A client can send either synchronous or asynchronous transactions. In a synchronous transaction, the client specifies a timeout value that indicates the maximum amount of time to wait for the server to process the transaction. The DdeClientTransaction function does not return until the server processes the transaction, the transaction fails, or the timeout value expires. The client speci-fies the timeout value when it calls DdeClientTransaction.
During a synchronous transaction, the client enters a modal loop while waiting for the transaction to be processed. The client can still process user input but cannot send another synchronous transaction until the DdeClientTransaction function returns.
A client sends an asynchronous transaction by specifying the TIMEOUT_ASYNC flag in the DdeClientTransaction function. The function returns after the transaction is begun, passing a transaction identifier to the client. When the server finishes processing the asynchronous transaction, the DDEML sends an XTYP_XACT_COMPLETE transaction to the client. One of the parameters that the DDEML passes to the client during the XTYP_XACT_COMPLETE transaction is the transaction identifier. By comparing this transaction identifier with the identifier returned by the DdeClientTransaction function, the client identifies which asynchronous transaction the server has finished processing.
A client can use the DdeSetUserHandle function as an aid to processing an asynchronous transaction. This function makes it possible for a client to associate an application-defined doubleword value with a conversation handle and transac-tion identifier. The client can use the DdeQueryConvInfo function during the XTYP_XACT_COMPLETE transaction to obtain the application-defined doubleword value. This saves an application from having to maintain a list of active transaction identifiers.
If a server does not process an asynchronous transaction in a timely manner, the client can abandon the transaction by calling the DdeAbandonTransaction function. The DDEML releases all resources associated with the transaction and discards the results of the transaction when the server finishes processing it.
The asynchronous transaction method is provided for applications that must send a high volume of DDE transactions while simultaneously performing a substantial amount of processing, such as calculations. The asynchronous method is also useful in applications that need to stop processing DDE transactions temporarily so they can complete other tasks without interruption. In most other situations, an application should use the synchronous method.
Synchronous transactions are simpler to maintain and faster than asynchronous transactions. However, only one synchronous transaction can be performed at a time, whereas many asynchronous transactions can be performed simultaneously. With synchronous transactions, a slow server can cause a client to remain idle while waiting for a response. Also, synchronous transactions cause the client to enter a modal loop that could bypass message filtering in the application's own message loop.