opensearch-001: ImmutableCacheStatsHolder O(n²) levelsList.contains in filterLevels

Classification

CWE: CWE-407 (Inefficient Algorithmic Complexity)
Severity: MEDIUM
Path: Cache stats reporting — called on every stats API request

Location

server/src/main/java/org/opensearch/common/cache/stats/ImmutableCacheStatsHolder.java:232-235

Defect

private List<String> filterLevels(String[] levels, List<String> dimensionNames) {
    if (levels == null) {
        return dimensionNames;
    }
    List<String> levelsList = Arrays.asList(levels);       // backed array — O(n) contains
    List<String> result = new ArrayList<>();
    for (String dimensionName : dimensionNames) {
        if (levelsList.contains(dimensionName)) {           // O(levels.length) per iteration
            result.add(dimensionName);
        }
    }
    return result;
}

Arrays.asList() returns a fixed-size list backed by the array — contains() is a linear O(levels.length) scan. The outer loop runs dimensionNames.size() times.

Total: O(dimensionNames × levels) = O(n²) when both grow proportionally.

For a cluster with D dimensions and L requested levels this fires on every /_nodes/stats or cache stats request.

Fix

Set<String> levelsSet = new HashSet<>(Arrays.asList(levels));  // O(L) build
for (String dimensionName : dimensionNames) {
    if (levelsSet.contains(dimensionName)) {                    // O(1)
        result.add(dimensionName);
    }
}

Complexity

Metric	Before	After
contains()	O(L)	O(1)
Full filter	O(D×L)	O(D+L)

Affected Versions

All OpenSearch versions with the multi-tier cache stats feature.

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