[hotfix] Update comments in 'ChainingStrategy' and remove outdated 'FORCE_ALWAYS' constant

flink-streaming-java/src/main/java/org/apache/flink/streaming/api/graph/StreamingJobGraphGenerator.java

@@ -398,15 +398,12 @@ public class StreamingJobGraphGenerator {
 				&& headOperator != null
 				&& upStreamVertex.getSlotSharingID() == downStreamVertex.getSlotSharingID()
 				&& upStreamVertex.getSlotSharingID() != -1
-				&& (outOperator.getChainingStrategy() == ChainingStrategy.ALWAYS ||
-					outOperator.getChainingStrategy() == ChainingStrategy.FORCE_ALWAYS)
+				&& outOperator.getChainingStrategy() == ChainingStrategy.ALWAYS
 				&& (headOperator.getChainingStrategy() == ChainingStrategy.HEAD ||
-					headOperator.getChainingStrategy() == ChainingStrategy.ALWAYS ||
-					headOperator.getChainingStrategy() == ChainingStrategy.FORCE_ALWAYS)
+					headOperator.getChainingStrategy() == ChainingStrategy.ALWAYS)
 				&& (edge.getPartitioner() instanceof ForwardPartitioner)
 				&& upStreamVertex.getParallelism() == downStreamVertex.getParallelism()
-				&& (streamGraph.isChainingEnabled() ||
-					outOperator.getChainingStrategy() == ChainingStrategy.FORCE_ALWAYS);
+				&& streamGraph.isChainingEnabled();
 	}
 
 	private void setSlotSharing() {

flink-streaming-java/src/main/java/org/apache/flink/streaming/api/operators/ChainingStrategy.java

@@ -18,23 +18,22 @@
 
 package org.apache.flink.streaming.api.operators;
 
-
 /**
- * Defines the chaining scheme for the operator.
- * By default {@link #ALWAYS} is used, which means operators will be eagerly chained whenever possible.
+ * Defines the chaining scheme for the operator. When an operator is chained to the
+ * predecessor, it means that they run in the same thread. They become one operator
+ * consisting of multiple steps.
+ * 
+ * <p>The default value used by the {@link StreamOperator} is {@link #HEAD}, which means that
+ * the operator is not chained to its predecessor. Most operators override this with
+ * {@link #ALWAYS}, meaning they will be chained to predecessors whenever possible.
  */
 public enum ChainingStrategy {
 
-	/**
-	 * Chaining will happen even if chaining is disabled on the execution environment.
-	 * This should only be used by system-level operators, not operators implemented by users.
-	 */
-	FORCE_ALWAYS,
-
 	/** 
-	 * Operators will be eagerly chained whenever possible, for
-	 * maximal performance. It is generally a good practice to allow maximal
-	 * chaining and increase operator parallelism
+	 * Operators will be eagerly chained whenever possible.
+	 * 
+	 * <p>To optimize performance, it is generally a good practice to allow maximal
+	 * chaining and increase operator parallelism.
 	 */
 	ALWAYS,
 
@@ -42,7 +41,10 @@ public enum ChainingStrategy {
 	 * The operator will not be chained to the preceding or succeeding operators.
 	 */
 	NEVER,
-	
-	
+
+	/**
+	 * The operator will not be chained to the predecessor, but successors may chain to this
+	 * operator.
+	 */
 	HEAD
 }