Java Reference
In-Depth Information
Successive Update
Also known as Client Pull / Server Push
Pattern Properties
Type: Processing (Behavioral)
Level: Architectural
Purpose
To provide a way for clients to receive updates from a server on an ongoing basis. The updates generally reflect a
change in server data, a new or updated resource, or a change in the state of the business model.
Introduction
Suppose you wanted to use the Personal Information Manager as a way to coordinate work between multiple
users. For example, you could enable multiple users to share information about the part of the project that they
were currently working on. If you subdivided a project into
Task
and
Deliverable
objects, you might want to
allow interested users (such as the project manager) to get updates on the progress of one or more
Tasks
.
If you create a server, it is straightforward to centralize the project information—you would simply store the
Project
,
Task
and
Deliverable
objects on the server. But how should you manage the task of keeping the
clients up to date?
You can follow two strategies, each of which is a form of the Successive Update pattern. Make the client
responsible for regularly querying the server, requesting updates on a
Task
; or alternatively, make the server
responsible for sending
Task
updates out to clients.
In both cases, you expand the role of server and enable a groupware-style solution. You can send periodic updates
to clients and ensure that they are coordinated in the work that they do.
Applicability
Use Successive Update for client-server systems when:
Server-side resources or data are in flux, changing due to interactions with multiple clients or external updates.
You want a client to receive updates without forcing the user to do a manual refresh.
Description
Successive Update is used for applications that require an ongoing refresh of the client's state. While it is possible
to manually update the data, it is tedious and frustrating for an end user who must frequently refresh
information— imagine having to manually send a request to your server every minute that you used an
application. Although such a system might gives its users strong, muscular fingers, overall it would probably not
be popular in conventional IT businesses.
To provide an acceptable alternative, Successive Update automates what users would otherwise have to do
manually. The client and server establish an automatic update strategy, eliminating the need for the user to
become directly involved. You can implement Successive Update using a variety of approaches, but it is
frequently associated with two techniques at two ends of a spectrum— client pull and server push.
Client Pull
Client pull schedules a periodic refresh of information that is managed on the client side. The server performs its
normal task, providing information when requested by its clients. Client pull is essentially a regular series of
requests for server information.
Figure 4.8. Successive Update sequence diagram (client pull)
