Pagination for wlm/stats api

client/rest-high-level/src/test/java/org/opensearch/client/RestHighLevelClientTests.java

@@ -882,7 +882,8 @@ public void testApiNamingConventions() throws Exception {
             "cluster.delete_weighted_routing",
             "cluster.put_decommission_awareness",
             "cluster.get_decommission_awareness",
-            "cluster.delete_decommission_awareness", };
+            "cluster.delete_decommission_awareness",
+            "wlm_stats_list" };
         List<String> booleanReturnMethods = Arrays.asList("security.enable_user", "security.disable_user", "security.change_password");
         Set<String> deprecatedMethods = new HashSet<>();
         deprecatedMethods.add("indices.force_merge");

rest-api-spec/src/main/resources/rest-api-spec/api/wlm_stats_list.json

@@ -0,0 +1,44 @@
+{
+  "wlm_stats_list": {
+    "stability": "experimental",
+    "documentation": {
+      "url": "https://docs.opensearch.org/docs/latest/tuning-your-cluster/availability-and-recovery/workload-management/wlm-feature-overview/",
+      "description": "This API endpoint returns a list of WLM stats with pagination support."
+    },
+    "url": {
+      "paths": [
+        {
+          "path": "/_list/wlm_stats",
+          "methods": ["GET"]
+        }
+      ]
+    },
+    "params": {
+      "size": {
+        "type": "int",
+        "required": false,
+        "description": "Number of results per page"
+      },
+      "next_token": {
+        "type": "string",
+        "required": false,
+        "description": "Pagination token for next page"
+      },
+      "sort": {
+        "type": "string",
+        "required": false,
+        "description": "Sort field"
+      },
+      "order": {
+        "type": "string",
+        "required": false,
+        "description": "Sort order (asc or desc)"
+      },
+      "v": {
+        "type": "boolean",
+        "required": false,
+        "description": "Whether to include headers"
+      }
+    }
+  }
+}

rest-api-spec/src/main/resources/rest-api-spec/test/wlm_stats/10_basic.yml

@@ -0,0 +1,48 @@
+---
+"Custom size param still returns headers":
+  - do:
+      wlm_stats_list:
+        size: 1
+        v: true
+  - is_true: $body
+  - match:
+      $body: /TOTAL_REJECTIONS/
+
+---
+"Sort by node_id asc does not error":
+  - do:
+      wlm_stats_list:
+        sort: node_id
+        order: asc
+  - is_true: $body
+  - match:
+      $body: /DEFAULT_WORKLOAD_GROUP/
+
+---
+"Sort by workload_group_id desc does not error":
+  - do:
+      wlm_stats_list:
+        sort: workload_group
+        order: desc
+  - is_true: $body
+  - match:
+      $body: /DEFAULT_WORKLOAD_GROUP/
+
+---
+"Invalid sort field returns error":
+  - do:
+      catch: bad_request
+      wlm_stats_list:
+        sort: memory_usage
+        order: desc
+  - match:
+      error.reason: "Invalid value for 'sort'. Allowed: 'node_id', 'workload_group'"
+
+---
+"Invalid sort order returns error":
+  - do:
+      catch: bad_request
+      wlm_stats_list:
+        order: upside_down
+  - match:
+      error.reason: "Invalid value for 'order'. Allowed: 'asc', 'desc'"

server/src/main/java/org/opensearch/action/admin/cluster/wlm/WlmStatsResponse.java

@@ -33,7 +33,7 @@ public class WlmStatsResponse extends BaseNodesResponse<WlmStats> implements ToX
         super(in);
     }
 
-    WlmStatsResponse(ClusterName clusterName, List<WlmStats> nodes, List<FailedNodeException> failures) {
+    public WlmStatsResponse(ClusterName clusterName, List<WlmStats> nodes, List<FailedNodeException> failures) {
         super(clusterName, nodes, failures);
     }
 

server/src/main/java/org/opensearch/action/pagination/SortBy.java

@@ -0,0 +1,51 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.action.pagination;
+
+import org.opensearch.wlm.stats.WlmStats;
+
+import java.util.Comparator;
+import java.util.Locale;
+
+/**
+ * Represents the different fields by which WLM statistics can be sorted.
+ */
+public enum SortBy {
+    WORKLOAD_GROUP {
+        @Override
+        public Comparator<WlmStats> getComparator() {
+            return Comparator.comparing(
+                (WlmStats wlmStats) -> wlmStats.getWorkloadGroupStats().getStats().isEmpty()
+                    ? ""
+                    : wlmStats.getWorkloadGroupStats().getStats().keySet().iterator().next()
+            ).thenComparing(wlmStats -> wlmStats.getNode().getId());
+        }
+    },
+    NODE_ID {
+        @Override
+        public Comparator<WlmStats> getComparator() {
+            return Comparator.comparing((WlmStats wlmStats) -> wlmStats.getNode().getId())
+                .thenComparing(
+                    wlmStats -> wlmStats.getWorkloadGroupStats().getStats().isEmpty()
+                        ? ""
+                        : wlmStats.getWorkloadGroupStats().getStats().keySet().iterator().next()
+                );
+        }
+    };
+
+    public abstract Comparator<WlmStats> getComparator();
+
+    public static SortBy fromString(String input) {
+        try {
+            return SortBy.valueOf(input.toUpperCase(Locale.ROOT));
+        } catch (IllegalArgumentException e) {
+            throw new IllegalArgumentException("Invalid sort field: " + input + ". Allowed values: workload_group, node_id");
+        }
+    }
+}

server/src/main/java/org/opensearch/action/pagination/SortOrder.java

@@ -0,0 +1,43 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.action.pagination;
+
+import org.opensearch.wlm.stats.WlmStats;
+
+import java.util.Comparator;
+import java.util.Locale;
+
+/**
+ * Represents the sort order for WLM statistics.
+ * The sort order can be either ascending or descending.
+ */
+public enum SortOrder {
+    ASC {
+        @Override
+        public Comparator<WlmStats> apply(Comparator<WlmStats> baseComparator) {
+            return baseComparator;
+        }
+    },
+    DESC {
+        @Override
+        public Comparator<WlmStats> apply(Comparator<WlmStats> baseComparator) {
+            return baseComparator.reversed();
+        }
+    };
+
+    public abstract Comparator<WlmStats> apply(Comparator<WlmStats> baseComparator);
+
+    public static SortOrder fromString(String input) {
+        try {
+            return SortOrder.valueOf(input.toUpperCase(Locale.ROOT));
+        } catch (IllegalArgumentException e) {
+            throw new IllegalArgumentException("Invalid sort order: " + input + ". Allowed values: asc, desc");
+        }
+    }
+}