HDDS-7544. Cache replica set in ContainerEntry (#8990)

Author: cchung100m

hadoop-hdds/server-scm/src/main/java/org/apache/hadoop/hdds/scm/container/states/ContainerEntry.java

@@ -17,10 +17,12 @@
 
 package org.apache.hadoop.hdds.scm.container.states;
 
+import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.Map;
 import java.util.Set;
+import java.util.function.Function;
 import org.apache.hadoop.hdds.protocol.DatanodeID;
 import org.apache.hadoop.hdds.scm.container.ContainerInfo;
 import org.apache.hadoop.hdds.scm.container.ContainerReplica;
@@ -31,7 +33,8 @@
  */
 public class ContainerEntry {
   private final ContainerInfo info;
-  private final Map<DatanodeID, ContainerReplica> replicas = new HashMap<>();
+  private final Map<DatanodeID, ContainerReplica> replicasMap = new HashMap<>();
+  private Set<ContainerReplica> replicas = Collections.emptySet();
 
   ContainerEntry(ContainerInfo info) {
     this.info = info;
@@ -42,14 +45,20 @@ public ContainerInfo getInfo() {
   }
 
   public Set<ContainerReplica> getReplicas() {
-    return new HashSet<>(replicas.values());
+    return replicas;
   }
 
   public ContainerReplica put(ContainerReplica r) {
-    return replicas.put(r.getDatanodeDetails().getID(), r);
+    return copyAndUpdate(map -> map.put(r.getDatanodeDetails().getID(), r));
   }
 
   public ContainerReplica removeReplica(DatanodeID datanodeID) {
-    return replicas.remove(datanodeID);
+    return copyAndUpdate(map -> map.remove(datanodeID));
+  }
+
+  private <T> T copyAndUpdate(Function<Map<DatanodeID, ContainerReplica>, T> update) {
+    T result = update.apply(replicasMap);
+    replicas = Collections.unmodifiableSet(new HashSet<>(replicasMap.values()));
+    return result;
   }
 }