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Asynchronous Web Services

This comprehensive guide explores the fundamentals of asynchronous web services, focusing on service invocations, client-side and server-side asynchrony, and real-world use cases. It distinguishes between synchronous and asynchronous service invocations, discussing patterns like fire-and-forget, polling, and callbacks. The document examines scenarios involving long-running processes, event-driven applications, and batch submissions, highlighting the importance of non-blocking operations. Additionally, it provides insights into existing technologies that support asynchrony, such as JMS, .NET, and WS-Addressing.

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Asynchronous Web Services

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  1. Asynchronous Web Services Jaliya N. Ekanayake

  2. Basics of Web Services

  3. Simple Web Service Invocations • Example of Synchronous Service Invocation. • Service takes some time to complete • Client is waiting for the response • Example of Asynchronous Service Invocation • Service takes some time to complete • Client is waiting for the response

  4. Some Use Cases • Time consuming web services • long-lived operations • batch processing • peer to peer programs • event driven application models. • Web services that leverage The client can be a Reactive Application • Client can be a server to some other clients. So the blocking is not acceptable.

  5. Some Use Cases contd.. • User may not need to wait for the responses • User is doing some batch submission. • E.g. A bank submitting credit card information to a service provider. • User can check for responses after sending few invocations. • The response from a service represents a collective information. • User need the response after some time. • E.g. A batch submission process, the result will be available in the next day.

  6. Supporting the Asynchrony • Protocol level asynchrony • E.g. JMS can be used to invoke the services asynchronously. • Client subscribes to a topic and return • Response will be delivered to client through the JMS Queue • Client Asynchrony Patterns • Fire and Forget • Callbacks • Polling • Service Asynchrony

  7. Client Machine Server 1 Invoke 2 Send Client Proxy Client Service 3 Return Client Side Asynchrony • Fire and Forget. • Invokes the service and return immediately without ever bothering about a response. • Issues • No waiting. Client can immediately resume the thread. • Easy for the developers • Loosely coupled • No way to verify whether the request has been sent or not.

  8. Fire and Forget • Code Snippet • Example>>

  9. Client Machine Server 2 Send 1 Invoke Client Proxy Client Service 3 Return a Poll Object Polling and getting back the result Poll Object Set the Response Client Side Asynchrony • Polling • Client repeatedly poll for the availability of the results, while performing some other task. • Issues • Client has to wait polling • Client has to handle the complexity of this polling operation • Response can be retrieved asynchronously

  10. Polling • Code Snippet • Example>>

  11. Client Machine Server 3 Send 2 Invoke Client Proxy Client Service 1 Creates a Callback Object Callback 4 Response is dispatched to the callback by the client proxy Client Side Asynchrony • Result Callbacks • Client provides a callback method. Proxy will dispatch the result using the callback method. • Issues • Client has to provide the callback method • Client has to handle the additional complexity • Response can be retrieved asynchronously

  12. Callback • Code Snippet • Example>>

  13. Asynchronous Web Service Model • The service does not “return” anything. • It can send the response (if any) by itself. • Transport may or may not be synchronous • E.g. 1 (HTTP Transport) • Service has a void return type and it will send a response after a while using different HTTP connection • E.g. 2 (One way transport -JMS) • Service sends a response using a new one way transport. • What is required. • Service should have the capability to send the response by itself. • Client should be able to correlate the request and the response • Client should have an addressable endpoint. • Can use WS-Addressing <wsa:relatesTo> for standard correlation.

  14. Client Sender M Service Receiver M Asynchronous Web Service Model contd.. • The model • Features • Service sends the response by itself. • Correlation information contains in the SOAP message itself. • Client has both a Sender and a Receiver.

  15. Support from the Existing Technologies • Microsoft .NET supports the client side asynchrony. • With the WSE 2.0 the service asynchrony is also supported. • Systinet WASP supports the client side asynchrony and WS-Addressing. • Axis 2 (Currently under development) will support both.

  16. Summery • There are many advantages of using asynchronous patterns in implementing web services for B2B, EAI scenarios. • Client Side Asynchrony • Fire & Forget • Polling • Callbacks • Service Asynchrony • Technology support

  17. References • http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnmaj/html/aj2mpsoarch.asp • http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpconinvokingwebservicesasynchronously.asp • www.fawcette.com/xmlmag/ 2003_02/magazine/practice/dchappell/ • http://www.c-sharpcorner.com/Code/2004/April/AsyncWebServices.asp • http://www.esecurityplanet.com/prodser/article.php/2235201 • http://xml.coverpages.org/ni2004-04-15-a.html

  18. Thank You!

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