Search only replicas (scale to zero) with Reader/Writer Separation

[prudhvigodithi]

Description

• The primary goal is to allow users to designate an index as search-only allowing only to have the search only replicas running when enabled via an API call _searchonly/enable (can be disabled as _searchonly/disable).

• With _searchonly/enable for an index the process has Two-Phase Scale-Down with a temporary block for the duration of the scale-down operation and then explicitly replace it with a permanent block once all prerequisites (e.g., shard sync, flush, metadata updates) have been met.

From #17299 (comment) using _scale API with search_only set to true or false.

curl -X POST "http://localhost:9200/my-index/_scale" \
-H "Content-Type: application/json" \
-d '{
  "search_only": true
}' 

curl -X POST "http://localhost:9200/my-index/_scale" \
-H "Content-Type: application/json" \
-d '{
  "search_only": false
}'

• Eliminates the need for users to manually invoke the _remotestore/_restore API to recover search-only replicas with _searchonly/enable, has automatic recovery of search-only replicas from the remote store during cluster recovery. The default behavior is still honored in normal conditions https://opensearch.org/docs/latest/tuning-your-cluster/availability-and-recovery/remote-store/index/#restoring-from-a-backup. More testing scenarios added here Search only replicas (scale to zero) with Reader/Writer Separation #17299 (comment).

• To Do (Work on the cluster health, coming from [META] Reader/Writer Separation #15306 (comment) add this similar implementation) This PR also has updated cluster health code Search only replicas (scale to zero) with Reader/Writer Separation #17299 (comment).

Related Issues

#16720 and part of #15306

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

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[prudhvigodithi]

While I refactor the code and add additional tests, I’m creating this PR to gather early feedback please take a look and add your thoughts. I will share the testing results in the comments. Thanks!
@mch2 @shwetathareja @msfroh @getsaurabh02

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[prudhvigodithi]

I went through and tested the following scenarios

Scenario 1: Search-Only Replicas Recovery with Persistent Data Directory and when cluster.remote_store.state.enabled is set to false

With the following settings, OpenSearch was started using:

./gradlew clean run -PnumNodes=6 --data-dir=/tmp/foo

OpenSearch settings

    
setting 'path.repo', '["/tmp/my-repo"]'
setting 'opensearch.experimental.feature.read.write.split.enabled', 'true'
setting 'node.attr.remote_store.segment.repository', 'my-repository'
setting 'node.attr.remote_store.translog.repository', 'my-repository'
setting 'node.attr.remote_store.repository.my-repository.type', 'fs'
setting 'node.attr.remote_store.state.repository', 'my-repository'
setting 'node.attr.remote_store.repository.my-repository.settings.location', '/tmp/my-repo'
    
  

Shard Allocation Before Recovery

curl -X GET "localhost:9200/_cat/shards/my-index?v&h=index,shard,prirep,state,unassigned.reason,node,searchOnly"

index    shard prirep state   unassigned.reason node
my-index 0     p      STARTED                   runTask-0
my-index 0     s      STARTED                   runTask-4
my-index 0     r      STARTED                   runTask-2
my-index 1     p      STARTED                   runTask-3
my-index 1     r      STARTED                   runTask-1
my-index 1     s      STARTED                   runTask-5

On restart (terminate the process) everything comes back as running. With search only enabled (/_searchonly/enable) after restart only search replicas are up as running and works as expected.

curl -X GET "localhost:9200/_cat/shards/my-index?v&h=index,shard,prirep,state,unassigned.reason,node,searchOnly"
index    shard prirep state   unassigned.reason node
my-index 0     s      STARTED                   runTask-2
my-index 1     s      STARTED                   runTask-1

Scenario 2: No Data Directory Preservation and when cluster.remote_store.state.enabled is set t o false – Index Lost After process Restart (Recovery)

In this scenario, OpenSearch is started without preserving the data directory, meaning that all local shard data is lost upon Recovery.

