Java Reference
In-Depth Information
The Callback sequence diagram is shown in
Figure 4.7
.
Figure 4.7. Callback sequence diagram
Callback imposes some basic requirements on both client and server:
Client - The client must provide a callback interface for the server, so the server can contact the client when its
request is done.
Server - Beyond the traditional calling interface for the client, the server needs some way to notify clients when
processing is complete. In addition, the server must be able to process and possibly store client requests.
Benefits and Drawbacks
The Callback pattern's major advantage is its improvement of a system's operating efficiency, especially server
performance. You can see most of the improvement in two areas:
Server-side processing - The server does not have to maintain communication threads to service waiting clients,
so it can channel those resources into processing client requests or servicing other callers. Furthermore the
processing can be performed when the server sees fit. It doesn't have to process the request immediately.
Server communication - The server does not have to maintain an open connection while the client waits for its
results. This means that a server can support a greater number of callers with its limited communication resources,
such as sockets.
This is a major motivation for some to choose the Callback pattern for their systems. In cases where server load is
large or unpredictable (such as on the Web), this capability offers substantial advantages to designers. In the most
extreme cases, it can mean the difference between running a group of servers in parallel and using a single
machine to service client requests.
