Synchronous And Asynchronous Services

A client's interaction with a web service can be synchronous or asynchronous.

Synchronous services - With synchronous services, clients invoke a request on a service and then suspend their processing while they wait for a response. e.g.


Synchronous services are best suited when applications require a more immediate response to a request. Web services that rely on synchronous communication are usually RPC-oriented.

Asynchronous services - With asynchronous services (also dubbed document-style web services), clients initiate a request to a service and then resume their processing without waiting for a response. The service handles the client request and returns a response at some later point, at which time the client retrieves the response and proceeds with its processing.

A document-oriented approach is used for asynchronous class of services. Services which process documents tend to use an asynchronous architecture. A document-oriented Web service receives a document as a service request. The document content determines the processing workflow for the Web service. There can be a number of processing steps required to fulfill the request.


There are two ways to obtain the response -

• The client that invoked the service periodically checks the status of the request using the ID that was provided at the time the request was submitted. (This is also known as polling.)

• Or, The web service can then respond, at some later time, by making a call back to the client with the response.Therefore, a client of a web service that contains a callback operation is special in that the client itself must provide an endpoint. We call this the callback endpoint and define an endpoint as a unique address identified by a URI to which SOAP request messages can be sent.

Web service callbacks refer to scenarios that involve the web service provider sending a SOAP message back to its clients. The Web Services Description Language (WSDL) specifications define such operations as being of the type “solicit/response.” Clients of web services that support callback operations must have a web service endpoint themselves, which the web service can use for sending callback requests at any point in time, or, in other words, asynchronously.
The first approach is non-intrusive and an independent polling module can be developed that is responsible for getting the results.

The second approach requires an implementation of event-listener.

Best practices suggest that you should support both polling and callbacks when implementing an asynchronous web service. Supporting both approaches will make your service accessible to the widest audience of clients.