Chapter 3: Programming Web Services

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

2. Highlights of this Chapter • Representational State Transfer (ReST or REST) • Developing and Using Web Services • Web Service Interoperability Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael 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 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 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 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) • ERROR: Too fine a granularity (see chapter 4) • Invoke them as objects even though they are Web services • Poor performance • Tight coupling 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 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? Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

9. Traditional 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! Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

10. Approach for Open Environment • Server application could send email 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 email when the transaction is complete The server behaves like an agent! Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns

11. Requirements • Cross-enterprise processes • Autonomy (whim) • Exception handling • Revisions (necessity) • How to deal with the above • Conversations and long-lived transactions • Contracts among parties involved 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 Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns