The cluster.blocks.read_only
setting in Elasticsearch is a cluster-wide setting that controls whether the entire cluster is in read-only mode. When enabled, it prevents any write operations on the cluster, including indexing new documents, updating existing ones, or making changes to cluster settings.
- Default Value:
false
- Possible Values:
true
orfalse
- Recommendations: This setting should be used with caution and typically only in specific scenarios such as during maintenance or to prevent data modifications in critical situations.
When set to true
, the cluster enters a read-only state where:
- Read operations (searches, gets) are still allowed
- Write operations (index, update, delete) are blocked
- Cluster state changes are prevented
Example
To enable read-only mode for the entire cluster:
PUT _cluster/settings
{
"persistent": {
"cluster.blocks.read_only": true
}
}
This might be used when you need to perform maintenance on the cluster or when you want to prevent any accidental writes during a critical operation.
To disable read-only mode:
PUT _cluster/settings
{
"persistent": {
"cluster.blocks.read_only": null
}
}
Common Issues and Misuses
- Forgetting to disable read-only mode after maintenance, leading to unexpected write failures
- Enabling read-only mode without proper communication to users or applications, causing confusion and potential data loss
- Using this setting instead of more granular index-level read-only blocks when only specific indices need protection
Do's and Don'ts
Do's:
- Use this setting for cluster-wide maintenance or emergency situations
- Communicate clearly with all stakeholders before enabling read-only mode
- Have a plan and timeline for disabling read-only mode
- Monitor the cluster state to ensure it's not left in read-only mode unintentionally
Don'ts:
- Don't use this setting for long-term read-only scenarios; consider other options like index-level blocks
- Don't enable read-only mode without a clear purpose and plan
- Don't forget to disable it after the maintenance or emergency situation is resolved
Frequently Asked Questions
Q: How does cluster.blocks.read_only differ from index blocks?
A: While cluster.blocks.read_only affects the entire cluster, index blocks can be applied to specific indices. Index blocks offer more granular control, allowing you to set read-only status on individual indices rather than the whole cluster.
Q: Can I still perform cluster state changes when cluster.blocks.read_only is true?
A: No, when cluster.blocks.read_only is set to true, it prevents most cluster state changes along with write operations. This includes changes to settings, mappings, and other cluster-level configurations.
Q: Will setting cluster.blocks.read_only to true affect ongoing ingestion or indexing jobs?
A: Yes, enabling this setting will cause all ongoing write operations, including ingestion and indexing jobs, to fail. It's important to coordinate the timing of enabling this setting to minimize disruption.
Q: How can I check if my cluster is currently in read-only mode?
A: You can check the current cluster settings using the Cluster Get Settings API:
GET _cluster/settings?include_defaults=true
Look for the "cluster.blocks.read_only" setting in the response.
Q: Is there a way to temporarily enable read-only mode without making it persistent?
A: Yes, you can use transient settings instead of persistent ones. Replace "persistent" with "transient" in the API call. Transient settings are lost upon a full cluster restart, while persistent settings are not.