Transaction Control

An application can suspend transactions to its DDE callback function — either those transactions associated with a specific conversation handle or all transactions regardless of the conversation handle. This capability is useful when an application receives a transaction that requires lengthy processing. In such a case, the application can return the CBR_BLOCK return code to suspend future transactions associated with the transaction's conversation handle, so that the application is free to process other conversations.

When processing has been completed, the application calls the DdeEnableCallback function to resume transactions associated with the suspended conversation. Calling DdeEnableCallback causes the DDEML to resend the transaction that resulted in the application suspending the conversation. Therefore, the application should store the result of the transaction in such a way that it can obtain and return the result without reprocessing the transaction.

An application can suspend all transactions associated with a specific conversation handle by specifying the handle and the EC_DISABLE flag in a call to DdeEnableCallback. By specifying a NULL handle, an application can suspend all transactions for all conversations.

When a conversation has been suspended, the DDEML saves transactions for the conversation in a transaction queue. When the application reenables the conversation, the DDEML removes the saved transactions from the queue and passes each transaction to the appropriate callback function. The capacity of the transaction queue is large, but an application should reenable a suspended conversation as soon as possible to avoid losing transactions.

An application can resume usual transaction processing by specifying the EC_ENABLEALL flag in DdeEnableCallback. For a more controlled resumption of transaction processing, the application can specify the EC_ENABLEONE flag. This flag removes one transaction from the transaction queue and passes it to the appropriate callback function; after that transaction has been processed, any conversations are again disabled.

If the EC_ENABLEONE flag and a conversation handle are specified in the call to DdeEnableCallback, only that conversation is blocked after the transaction has been processed. If a NULL conversation handle is specified, all conversations are blocked after a transaction has been processed in any conversation.