SOAP uses XML to marshal data that is transported to a software application.
Since SOAP’s introduction, three SOAP encoding styles have become popular and are reliably implemented across software vendors and technology providers:
- SOAP Remote Procedure Call (RPC) encoding, also known as Section 5 encoding, which is defined by the SOAP 1.1 specification
- SOAP Remote Procedure Call Literal encoding (SOAP RPC-literal), which uses RPC methods to make calls but uses an XML do-it-yourself method for marshalling the data
- SOAP document-style encoding, which is also known as message-style or document-literal encoding.
There are other encoding styles, but software developers have not widely adopted them, mostly because their promoters disagree on a standard. For example, Microsoft is promoting Direct Internet Message Exchange (DIME) to encode binary file data, while the rest of the world is promoting SOAP with Attachments. SOAP RPC encoding, RPC-literal, and document-style SOAP encoding have emerged as the encoding styles that a software developer can count on.
SOAP RPC is the encoding style that offers you the most simplicity. You make a call to a remote object, passing along any necessary parameters. The SOAP stack serializes the parameters into XML, moves the data to the destination using transports such as HTTP and SMTP, receives the response, deserializes the response back into objects, and returns the results to the calling method. Whew! SOAP RPC handles all the encoding and decoding, even for very complex data types, and binds to the remote object automatically.
Now, imagine that you have some data already in XML format. SOAP RPC also allows literal encoding of the XML data as a single field that is serialized and sent to the Web service host. This is what’s referred to as RPC-literal encoding. Since there is only one parameter — the XML tree — the SOAP stack only needs to serialize one value. The SOAP stack still deals with the transport issues to get the request to the remote object. The stack binds the request to the remote object and handles the response.
Lastly, in a SOAP document-style call, the SOAP stack sends an entire XML document to a server without even requiring a return value. The message can contain any sort of XML data that is appropriate to the remote service. In SOAP document-style encoding, the developer handles everything, including determining the transport (e.g., HTTP, MQ, SMTP), marshaling and unmarshaling the body of the SOAP envelope, and parsing the XML in the request and response to find the needed data.
The three encoding systems are compared here:
SOAP RPC encoding is easiest for the software developer; however, all that ease comes with a scalability and performance penalty. In SOAP RPC-literal encoding, you are more involved with handling XML parsing, but it requires there to be overhead for the SOAP stack to deal with. SOAP document-literal encoding is most difficult for the software developer, but consequently requires little SOAP overhead.
Why is SOAP RPC easier? With this encoding style, you only need to define the public object method in your code once; the SOAP stack unmarshals the request parameters into objects and passes them directly into the method call of your object. Otherwise, you are stuck with the task of parsing through the XML tree to find the data elements you need before you get to make the call to the public method.
There is an argument for parsing the XML data yourself: since you know the data in the XML tree best, your code will parse that data more efficiently than generalized SOAP stack code. You will find this when measuring scalability and performance in SOAP encoding styles.
1. Discover SOAP encoding’s impact on Web service performance (http://www.ibm.com/developerworks/webservices/library/ws-soapenc/)