Add deprecation warning for GET/POST/PUT cache

src/main/java/org/opensearch/security/dlic/rest/api/FlushCacheApiAction.java

@@ -19,6 +19,7 @@
 
 import org.opensearch.cluster.service.ClusterService;
 import org.opensearch.common.inject.Inject;
+import org.opensearch.common.logging.DeprecationLogger;
 import org.opensearch.core.action.ActionListener;
 import org.opensearch.rest.RestRequest;
 import org.opensearch.rest.RestRequest.Method;
@@ -29,13 +30,16 @@
 import org.opensearch.threadpool.ThreadPool;
 
 import static org.opensearch.security.dlic.rest.api.Responses.internalServerError;
+import static org.opensearch.security.dlic.rest.api.Responses.methodNotImplemented;
 import static org.opensearch.security.dlic.rest.api.Responses.ok;
 import static org.opensearch.security.dlic.rest.support.Utils.addRoutesPrefix;
 
 public class FlushCacheApiAction extends AbstractApiAction {
 
     private final static Logger LOGGER = LogManager.getLogger(FlushCacheApiAction.class);
 
+    private static final DeprecationLogger deprecationLogger = DeprecationLogger.getLogger(FlushCacheApiAction.class);
+
     private static final List<Route> routes = addRoutesPrefix(
         ImmutableList.of(
             new Route(Method.DELETE, "/cache"),
@@ -91,7 +95,23 @@
 
                     }
                 )
-            );
+            )
+            .override(Method.GET, (channel, request, client) -> {
+                deprecationLogger.deprecate("GET", "GET is not supported on this endpoint and will be removed in the next major release.");
+                methodNotImplemented(channel, Method.GET);
+            })
+            .override(Method.POST, (channel, request, client) -> {
+                deprecationLogger.deprecate(
+                    "POST",
+                    "POST is not supported on this endpoint and will be removed in the next major release."
+                );
+                methodNotImplemented(channel, Method.POST);
+            })
+            .override(Method.PUT, (channel, request, client) -> {
+                deprecationLogger.deprecate("PUT", "PUT is not supported on this endpoint and will be removed in the next major release.");
+                methodNotImplemented(channel, Method.PUT);
+            });
+
     }
 
     @Override