./gradlew clean run -PnumNodes=6

OpenSearch settings

    
setting 'path.repo', '["/tmp/my-repo"]'
setting 'opensearch.experimental.feature.read.write.split.enabled', 'true'
setting 'node.attr.remote_store.segment.repository', 'my-repository'
setting 'node.attr.remote_store.translog.repository', 'my-repository'
setting 'node.attr.remote_store.repository.my-repository.type', 'fs'
setting 'node.attr.remote_store.state.repository', 'my-repository'
setting 'node.attr.remote_store.repository.my-repository.settings.location', '/tmp/my-repo'
    
  

Behavior After Recovery:

• Upon terminating the process and restarting OpenSearch, the index is completely lost.
• Any attempt to retrieve the shard state results in an index not found exception.

{"error":{"root_cause":[{"type":"index_not_found_exception","reason":"no such index [my-index]","index":"my-index","resource.id":"my-index","resource.type":"index_or_alias","index_uuid":"_na_"}],"type":"index_not_found_exception","reason":"no such index [my-index]","index":"my-index","resource.id":"my-index","resource.type":"index_or_alias","index_uuid":"_na_"},"status":404}%

• Even With Remote Restore _remotestore/_restore?restore_all_shards=true, index remains unavailable.
• Even after recreating the index manually and attempting to restore, documents do not get picked up.
• Since during testing --data-dir was not used, local data (including cluster metadata) is wiped on recovery.
• Because the cluster state is lost, OpenSearch no longer has any reference to index.

Scenario 3: Cluster Remote Store State Enabled (cluster.remote_store.state.enabled is set to true and with no persistent data directory) – Primary Shards Remain Unassigned After Recovery.

./gradlew clean run -PnumNodes=6

OpenSearch settings

    
setting 'path.repo', '["/tmp/my-repo"]'
setting 'opensearch.experimental.feature.read.write.split.enabled', 'true'
setting 'node.attr.remote_store.segment.repository', 'my-repository'
setting 'node.attr.remote_store.translog.repository', 'my-repository'
setting 'node.attr.remote_store.repository.my-repository.type', 'fs'
setting 'node.attr.remote_store.state.repository', 'my-repository'
setting 'node.attr.remote_store.repository.my-repository.settings.location', '/tmp/my-repo'
setting 'cluster.remote_store.state.enabled', 'true'
    
  

Shard Allocation After Recovery

curl -X GET "localhost:9200/_cat/shards/my-index?v&h=index,shard,prirep,state,unassigned.reason,node,searchOnly"
index    shard prirep state      unassigned.reason node
my-index 0     p      UNASSIGNED CLUSTER_RECOVERED 
my-index 0     s      UNASSIGNED CLUSTER_RECOVERED 
my-index 0     r      UNASSIGNED CLUSTER_RECOVERED 
my-index 1     p      UNASSIGNED CLUSTER_RECOVERED 
my-index 1     r      UNASSIGNED CLUSTER_RECOVERED 
my-index 1     s      UNASSIGNED CLUSTER_RECOVERED 

Issue: Primary Shards Remain Unassigned
Despite cluster.remote_store.state.enabled is true, the primary shards are not automatically assigned after restart ( replicating the recovery). The error message states:

"allocate_explanation": "cannot allocate because a previous copy of the primary shard existed but can no longer be found on the nodes in the cluster"

• Remote store only contains segments and translogs, NOT active shard copies.
• Since --data-dir was not used (data directory is not used), local copies of primary shards are lost.
• OpenSearch does not automatically restore primaries from the remote store without explicit intervention.

curl -X POST "http://localhost:9200/_remotestore/_restore" -H 'Content-Type: application/json' -d'  
{                
  "indices": ["my-index"]
}
'

• However with this PR, when _searchonly is enabled Search-Only Replicas Recover Without a Primary. Since cluster.remote_store.state.enabled is true, OpenSearch remembers the index exists after restart. The allocation logic skips checking for an active primary for search-only replicas.This allows search-only replicas to be assigned to a node, even without an existing primary. However without _searchonly the behavior is same for all replicas, wanted to give an advantage for users with _searchonly enabled indicies, for these indices they should not care _remotestore/_restore as we are not dealing with primaries.

  • Search-only replicas can recover automatically from the remote store.
  • Search queries remain functional.
  • Cluster state correctly remembers the index, but does not bring up primaries as _searchonly is enabled.

• The default behavior is OpenSearch does not assume lost primaries should be re-created from remote storage.It waits for explicit user intervention to restore primary shards (_remotestore/_restore). Is this by design ?

