Once you have opened a DDE conversation, you can converse on the topic using four types of exchanges. These exchange types include:

• Execute — The DDE client executes server commands using the DDE EXECUTE statement.
• Request — The DDE client retrieves data from the server using the DDE REQUEST statement.
• Send — The DDE client sends data to the server using the DDE SEND statement.
• Advise — The DDE client creates advise links to server data items using the DDE ADVISE statement. Advise links direct the DDE server to monitor the data item. When any linked data item changes value, your OpenEdge application is notified so it can retrieve the value using the DDE GET statement.

Execute, request, and send exchanges are each demand-driven. When you invoke the exchange, ABL waits for the exchange to complete. A demand-driven exchange works like a FIND statement, which waits until ABL retrieves the desired record from the database. Advise exchanges are event-driven. Once an advise link is created for a data item, the OpenEdge client continues execution and retrieves the data item value "automatically" as it changes. You can invoke both demand, and event-driven exchanges in the same conversation and for the same data item. The following sections describe how to implement these exchanges.