Cancelling and closing transactions

Transactions that have been submitted may be cancelled at any time, using RHTTPTransaction::Cancel(). When this is done, no further events will be received for that transaction as originally submitted.

An example from HTTPEXAMPLECLIENT:

void CHttpEventHandler::MHFRunL(RHTTPTransaction aTransaction, THTTPEvent aEvent)
    {
    switch (aEvent.iStatus)
        {
        case THTTPEvent::EGotResponseHeaders:
            {
            ...
            // If we're cancelling, do it now...
            if (cancelling)
                {
                aTransaction.Cancel();
                Printf(_L("\nTransaction Cancelled\n"));
                ...
                }
...

In this case, the transaction is left in a cancelled state. The client has the option, at this time, to modify the request if desired, and to resubmit the transaction. This might be done, for example, if the HTTP server has indicated with a 400-series status code that there was a client error.

When a transaction is resubmitted following cancellation, it is exactly as if it had been submitted for the first time: for example, no preferential treatment as given it in terms of time-to-service.

The practice of cancelling in an MHFRunL() method, modifying the request, and resubmitting is common in filter design. See Filters for more information.

If the client wishes to abort a transaction, it may close the transaction using RHTTPTransaction::Close(). It is not necessary to cancel first, since a Cancel() is done automatically during the Close(). Closing the transaction enables HTTP to clean up resources allocated during the execution of that transaction.

Once a transaction is closed, it must not be reused. Doing so will cause the client to panic, or behave unpredictably.