To RPC, or not to RPC: that is the question. Whether 'tis nobler in the mind to suffer the control and dependency of coupling, or to take arms against a sea of troubles, and by opposing, end them?
The Simple Object Access Protocol (SOAP) offers two messaging styles: RPC (Remote Procedure Call) and document style. One is for creating tightly coupled, inter-object style interfaces for Web services components; the other is for developing loosely coupled, application-to-application and system- to-system interfaces. Some of you may have questions about the differences in the styles or the problems they are designed to solve. My goal here is to answer those questions. I'll first present the two styles in enough detail for you to gain an appreciation of their relative strengths and weaknesses; I'll then look at guidelines for their use.
The first question you may have is what is an RPC? An RPC is a way for an application running in one execution thread on a system to call a procedure belonging to another application running in a different execution thread on the same or a different system. RPC interfaces are based on a request-response model where one program calls, or requests a service of, another across a tightly coupled interface. In Web services applications, one service acts as a client, requesting a service; the other as a server, responding to that request. RPC interfaces have two parts: the call-level interface seen by the two applications, and the underlying protocol for moving data from one application to the other.
The call-level interface to an RPC procedure looks just like any other method call in the programming language being used. It consists of a method name and a parameter list. The parameter list is made up of the variables passed to the called procedure and those returned as part of its response. This is true on both sides of the interface. Both sides believe they are calling, or are being called by, a locally running procedure. Wiring in between hides the complexity of moving data between the two applications.
For Web services, SOAP defines the wiring between the calling and called procedures. At the SOAP level, the RPC interface appears as a series of highly structured XML messages moving between the client and the server where the <Body> of each SOAP message contains an XML representation of the call or return stack.
The transformation from call-level interface to XML and back occurs through the magic of two processes – marshaling and serialization. Figure 1 illustrates the major components and steps involved in this process.
The process begins with the client calling a method implemented as a remote procedure. The client actually calls a proxy stub that acts as a surrogate for the real procedure. The proxy stub presents the same external interface to the caller as would the real procedure, but instead of implementing the procedure's functionality, implements the processes necessary for preparing and transporting data across the interface.
The proxy stub gathers the parameters it receives through its parameter list into a standard form, in this case, into a SOAP message, through a process called marshaling.
The proxy stub encodes the parameters as appropriate during the marshaling process to ensure the recipient can correctly interpret their values. Encoding may be as simple as identifying the correct structure and data type as attributes on the XML tag enclosing the parameter's value or as complex as converting the content to a standard format such as Base64. The final product of the marshaling process is a SOAP message representation of the call stack.
The proxy stub serializes the SOAP message across the transport layer to the server. Serialization involves converting the SOAP message into a TCP/IP buffer stream and transporting that buffer stream between the client and the server.
The server goes through the reverse process to extract the information it needs. A listener service on the server deserializes the transport stream and calls a proxy stub on the server that unmarshals the parameters, decodes and binds them to internal variables and data structures, and invokes the called procedure. The listener process may be, for example, a J2EE servlet, JSP (JavaServer Page), or Microsoft ASP (Active Server Page). The client and server reverse roles and the inverse process occurs to return the server's response to the client.
You may be curious about the distinction I make between marshaling and serialization, having seen the terms used interchangeably. I distinguish between them because with Web services different standards define the rules for the two processes. SOAP defines the rules for marshaling and encoding data into XML messages, but doesn't specify how data is actually serialized across the interface. SOAP can bind to any protocol (usually either HTTP or Simple Mail Transport Protocol [SMTP]) for serialization, which means the specifications for those protocols actually define the serialization rules.
Section 7 of the SOAP specification defines the rules for marshaling RPC calls into XML messages (the most recent version of the SOAP 1.2 specification moves this information to the Adjuncts section, but the rules remain the same). Section 7 says to encode RPC method calls and responses as hierarchical XML elements, or structures, where the rootlevel element name is the method name in the case of the request and an arbitrary value in the case of the response, the structure's child elements are the method's parameters or return values; and each parameter or return value's elements are the data value or values it represents.
Section 5 of the SOAP specification defines SOAP's built-in rules for encoding data values. Encoding is necessary any time the recipient needs to interpret an element's value as something other than a literal string, i.e. as an integer, floating point number, or MIME type. XML Schema offers an increasingly popular alternative that has all but obsolesced Section 5 encoding. Listings 1 and 2 illustrate the two options for a skeletal RPC method call; the encodingStyle attribute tells the recipient which scheme is being used.
With this background on RPC style in place, the next question is how does document- style messaging differ? The difference is primarily in the control you have over the marshaling process. With RPC-style messaging, standards govern that process. With document- style messaging, you make the decisions: you convert data from internal variables into XML; you place the XML into the <Body> element of the encapsulating SOAP document; you determine the schema(s), if any, for validating the document's structure; and you determine the encoding scheme, if any, for interpreting data item values. The SOAP document simply becomes a wrapper containing whatever content you decide. For example, the SOAP document shown in Listing 3 contains an XML namespace reference, http://www.xyz.com/genealogy, that presumably includes all the information a receiving program needs for validating the message's structure and content, and for correctly interpreting data values.
Figure 2 illustrates the steps in a typical document-style message exchange. If you compare the steps involved in this process with those involved in processing an RPCstyle message from Figure 1, you will notice they are essentially parallel processes.
The SOAP client uses an Extensible Stylesheet Language Transformation (XSLT) and the DOM parser, or some other means, to create an XML document.
