web services - What is WS-Addressing good for?

Question

I am just getting started with SOAP web services and stumbled across WS-Addressing.

I have read the Wikipedia page, but I'm having a hard time understanding just what the point of WS-Addressing is.

According to Wikipedia and various sources on the web, WS-Addressing allows to put "addressing information" or "routing information" into the header of a SOAP request.

Why is this useful? If I send a request via HTTP (or even via SMTP or UDP), then the address I send to is the address of the server which will process my request, and the server can simply reply by the same channel. So why is addressing/routing information needed?

I'd be particularly interested in some real-world (more or less) example where WS-Addressing is helpful.

Answer 1

I've found WS-Addressing particularly useful in situations where the SOAP response can't be served immediately. Either the resources to form the response are not available right away or the result itself takes a long time to be generated.

That can happen when your business process involves "a human touch" for example (processes like the ones WS-HumanTask is targeting). You can stick web services in front of your business, but sometimes business takes time. It might be a subscription that must be manually verified, something to be approved, whatever, but it takes days to do it. Are you going to keep the connection opened all that time? Are you going to do nothing else than wait for the response? No! That is inefficient.

What you need is a notification process. The client makes a requests but does not wait for the response. It instead instructs the server where to send the response by use of a "reply to" address. Once the response is available, the server connects to that address and sends the response.

And voila... asynchronous interactions between web services, decoupling the lifetime of the communication process from the lifetime of the HTTP connection. Very useful...

But wait... HTTP connection? Why should I care about that? What if I want the response sent back on another type of protocol? (which SOAP kindly provides as it is not tied to any protocol).

With normal request/response flow, the response comes on the same channel as the request 'cose it's a connection you know.... So for example you have a HTTP connection... that means HTTP in and HTTP out.

But with WS-Addressing you are not tied to that. You can demand the response on another type of channel. Request comes on HTTP for example, but you can instruct the server to send the response back over SMTP, for example.

In this way WS-Addressing defines standard ways to route a message over multiple transports. As the wiki page is saying:

instead of relying on network-level transport to convey routing information, a message utilizing WS-Addressing may contain its own dispatch metadata in a standardized SOAP header.

and as for your observation:

and the server can simply reply by the same channel

... what works for some, might not work for others, and for others we have WS-Addressing :D.