A Web Services Description Language (WSDL) binding style can be RPC or document. The use can be encoded or literal. How do you determine which combination of style and use to use? The author describes the WSDL and SOAP messages for each combination to help you decide.
A WSDL document describes a Web service. A WSDL binding describes how the service is bound to a messaging protocol, particularly the SOAP messaging protocol. A WSDL SOAP binding can be either a Remote Procedure Call (RPC) style binding or a document style binding. A SOAP binding can also have an encoded use or a literal use. This gives you four style/use models:
RPC/encoded
RPC/literal
Document/encoded
Document/literal
Add to this collection a pattern which is commonly called the document/literal wrapped pattern and you have five binding styles to choose from when creating a WSDL file. Which one should you choose?
Before I go any further, let me clear up some confusion that many of us have stumbled over. The terminology here is very unfortunate: RPC versus document. These terms imply that the RPC style should be used for RPC programming models and that the document style should be used for document or messaging programming models. That is not the case at all. The style has nothing to do with a programming model. It merely dictates how to translate a WSDL binding to a SOAP message. Nothing more. You can use either style with any programming model.
Likewise, the terms encoded and literal are only meaningful for the WSDL-to-SOAP mapping, though, at least here, the traditional meanings of the words make a bit more sense.
For this discussion, let's start with the Java method in Listing 1 and apply the JAX-RPC Java-to-WSDL rules to it (see Resources for the JAX-RPC 1.1 specification).
Take the method in Listing 1 and run it through your favorite Java-to-WSDL tool, specifying that you want it to generate RPC/encoded WSDL. You should end up with something like the WSDL snippet in Listing 2.
<binding .../> <!-- I won't bother with the details, just assume it's RPC/encoded. -->
Now invoke this method with "5" as the value for parameter x and "5.0" for parameter y. That sends a SOAP message which looks something like Listing 3.
For the most part, for brevity, I ignore namespaces and prefixes in the listings in this article. I do use a few prefixes that you can assume are defined with the following namespaces:
The various Web services specifications are sometimes inconsistent and unclear. The WS-I organization was formed to clear up the issues with the specs. It has defined a number of profiles which dictate how you should write your Web services to be interoperable. For more information on WS-I, see the WS-I links in Resources.
The type encoding info (such as xsi:type="xsd:int") is usually just overhead which degrades throughput performance.
You cannot easily validate this message since only the <x ...>5</x> and <y ...>5.0</y> lines contain things defined in a schema; the rest of the soap:body contents comes from WSDL definitions.
Although it is legal WSDL, RPC/encoded is not WS-I compliant.
Is there a way to keep the strengths and remove the weaknesses? Possibly. Let's look at the RPC/literal style.
The RPC/literal WSDL for this method looks almost the same as the RPC/encoded WSDL (see Listing 4). The use in the binding is changed from encoded to literal. That's it.
Although in normal circumstances xsi:type does not appear in a literal WSDL's SOAP message, there are still cases when type information is necessary and it will appear -- in polymorphism, for instance. If the API expects a base type and an extension instance is sent, the type of that instance must be provided for proper deserialization of the object.
Here are the strengths and weaknesses of this approach:
You still cannot easily validate this message since only the <x ...>5</x> and <y ...>5.0</y> lines contain things defined in a schema; the rest of the soap:body contents comes from WSDL definitions.
What about the document styles? Do they help overcome this weakness?
I could have changed only the binding, as I did from RPC/encoded to RPC/literal. It would have been legal WSDL. However, the WS-I Basic Profile dictates that document/literal message parts refer to elements rather than types, so I'm complying with WS-I. (And using an element part here leads well into the discussion about the document/literal wrapped pattern.)
Here are the strengths and weaknesses of this approach:
The WSDL is getting a bit more complicated. This is a very minor weakness, however, since WSDL is not meant to be read by humans.
The operation name in the SOAP message is lost. Without the name, dispatching can be difficult, and sometimes impossible.
WS-I only allows one child of the soap:body in a SOAP message. As you can see in Listing 7, this example's soap:body has two children.
The document/literal style seems to have merely rearranged the strengths and weaknesses from the RPC/literal model. You can validate the message, but you lose the operation name. Is there anything you can do to improve upon this? Yes. It's called the document/literal wrapped pattern.
Notice that this SOAP message looks remarkably like the RPC/literal SOAP message in Listing 5. You might say it looks exactly like the RPC/literal SOAP message, but there's a subtle difference. In the RPC/literal SOAP message, the <myMethod> child of <soap:body> was the name of the operation. In the document/literal wrapped SOAP message, the <myMethod> clause is the name of the wrapper element which the single input message's part refers to. It just so happens that one of the characteristics of the wrapped pattern is that the name of the input element is the same as the name of the operation. This pattern is a sly way of putting the operation name back into the SOAP message.
