1. Chain of Responsibility

2. A Day in IT-support Front-line Support Second-line Support
Internal support
IT Issue
(= easy)
IT Issue
(= hard)
IT Issue
(= very hard)
Hmmmm…
IT Issue
(> hard)
IT Issue
(> very hard)
IT Issue
(> easy)
IT Issue
Front-line
Support
Manager
Second-line
Support
Manager
Internal
Support
Manager
On-site
Supporter
DCS – SWC

3. A Day in IT-support
Even though the involved managers are different, they essentially handle issues the same way:
Can my department handle the issue?
If yes, pass it to an actual handler in my department, and fix the issue
If no, pass it to the next department manager
DCS – SWC

4. Chain of Responsibility
In more abstract terms:
We have a chain of Handlers available
Each handler can handle a Request
A Request is handled the same way by all Handlers:
Can I handle the Request?
If yes, then process it (and thereby consume it)
If no, pass it on to the next handler
DCS – SWC

5. Chain of Responsibility
Handler
handleRequest(Request r)
ConcreteHandlerA
handleRequest(Request r)
ConcreteHandlerB
handleRequest(Request r)
DCS – SWC

6. Chain of Responsibility
public void handleRequest(Request r) {
if (canHandleRequest(r))
processRequest(r)
else
forwardRequest(r); }
DCS – SWC

7. Chain of Responsibility
public void forwardRequest(Request r) {
if (successor != null)
successor.handleRequest(r)
else
throw (new UnhandledRequestException()); }
DCS – SWC

8. Chain of Responsibility
Notice that a Handler has a reference to Handler – the successor in the chain of Handler objects
One Handler does not know about details of other handlers
Order of handlers can be changed dyna-mically, since handlers do not depend on each other
DCS – SWC

9. Chain of Responsibility
Issues to consider in practice
Can we be sure all requests are handled?
What happens if a request is not handled by the final handler?
Should the behavior of a handler depend on whether or not it has a successor?
Who manages the chain of handlers during run-time?
DCS – SWC

10. Chain of Responsibility
Issues to consider in practice
How does a Handler actually decide, if it can handle a specific request
Is it acceptable if two handlers can handle the same request?
What if a request should return some value?
DCS – SWC