benefit from the latest stability improvements

Frequently Asked Questions

Q: What is the minimum number of master-eligible nodes recommended for a production cluster?
A: It's recommended to have at least 3 master-eligible nodes in a production cluster to prevent split brain scenarios and ensure high availability.

Q: Can a split brain scenario occur in Elasticsearch 7.x and later versions?
A: While less likely, split brain scenarios can still occur in newer versions. Elasticsearch 7.x and later use a new cluster coordination algorithm that significantly reduces the risk, but proper configuration is still crucial.

Q: How does the cluster.no_master_block setting help in split brain scenarios?
A: The cluster.no_master_block: all setting prevents both read and write operations when no master is detected, reducing the risk of data inconsistencies during a split brain scenario.

Q: Can increasing the discovery.zen.ping_timeout setting help prevent split brain scenarios?
A: Increasing discovery.zen.ping_timeout can help in environments with slower networks, giving nodes more time to respond before being considered offline. However, it's not a solution for underlying network issues or misconfigurations.

Q: How can I recover data if a split brain scenario has caused data inconsistencies?
A: Recovering from data inconsistencies caused by a split brain scenario can be complex. It may involve identifying the most up-to-date data set, reindexing from backups, or using tools like the Elasticsearch Tribe node to compare and merge data from different parts of the split cluster.