Scenario 4: Persistent Data Directory with Remote Store State – Seamless Recovery of Primaries, replicas and search-only replicas

./gradlew clean run -PnumNodes=6 --data-dir=/tmp/foo

OpenSearch settings

    
setting 'path.repo', '["/tmp/my-repo"]'
setting 'opensearch.experimental.feature.read.write.split.enabled', 'true'
setting 'node.attr.remote_store.segment.repository', 'my-repository'
setting 'node.attr.remote_store.translog.repository', 'my-repository'
setting 'node.attr.remote_store.repository.my-repository.type', 'fs'
setting 'node.attr.remote_store.state.repository', 'my-repository'
setting 'node.attr.remote_store.repository.my-repository.settings.location', '/tmp/my-repo'
setting 'cluster.remote_store.state.enabled', 'true'
    
  

Upon recovery (no intervention is required )

curl -X GET "localhost:9200/_cat/shards/my-index?v&h=index,shard,prirep,state,unassigned.reason,node,searchOnly"
index    shard prirep state   unassigned.reason node
my-index 0     p      STARTED                   runTask-0
my-index 0     r      STARTED                   runTask-2
my-index 0     s      STARTED                   runTask-4
my-index 1     p      STARTED                   runTask-5
my-index 1     r      STARTED                   runTask-3
my-index 1     s      STARTED                   runTask-1

• All primary and replica shards successfully recover since the cluster metadata is retained in the persistent data directory.

If search-only mode is enabled to index, OpenSearch correctly brings up only search replicas while removing primary and regular replicas.

curl -X GET "localhost:9200/_cat/shards/my-index?v&h=index,shard,prirep,state,unassigned.reason,node,searchOnly"
index    shard prirep state   unassigned.reason node
my-index 0     s      STARTED                   runTask-3
my-index 1     s      STARTED                   runTask-3

• Only search replicas (SORs) are restored, as expected.

[prudhvigodithi]

Coming from #17299 (comment) @shwetathareja can you please go over scenario 2 and 3 and if it make sense. I wanted to understand why _remotestore/_restore is required in these scenarios and I wanted to give advantage for users ti remove this intervention for search only indices.
Thanks
@mch2

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❌ Gradle check result for 7fa5133: FAILURE

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[prudhvigodithi]

I have updated the PR to adjust the cluster health configuration using only search replicas and to incorporate the changes made when _searchonly is enabled, the change is not too big hence going with the same PR.

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❌ Gradle check result for 64bb954: FAILURE

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[prudhvigodithi]

Adding @sachinpkale can you please take a look at this comment #17299 (comment) and provide your thoughts to understand why _remotestore/_restore is required (Scenario 3 from #17299 (comment)) and why the cluster cannot be auto recovered, is there any strong reason for this manual intervention to run the API?

curl -X GET "localhost:9200/_cat/shards/my-index?v&h=index,shard,prirep,state,unassigned.reason,node,searchOnly"
index    shard prirep state      unassigned.reason node
my-index 0     p      UNASSIGNED CLUSTER_RECOVERED 
my-index 0     s      UNASSIGNED CLUSTER_RECOVERED 
my-index 0     r      UNASSIGNED CLUSTER_RECOVERED 
my-index 1     p      UNASSIGNED CLUSTER_RECOVERED 
my-index 1     s      UNASSIGNED CLUSTER_RECOVERED 
my-index 1     r      UNASSIGNED CLUSTER_RECOVERED 

I dint get much info from the docs https://opensearch.org/docs/latest/tuning-your-cluster/availability-and-recovery/remote-store/index/#restoring-from-a-backup.

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✅ Gradle check result for 4f3b2a5: SUCCESS

[mch2]

I think this is in a good state, I'd like to get this merged so we can bake this and iterate if needed before 3.0 cutoff.
@shwetathareja @Bukhtawar @msfroh Wondering if any of you would like to make another pass here, will hold for another day given the size of this.

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✅ Gradle check result for ec3cde7: SUCCESS

[prudhvigodithi]

Thanks @mch2 I have resolved the conversations/comments which should now allow maintainers to merge the PR.
Thanks