1. Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Chapter 3: Programming Web Services
Service-Oriented Computing: Semantics, Processes, Agents – Munindar P. Singh and Michael N. Huhns, Wiley, 2005
© Singh & Huhns

2. Highlights of this Chapter
Service-Oriented Computing: Semantics, Processes, Agents
August 2004
Highlights of this Chapter
Representational State Transfer (ReST or REST)
Developing and Using Web Services
Web Service Interoperability
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
© Singh & Huhns

3. Representational State Transfer
REST is an architectural style for networked systems that constrains the connector semantics (not the component semantics)
The Web is a network of hyperlinked resources
A resource is anything identified by a URI
A Web application works as a state machine
A client selecting a link is a state transition, resulting in receiving the next page (next state) of the application
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

4. Characteristics of REST
Client-Server
Statelessness: requests cannot take advantage of stored contexts on a server
Great for load balancing
Caching: responses can be labeled as cacheable
Uniform interface: URIs, hypermedia
Layered components
Focus on resources as opposed to methods:
Read, construct, update a resource representation
Well-suited to hypermedia applications
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

5. HTTP Verbs Popular verbs
Get: query (no side effects)
Post: update
Put
Delete
In particular, can use Get and Post instead of much of SOAP
Idempotent operations
Multiple executions = one execution
Challenge: the specification imposes requirements but with no way to judge compliance without understanding the designer’s intent
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

6. Software for Open Environments
Tempting to simply lift current database and programming techniques for open environments
Popular pitfall (blame the tools for it)
Think of objects (get and set methods)
Invoke them as before even though they are Web services
Poor performance
Tight coupling
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

7. Simple B2C Web Service Example
Suppose you want to sell cameras over the Web, debit a credit card, and guarantee next-day delivery
Your application must
update sales database
debit the credit card
send an order to the shipping department
receive an OK from the shipping department for next-day delivery
update an inventory database
Problems: Some steps complete but not all
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

8. “Traditional” B2C Problems
What if the order is shipped, but the debit fails?
What if the debit succeeds, but the order was never entered or shipped?
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

9. Database Approach
A traditional database approach works only for a closed environment:
Transaction processing (TP) monitors (such as IBM’s CICS, Transarc’s Encina, BEA System’s Tuxedo) can ensure that all or none of the steps are completed, and that systems eventually reach a consistent state
But what if the user’s modem is disconnected right after he clicks on OK? Did the order succeed? What if the line went dead before the acknowledgement arrives? Will the user order again?
The TP monitor cannot get the user into a consistent state!
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

10. Approach for Open Environment
Server application could send about credit problems, or detect duplicate transactions
Downloaded applet could synchronize with server after broken connection was restored, and recover transaction; applet could communicate using http, or directly with server objects via CORBA/IIOP or RMI
If there are too many orders to process synchronously, they could be put in a message queue, managed by a Message Oriented Middleware server (which guarantees message delivery or failure notification), and customers would be notified by when the transaction is complete
The server behaves like an agent!
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

11. Requirements Cross-enterprise processes Exception handling
Conversations and long-lived transactions
Contracts among parties involved
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

12. Chapter 3 Summary Tools help with low-level details
Agreeing on standards is more important than the standards themselves
Should conceptualize interactions in a manner compatible with the Web architecture
Can simplify from SOAP in many cases
The above is a small point anyway
Bigger challenges are in ensuring larger-scale interactions, ensuring integrity, handling exceptions, …
Sophisticated programming models are emerging
Chapter 3
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns