Learning DCOM

Performance Impact of Connection Points

Generally, you want to minimize network round-trips because round-trips are very expensive. We discussed the connection points technique in this chapter because it is a standard way to support events in COM. In fact, if you want VBScript clients to trap events, you must use connection points. In addition, as we have mentioned earlier, the callback interface must also be a dispinterface.

It is generally a good practice to conform to a standard, but connection points are extremely expensive if you take round-trips into account. For example, it takes four round-trips to set up a connection: (1) Client queries for the IConnectionPointContainer interface; (2) Client finds the connection point; (3) Client registers interest by calling IConnectionPoint::Advise; and (4) Server makes a QueryInterface back to the client to verify that the client indeed supports the interface associated with the connection point. This must be done because IConnectionPoint::Advise takes an IUnknown pointer, instead of a specific pointer type (such as an IReceiveMessage pointer).

If you were to implement callbacks yourself without using connection points, you could reduce connection setup to a single round-trip. For instance, simply provide a method in the server component to accept an interface pointer of a specific type (i.e., not IUnknown but the callback interface’s type). To set up a connection, a client calls this method to pass the callback interface to the server ...