Asynchronous Web Services Jaliya N. Ekanayake

Basics of Web Services

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

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.

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.

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

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. Client Proxy Client Service 1 Invoke 3 Return 2 Send Client MachineServer

Fire and Forget Code Snippet Example>>

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 Client Proxy Client Service 1 Invoke 3 Return a Poll Object 2 Send Client MachineServer Poll Object Polling and getting back the result Set the Response

Polling Code Snippet Example>>

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 Client Proxy Client Service 2 Invoke 3 Send Client MachineServer Callback 1 Creates a Callback Object 4 Response is dispatched to the callback by the client proxy

Callback Code Snippet Example>>

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 for standard correlation.

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. Client Sender Receiver Service M M

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.

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

References us/dnmaj/html/aj2mpsoarch.asp us/dnmaj/html/aj2mpsoarch.asp us/cpguide/html/cpconinvokingwebservicesasynchronously.asp us/cpguide/html/cpconinvokingwebservicesasynchronously.asp _02/magazine/practice/dchappell/

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