add comments class in selector package

skywalking-collector/skywalking-collector-cluster/src/main/java/com/a/eye/skywalking/collector/actor/selector/AbstractHashMessage.java

 package com.a.eye.skywalking.collector.actor.selector;
 
 /**
+ * The <code>AbstractHashMessage</code> should be implemented by any class whose instances
+ * are intended to provide send message with {@link HashCodeSelector}.
+ * <p>
+ * Usually the implemented class used to persistence data to database
+ * or aggregation the metric,
+ *
  * @author pengys5
  */
 public abstract class AbstractHashMessage {

skywalking-collector/skywalking-collector-cluster/src/main/java/com/a/eye/skywalking/collector/actor/selector/HashCodeSelector.java

 package com.a.eye.skywalking.collector.actor.selector;
 
+import com.a.eye.skywalking.collector.actor.AbstractWorker;
 import com.a.eye.skywalking.collector.actor.WorkerRef;
 
 import java.util.List;
 
 /**
+ * The <code>HashCodeSelector</code> is a simple implementation of {@link WorkerSelector}.
+ * It choose {@link WorkerRef} by message {@link AbstractHashMessage} key's hashcode, so it can use to send the same hashcode
+ * message to same {@link WorkerRef}. Usually, use to database operate which avoid dirty data.
+ *
  * @author pengys5
  */
 public class HashCodeSelector implements WorkerSelector<WorkerRef> {
 
+    /**
+     * Use message hashcode to select {@link WorkerRef}.
+     *
+     * @param members given {@link WorkerRef} list, which size is greater than 0;
+     * @param message the {@link AbstractWorker} is going to send.
+     * @return the selected {@link WorkerRef}
+     */
     @Override
     public WorkerRef select(List<WorkerRef> members, Object message) {
         if (message instanceof AbstractHashMessage) {

skywalking-collector/skywalking-collector-cluster/src/main/java/com/a/eye/skywalking/collector/actor/selector/RollingSelector.java

 package com.a.eye.skywalking.collector.actor.selector;
 
+import com.a.eye.skywalking.collector.actor.AbstractWorker;
 import com.a.eye.skywalking.collector.actor.WorkerRef;
 
 import java.util.List;
 
 /**
+ * The <code>RollingSelector</code> is a simple implementation of {@link WorkerSelector}.
+ * It choose {@link WorkerRef} nearly random, by round-robin.
+ *
  * @author pengys5
  */
 public class RollingSelector implements WorkerSelector<WorkerRef> {
 
     private int index = 0;
 
+    /**
+     * Use round-robin to select {@link WorkerRef}.
+     *
+     * @param members given {@link WorkerRef} list, which size is greater than 0;
+     * @param message message the {@link AbstractWorker} is going to send.
+     * @return the selected {@link WorkerRef}
+     */
     @Override
     public WorkerRef select(List<WorkerRef> members, Object message) {
         int size = members.size();

skywalking-collector/skywalking-collector-cluster/src/main/java/com/a/eye/skywalking/collector/actor/selector/WorkerSelector.java

 package com.a.eye.skywalking.collector.actor.selector;
 
+import com.a.eye.skywalking.collector.actor.AbstractWorker;
 import com.a.eye.skywalking.collector.actor.WorkerRef;
 
 import java.util.List;
 
 /**
+ * The <code>WorkerSelector</code> should be implemented by any class whose instances
+ * are intended to provide select a {@link WorkerRef} from a {@link WorkerRef} list.
+ * <p>
+ * Actually, the <code>WorkerRef</code> is designed to provide a routing ability in the collector cluster
+ *
  * @author pengys5
  */
 public interface WorkerSelector<T extends WorkerRef> {
+
+    /**
+     * select a {@link WorkerRef} from a {@link WorkerRef} list.
+     *
+     * @param members given {@link WorkerRef} list, which size is greater than 0;
+     * @param message the {@link AbstractWorker} is going to send.
+     * @return the selected {@link WorkerRef}
+     */
     T select(List<T> members, Object message);
 }