Improve regex parsing in GET /_alias/ or GET /_alias/prefix

[mayya-sharipova]

Elasticsearch Version

8.14

Installed Plugins

No response

Java Version

bundled

OS Version

Linux

Problem Description

A user has a huge amount of aliases for their indices (could be > 1000 aliases per index), defined like this:

    "cf_index": {
        "aliases": {
            "cf_0001": {
                "filter": {
                    "term": {
                        "cf": "0001"
                    }
                }
            },
            "cf_0002": {
                "filter": {
                    "term": {
                        "cf": 0002"
                    }
                }
            },
            "cf_0003": {
                "filter": {
                    "term": {
                        "cf": "003"
                    }
                }
            },

when GET _alias API is requested (through monitoring or user based? ), there is a lot CPU spent on processing Regex expressions:

   100.4% [cpu=100.4%, other=0.0%] (502.2ms out of 500ms) cpu usage by thread 'elasticsearch[instance-0000000047][management][T#2]'
     10/10 snapshots sharing following 16 elements
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.common.regex.Regex.simpleMatchWithNormalizedStrings(Regex.java:170)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.common.regex.Regex.simpleMatch(Regex.java:164)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.common.regex.Regex.simpleMatch(Regex.java:143)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.cluster.metadata.Metadata.findAliasInfo(Metadata.java:896)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.cluster.metadata.Metadata.findAliases(Metadata.java:823)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.action.admin.indices.alias.get.TransportGetAliasesAction.localClusterStateOperation(TransportGetAliasesAction.java:98)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.action.admin.indices.alias.get.TransportGetAliasesAction.localClusterStateOperation(TransportGetAliasesAction.java:47)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.action.support.TransportLocalClusterStateAction.lambda$doExecute$0(TransportLocalClusterStateAction.java:60)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.action.support.TransportLocalClusterStateAction$$Lambda/0x00007f1f2ce2bcc8.accept(Unknown Source)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.action.ActionRunnable$4.doRun(ActionRunnable.java:100)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:984)
       app/org.elasticsearch.server@8.14.1/org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:26)
       java.base@22.0.1/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
       java.base@22.0.1/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
       java.base@22.0.1/java.lang.Thread.runWith(Thread.java:1583)
       java.base@22.0.1/java.lang.Thread.run(Thread.java:1570)

Steps to Reproduce

Steps (I have not reproduced myself):

• Create an index with > 1000 filter aliases
• Request GET _alias API or GET /_alias/prefix API

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)