Add support for session save mode

This commit introduces SaveMode enum that individual SessionRepository implementations use to allow customization of the way they handle save operation in terms of tracking delta of changes.

Resolves: #1466

spring-session-core/src/main/java/org/springframework/session/SaveMode.java

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2014-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.session;
+
+/**
+ * Supported modes of tracking and saving session changes to session store.
+ *
+ * @author Rob Winch
+ * @author Vedran Pavic
+ * @since 2.2.0
+ */
+public enum SaveMode {
+
+	/**
+	 * Save only changes made to session, for instance using
+	 * {@link Session#setAttribute(String, Object)}. In highly concurrent environments,
+	 * this mode minimizes the risk of attributes being overwritten during processing of
+	 * parallel requests.
+	 */
+	ON_SET_ATTRIBUTE,
+
+	/**
+	 * Same as {@link #ON_SET_ATTRIBUTE} with addition of saving attributes that have been
+	 * read using {@link Session#getAttribute(String)}.
+	 */
+	ON_GET_ATTRIBUTE,
+
+	/**
+	 * Always save all session attributes, regardless of the interaction with the session.
+	 * In highly concurrent environments, this mode increases the risk of attributes being
+	 * overwritten during processing of parallel requests.
+	 */
+	ALWAYS
+
+}