These are the basic characteristics of the document/literal wrapped pattern:
The input message has a single part.
The part is an element.
The element has the same name as the operation.
The element's complex type has no attributes.
Here are the strengths and weaknesses of this approach:
From a WSDL point of view, there's no reason the wrapped pattern is tied only to document/literal bindings. It could just as easily be applied to an RPC/literal binding. But this would be rather silly. The SOAP message would contain a myMethod element for the operation and a child myMethod element for the element name. Also, even though it's legal WSDL, an RPC/literal part should be a type; an element part is not WS-I compliant.
This wrapped style originates from Microsoft®. There is no specification that defines this style; so while this style is a good thing, unfortunately, the only choice right now, in order to interoperate with Microsoft's and other's implementations, is to make educated guesses as to how it works based on the output of the Microsoft WSDL generator. This pattern has been around for awhile, and the industry has done a good job of understanding it, but while the pattern is fairly obvious in this example, there are corner cases in which the proper thing to do is not particularly clear. Our best hope is that an independent group like the WS-I organization will help stabilize and standardize this in the future.
So far, this article has given the impression that the document/literal wrapped style is the best approach. Very often that's true. But there are still cases where you'd be better off using another style.
WSDL 2.0 will not allow overloaded operations. This is unfortunate for languages like the Java language which do allow them. Specs like JAX-RPC will have to define a name mangling scheme to map overloaded methods to WSDL. WSDL 2.0 merely moves the problem from the WSDL-to-SOAP mapping to the WSDL-to-language mapping.
WSDL allows overloaded operations. But when you add the wrapped pattern to WSDL, you require an element to have the same name as the operation, and you cannot have two elements with the same name in XML. So you must use the document/literal, non-wrapped style or one of the RPC styles.
Since the document/literal, non-wrapped style doesn't provide the operation name, there are cases where you'll need to use one of the RPC styles. For instance, say you have the set of methods in Listing 11.
public void myMethod(int x, float y); public void myMethod(int x); public void someOtherMethod(int x, float y);
Now assume that your server receives the document/literal SOAP message that you saw back in Listing 7. Which method should the server dispatch to? All you know for sure is that it's not myMethod(int x) because the message has two parameters and this method requires one. It could be either of the other two methods. With the document/literal style, you have no way to know which one.
Instead of the document/literal message, assume that the server receives an RPC/literal message such as the one in Listing 5. With this message it's fairly easy for a server to decide which method to dispatch to. You know the operation name is myMethod, and you know you have two parameters, so it must be myMethod(int x, float y).
Under any literal style, the href attribute is not available, so the graph linkage is lost (see Listing 14 and Figure 2). You still have a root node, A, which points to a node B to the left and another node B to the right. These B nodes are equal, but they are not the same node. Data have been duplicated instead of being referenced twice.
There are various ways you can do graphs in literal styles, but there are no standard ways; so anything you might do would probably not interoperate with the service on the other end of the wire.
So far I have been discussing request messages. But what about response messages? What do they look like? By now it should be clear to you what the response message looks like for a document/literal message. The contents of the soap:body are fully defined by a schema, so all you have to do is look at the schema to know what the response message looks like. For instance, see Listing 15 for the response for the WSDL in Listing 8.
But what is the child of the soap:body for the RPC style responses? The WSDL 1.1 specification is not clear. But WS-I comes to the rescue. WS-I's Basic Profile dictates that in the RPC/literal response message, the name of the child of soap:body is "... the corresponding wsdl:operation name suffixed with the string 'Response'." Surprise! That's exactly what the conventional wrapped pattern's response element is called. So Listing 15 applies to the RPC/literal message as well as the document/literal wrapped message. (Since RPC/encoded is not WS-I compliant, the WS-I Basic Profile doesn't mention what an RPC/encoded response looks like, but you can assume the same convention applies here that applies everywhere else.) So the contents of response messages are not so mysterious after all.
There are four binding styles (there are really five, but document/encoded is meaningless). While each style has its place, under most situations the best style is document/literal wrapped.
Be prepared for those times when you have to create your SOAP message by hand and deal with namespace issues without the help of a tool by reading "Handle namespaces in SOAP messages you create by hand" (developerWorks, May 2005).
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Russell Butek is an SOA and Web services consultant for IBM. He has been one of the developers of IBM WebSphere Web services engine. He has also been a member the JAX-RPC Java Specification Request (JSR) expert group. He was involved in the implementation of Apache's AXIS SOAP engine, driving AXIS 1.0 to comply with JAX-RPC 1.0.
本文探讨了Web Services Description Language (WSDL)中四种主要的绑定风格：RPC/encoded、RPC/literal、Document/encoded 和 Document/literal，并详细分析了它们各自的优缺点，特别是Document/literal wrapped风格的优势。
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