Chain of Responsibility

1
Chain of Responsibility

• Sheridan Thirsk
• Eric Thiessen
• Hayder Juhad

2
Introduction

• Behavioural object design pattern
• Promotes loose coupling
• Chain of handlers
• Only one handler can handle request
• Pass request down the chain
• Sender does not know handler explicitly

3
Action

• Client initiates request
• First object in chain attempts to handle request
• If it can, it does!
• If not, it passes the request to its successor
• Succesor attempts to handle request
• The pattern continues...

4
Participants

• Abstract Handler
• Defines common interface
• Links to successor
• Concrete Handler
• Extends abstract handler
• Each one can handle certain type of request
• Only access its successor
• Client
• Initiate request
• Not known which handler will handle request

5
Class Diagram

• Figure 1 Class Diagram of the Chain of
  Responsibility

6
Applicability

• Consider using Chain of Responsibility if
• Multiple possible handlers
• No receiver specified
• Dynamic specification of responsibility

7
Benefits

• Low Coupling
• Object does not know who handler is
• Reduces interaction
• Sender and receiver need not know each other
• Flexibility
• Reorder chain
• Add or remove handlers dynamically

8
Drawbacks

• Cannot guarantee receipt
• Request may get passed along by ALL handlers
• Request falls off the end of the chain

9
Real Example

• Security Monitoring System
• Various sensors
• Hierarchy of information passed up
• Passed up higher until object handles it
• Domain Name Server (DNS)
• Client connect to local DNS
• Server passes request until it is handled

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Known Uses

• Windows system to handle input
• GUI events
• From object where event occurred to Handler
• Pass events up container hierarchy
• Known as
• Responder in Java 1.0 AWT
• Event-Handler in MacApp
• Bureaucrat in Symantecs TCL library

11
Sample Code

• //Base Class
• class Handler
• protected
• Handler successor
• public
• void SetSuccessor(Handler s)
• virtual void HandleRequest(void r)

12
Sample Code

• //Domain Name Server
• class DNS public Handler
• private
• Char ServerName
• DomainNameEntry DNSTable
• int size
• public
• // Adds a entry in the DNSTable
• void Add(char DomainName, char b1, b2, b3,
  b4)
• void HandleRequest(char request)
• // if request in DNSTable then return entry
• // otherwise pass request to successor if
  // it exists

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Sample Code

• void main()
• // create servers
• DNS A("Server A")
• DNS B("Server B")
• DNS C("Server C")
• // setup a Chain of Responsibility
• A.SetSuccessor(B)
• B.SetSuccessor(C)
• C.SetSuccessor(NULL)
• //initialize the tables for the servers
• A.Add("google.com",124,32,4,1)
• A.Add("amazon.com",224,132,14,6)
• ...
• C.Add("i-like-puppies.com",12,23,4,114)

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Sample Code

• //get user request
• char input255
• cout ltlt"Input Request "
• cin gtgt input
• //pass request to server A
• A.HandleRequest(input)

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Related Patterns

• Composite
• Parent is successor to child
• Parent asks all children to perform operation
• Visitor
• Visitor Handle Request
• Visited cannot pass the request
• Iterator
• Same operation
• Structure is different
• Iterator Sequential chain
• CoR Many possibilities. Linked list, tree, etc.

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Conclusion

• Its just a chain!
• Each object in chain is potential handler
• Link to next object, successor
• Unknown handler
• Pros
• Reduced coupling
• Increased flexibility
• Con
• Can fall off the end
• Decouple requester and handler
• Especially with multiple handlers

17
Any Questions?

• Well, we have two!
• And we have answers

18
QA 1

• What can be done about requests falling off the
  end of the chain?!?
• Use a Controller with error handling
• Use exception handling

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QA 2

• What about an circular chain?!?
• Keep track of chain length
• Count times the request is passed
• If greater than length going in circles
• Error handling, throw an exception, etc.

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Thank you!

• Check out our website
• http//homepages.ucalgary.ca/sjthirsk/
• Contact us with any additional questions
• Eric
• ethiessen_at_gmail.com
• Sheridan
• sheridan_at_shaw.ca
• Hayder
• hjuhad_at_ucalgary.ca
• Or ask us in person!

21
References

• Cunningham. (2004). Chain of Responsibility
  Pattern - Cunningham Cunningham, Inc. July 14,
  2004.http//c2.com/cgi/wiki?ChainOfResponsibility
  Pattern
• (Gamma, Helm, Johnson Vlissides, 1995) E.
  Gamma, R.Helm, R.Johnson, J.Vlissides (1995).
  Design Patterns. Addison Wesley. ISBN
  0.201-63361-2.
• Roe, Cameron. (1998). Chain of Responsibility
  Design Pattern - SENG 609.04 Winter 1998.
  Software Engineering Research Network, University
  of Calgary. March 22, 1998.http//sern.ucalgary.c
  a/courses/SENG/609.04/W98/roec/Chain/chain.htmlDi
  sscussion
• Sarma, T. Kulathu. (2001). Chain of
  Responsibility - The Code Project. July 11,
  2001.http//www.codeproject.com/gen/design/chain_
  response.asp
• Tarr, Bob. (2004). The Chain of Responsibility
  Pattern - Design Patterns in Java. April 15,
  2004.http//www.research.umbc.edu/tarr/dp/lectur
  es/Chain.pdf
• To, Ha. (1999). Chain of Responsibility -
  Department of Computer Engineering, Santa Clara
  University. February 19, 1999.http//sol.students
  .engr.scu.edu/hto1/pattern/ddcor.html