SOAP gets mentioned every time people talk about Web services. SOAP is almost always considered an integral part of any “serious” Web services implementation, especially if there is also a desire to use WSDL.

But it does not have to be. WSDL 1.1 has HTTP binding support and this support will be beefed up in WSDL 2.0. Granted, the support for WSLD HTTP binding is not very good right now, for example JAX-RPC does not mandate it at all (SOAP binging and SOAP with Attachment are the only two requirements). Assuming the support for HTTP binding will improve or if using WSDL is not even a requirement (after all, a service can be described using “plain English”, as long as developers can understand this description), how should one decide whether to use SOAP?

To me, it all comes down to metadata. The main purpose of SOAP (in case of document-style services) is to provide a place where to put message metadata. This place is SOAP header. WS-Security digital signature, WS-Addressing headers or application-specific metadata all belong there. Today, many (if not most) Web services implementations don’t use SOAP headers and so for these implementations SOAP is just overhead and added complexity (just try get direct access to the InputStream of the message payload in any JAX-RPC-compliant implementaiton). Without anything in the header, SOAP does not have any advantage over good old XML over HTTP “post” (assuming that there is a schema for this XML).

One can argue that SOAP engines also provide XML mapping to the host language primitives, i.e., mapping to/from objects. I personally think that XML mapping capabilities must be de-coupled from SOAP processing. A good XML mapping framework should work with any XML source (in fact, this is exactly where JAX-WS, which is the next version of JAX-RPC is going). A developer must have a choice of receiving “raw” XML from SOAP engine or converting this XML into objects using a framework of his/her choice (e.g., what if I want to use Castor instead of JAXB), I really don’t understand why SOAP engine should make this choice for me.

Just to confirm my point that SOAP is not a requirement for a successful Web service implementation, here is the list of Internet heavyweights and their use of SOAP versus XML. Note that none of them (as far as I know) use WSDL HTTP binding, WSDL is only used when SOAP is used.

Yahoo(maps, search, Flickr, Upcoming, etc.) X
Google maps X (JavaScript)
Google searchX 
Blogger X
PayPalX X
MSN SearchX 

Of course, SOAP has some other benefits, such as solid tool support. SOAP also provides SOAP encoding (for RPC-style services), although the document style is quickly taking over. But is SOAP absolutely required to implement a “proper” Web service? I don’t think so.