add comments

java-client/src/main/java/org/apache/pegasus/rpc/async/ReplicaSession.java

@@ -229,16 +229,25 @@ public void operationComplete(ChannelFuture channelFuture) throws Exception {
   private void markSessionConnected(Channel activeChannel) {
     VolatileFields newCache = new VolatileFields(ConnState.CONNECTED, activeChannel);
 
-    //
+    // Note that actions in interceptor such as Negotiation might send request by
+    // ReplicaSession#asyncSend(), inside which ReplicaSession#tryConnect() would
+    // also be called since current state of this session is still CONNECTING.
+    // However, it would never create another connection, thus it is safe to do
+    // `fields = newCache` at the end.
     interceptorManager.onConnected(this);
 
     synchronized (pendingSend) {
       if (fields.state != ConnState.CONNECTING) {
-        // this session may have been closed or connected already
+        // This session may have been closed or connected already.
         logger.info("{}: session is {}, skip to mark it connected", name(), fields.state);
         return;
       }
 
+      // Once the authentication is enabled, any request except Negotiation such as
+      // query_cfg_operator for meta would be cached in authPendingSend and sent after
+      // Negotiation is successful. Negotiation would be performed first before any other
+      // request for the reason that AuthProtocol#isAuthRequest() would return true for
+      // negotiation_operator.
       sendPendingRequests(pendingSend, newCache);
       fields = newCache;
     }
@@ -284,9 +293,11 @@ void markSessionDisconnect() {
       }
     }
 
+    // Reset the authentication once the connection is closed.
     resetAuth();
   }
 
+  // After the authentication is reset, a new Negotiation would be launched.
   private void resetAuth() {
     synchronized (authPendingSend) {
       authSucceed = false;
@@ -340,6 +351,7 @@ void tryNotifyFailureWithSeqID(int seqID, error_code.error_types errno, boolean
   }
 
   private void write(final RequestEntry entry, VolatileFields cache) {
+    // Under some circumstances requests are not allowed to be sent or delayed.
     if (!interceptorManager.onSendMessage(this, entry)) {
       return;
     }