Home

DDE Hot Link

Following are the steps of exchanging the messages between client and server engaged in DDE hot link conversation:

1. A DDE hot link conversation start with client broadcasting WM_DDE_INITIATE message identifying the application and topic it requires. Application and topic are optional, if a client wants to talk with any server, application can be set to NULL. If the client wants to use any topic, topic can be set to NULL. A server application that supports the specified topic responds to the client with a WM_DDE_ACK message.
   
   
2. The client then indicates the data item it requires by posting a WM_DDE_ADVISE message. If the server can supply the requested data item, it replies back to the client by posting a WM_DDE_ACK message with "positive acknowledgment"; a "negative acknowledgment" indicates that the server cannot.

Now the server is responsible to notify the client whenever the value of the data item changes. The notification used is WM_DDE_DATA.

3. When the value of the data item is changes the server posts WM_DDE_DATA message, to which the client may or may not reply with WM_DDE_ACK message (The reply depends on the flag set in WM_DDE_DATA message).
   
   
4. When the client no more needs to get the notification of the update of data items, it posts a WM_DDE_UNADVISE message to the server and in reply to which the server acknowledges with WM_DDE_ACK message.
   
   
5. To terminate the conversation, client and server posts WM_DDE_TERMINATE messages to each other. In most of the cases its client who first post this message but this is not always the case. Sometimes the server may want to terminate the conversation, in which case he can first post the WM_DDE_TERMINATE message.

Following picture shows the sequence of message exchange between the client and server engaged in DDE hot link conversation.

   Client                                    Server

               ---- WM_DDE_INITIATE --->
                (application, topic)
              <----- WM_DDE_ACK -------

               ----- WM_DDE_ADVISE ---->
                       (item)
              <----- WM_DDE_ACK ------

               <----- WM_DDE_DATA ------
                       (item)
              ----- [WM_DDE_ACK] ----->

               ---- WM_DDE_UNADVISE --->
                       (item)
              <----- WM_DDE_ACK ------

               ---- WM_DDE_TERMINATE --->
              <-- WM_DDE_TERMINATE ----

DDE hot link solves the problem that one may face using the DDE cold link. The problem of not getting the updated data from the server automatically.