feat: add Microservices Load Shedding pattern (#3229) (#3599)

* feat: add Microservices Load Shedding pattern (#3229)

* fix: harden load shedder release and report the admitted count

release() clamps the in-flight count at zero, acquire() returns the admitted count that the service logs, and the demo summary reports the total shed. The class diagram is a rendered PNG.

microservices-load-shedding/src/main/java/com/iluwatar/loadshedding/RequestHandler.java

@@ -0,0 +1,41 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.loadshedding;
+
+/**
+ * The actual work a service performs for an admitted request. Kept as a functional interface so
+ * that the admission control in {@link ShedGuardedService} stays independent of the business logic.
+ */
+@FunctionalInterface
+public interface RequestHandler {
+
+  /**
+   * Processes the request.
+   *
+   * @param request an admitted request
+   * @return result to return to the caller
+   */
+  String handle(Request request);
+}