Implement concurrency limits plugin

[bpatelcs]

Description

Adds a new concurrency-limit module that provides adaptive per-action concurrency limiting for OpenSearch. Operators can dynamically configure concurrency limits on any transport action (e.g., search, bulk) via cluster settings — no code changes required to protect new actions.

Key features

• Adaptive algorithms: Vegas (TCP-Vegas-based, default), Gradient2, and AIMD — powered by Netflix's concurrency-limits library
• Three operating modes: disabled (default), monitor_only (tracks metrics without rejecting), enforced (actively rejects with HTTP 429)
• Burst capacity: Optional burst headroom above the adaptive base limit, with a configurable close/open state machine to absorb transient spikes
• Vegas baseline poisoning protection: Probe-based RTT baseline resets are gated by a configurable inflight threshold (vegas.baseline_reset_load_threshold), preventing heavy-load latency from corrupting the no-load baseline and causing a feedback loop
• Increase/decrease hysteresis: Configurable consecutive-sample thresholds (vegas.increase_barrier, vegas.decrease_barrier) to dampen limit oscillation
• Request partitioning: Divide the total limit across named sub-pools (e.g., premium/standard) using pluggable resolvers (byHeader, fixed, bySearchType)
• Warmup period: Configurable grace period after (re)configuration during which requests are not rejected, allowing the algorithm to calibrate
• _nodes/stats integration: Limiter state (current limit, in-flight, total rejected, RTT) exposed under concurrency_limiters in the node stats API
• Push telemetry: Observable gauges registered via MetricsRegistry mirror the pull-path stats for monitoring systems

Configuration example

PUT /_cluster/settings
{
  "persistent": {
    "concurrency_limit.action.search.action_name": "indices:data/read/search",
    "concurrency_limit.action.search.mode": "enforced",
    "concurrency_limit.action.search.algorithm": "vegas",
    "concurrency_limit.action.search.limit.initial": 20,
    "concurrency_limit.action.search.limit.max": 200,
    "concurrency_limit.action.search.vegas.baseline_reset_load_threshold": 0.5,
    "concurrency_limit.action.search.burst.capacity": 10,
    "concurrency_limit.action.search.burst.close_after": 5,
    "concurrency_limit.action.search.burst.open_after": 5
  }
}

Files changed

• New module modules/concurrency-limit/ — 8 source files, 6 test files, build.gradle, third-party licenses
• Server integration — NodeStats, NodeService, Node, NodesStatsRequest, TransportNodesStatsAction, TransportClusterStatsAction updated to plumb concurrency_limiter stats (version-gated to V_3_8_0)
• SPI interface — ConcurrencyLimiterStatsPlugin for decoupled stats collection
• Test framework — MockInternalClusterInfoService, InternalTestCluster updated for new NodeStats constructor parameter

Test plan

• ./gradlew :modules:concurrency-limit:check — precommit, unit tests, spotless, missingJavadoc, thirdPartyAudit all pass
• Manual verification: start a node, configure a limiter via cluster settings, confirm _nodes/stats?metric=concurrency_limiter returns limiter state

Related Issues

Resolves #22295

Check List

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

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[github-actions]

PR Code Analyzer ❗

AI-powered 'Code-Diff-Analyzer' found issues on commit 08aea12.

Path	Line	Severity	Description
gradle/libs.versions.toml	23	high	New third-party dependency version entry added: 'netflix_concurrency_limits = "0.4.0"' introduces com.netflix.concurrency-limits into the dependency catalog. Per mandatory supply chain rules, any new dependency must be flagged regardless of apparent legitimacy — namespace hijacking and typosquatting are common attack vectors.
modules/concurrency-limit/build.gradle	17	high	New implementation dependency declared: 'com.netflix.concurrency-limits:concurrency-limits-core:0.4.0'. This pulls a new external artifact into the build. The SHA1 file (9bf32c330cad19a6e564b460a65132aa765b6440) provides a checksum but cannot verify artifact authenticity or supply chain integrity — maintainers must independently verify the artifact origin and contents.

The table above displays the top 10 most important findings.

Total: 2 | Critical: 0 | High: 2 | Medium: 0 | Low: 0

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