The SOAP client places this XML document into the <Body> of a SOAP message.
The SOAP client optionally includes a namespace reference in the message that other applications can use for validating the encapsulated document's format and content. The namespace reference may be included as an attribute either on one of the SOAP elements or on the XML document's root element. If the document does not include a namespace reference, the client and server must agree on some other scheme for validating and interpreting the document's contents.
The SOAP client serializes the message to the SOAP server across either an HTTP or SMTP bound interface.
The SOAP server reverses the process, potentially using a different XSLT, to validate, extract, and bind the information it needs from the XML document to its own internal variables. The roles reverse and the two follow inverse processes for returning and accessing any response values. The rules guiding the marshaling process are the primary difference between this process and that for RPC-style messages. With document-style, you as the SOAP client's author create those rules.
Strengths and Weaknesses
Now that we've looked at both styles in some detail, we can discuss their relative strengths and weaknesses.
RPC-style messaging maps to the objectoriented, component-technology space. It is an alternative to other component technologies such as DCOM and CORBA where component models are built around programmable interfaces and languages such as Java and C#. RPC-style messaging's strength in this space lies in its platform independence. It offers a standards-based, platform-independent component technology, implemented over standard Internet protocols. One of the benefits of this style's XML layer is that clients and servers can use different programming languages, or technologies, to implement their respective side of the interface, which means one side can choose one set of technologies, such as J2EE's JAX-RPC, while the other chooses a completely different set, such as .NET's C#. RPC-style messaging's standards heritage can be an important consideration in hybrid environments (one using multiple technologies such as J2EE and .NET) and can provide a transition path between different technologies.
RPC-style messaging's weaknesses include:
Strong coupling: If you change the number, order, or data types of the parameters to the call-level interface, you must make the change on both sides of the interface.
Synchronicity:Most programming languages assume synchronous method calls: the calling program normally waits for the called program to execute and return any results before continuing. Web services are asynchronous by nature and, in comparison to technologies such as DCOM and CORBA, long running. You may want to take advantage of Web services' asynchronous nature to avoid the user having to wait for calls to complete by developing asynchronous RPC calls, but that adds another level of complexity to your application. Some tools hide this complexity using callbacks, or other techniques, to enable processing overlap between the request and the response. Check to see if the tools you're using let you choose between synchronous and asynchronous RPC calls.
Marshaling and serialization overhead: Marshaling and serializing XML is more expensive than marshaling and serializing a binary data stream. With XML, at least one side of the interface, and possibly both, involves some parsing in order to move data between internal variables and the XML document. There is also the cost of moving encoded text, which can be larger in size than its binary equivalent, across the interface.
How do these drawbacks compare to those found in other component technologies? The coupling and synchronicity issues are common to RPC-based component technologies. so they are really not discriminators when making comparisons between these technologies. The marshaling and serialization overhead is greater for RPC-style messaging and places this messaging style at a relative disadvantage. However, with today's high-speed processors and networks, performance is generally not an issue.
Document-style messaging is clearly an option in any situation where an XML document is one of the interface parameters. It is ideal for passing complex business documents, such as invoices, receipts, customer orders, or shipping manifests. Documentstyle messaging uses an XML document and a stylesheet to specify the content and structure of the information exchanged across the interface, making it an obvious choice in situations where a document's workflow involves a series of services where each service processes a subset of the information within the document. Each service can use an XSLT to validate, extract, and transform only the elements it needs from the larger XML document; with the exception of those elements, the service is insensitive to changes in other parts of the document. The XSLT insulates the service from changes in the number, order, or type of data elements being exchanged. As long as the service creating the document maintains backwards compatibility, it can add or rearrange the elements it places into a document without affecting other services. Those services can simply ignore any additional data. Document-style messaging is also agnostic on the synchronicity of the interface; it works equally well for both synchronous and asynchronous interfaces.
Document-style messaging's weaknesses include:
No standard service identification mechanism: With document-style messaging, the client and server must agree on a service identification mechanism: a way for a document's recipient to determine which service( s) need to process that document. SOAP header entries offer one option; you can include information in the document's header that helps identify the service(s) needed. WS-Routing makes just such a proposal. Another option is to name elements in the <Body> of the message for the services that need to process the payload the elements contain. You might ask how that differs from schema-based RPC-style messaging. You would be right in assuming there is little or no difference except possibly in terms of the number of "calls" that can be made per message. A third option is to perform either structure or content analysis as part of a service selection process in order to identify the services needed to process the document.
Marshaling and serialization overhead: Document-style messaging suffers from the same drawbacks as RPC-style messaging in this area. However, the problem may be more severe with document-style messaging. Document-style messaging incurs overhead in three areas: in using DOM, or another technique, to build XML documents; in using DOM, or SAX, to parse those documents in order to extract data values; and in mapping between extracted data values and internal program variables. Tools generating equivalent RPC-style interfaces optimize these transformations. You may have trouble achieving the same level of efficiency in your applications using standard tools.
Given these drawbacks, you may ask whether document-style messaging really is an alternative. The answer is yes. There are two compelling reasons to use documentstyle messaging. One is to gain the independence it provides. Its strength lies in decoupling interfaces between services to the point that they can change completely independently of one another. The other is that document-style messaging puts the full power of XML for structuring and encoding information at your disposal. The latter is one reason many consider document-style superior to RPC-style messaging.
Summary
Given their relative strengths and weaknesses, what guidelines should you use in choosing between the two messaging styles? RPC-style messaging's strength is as a bridging component technology. It is a good option for creating new components and for creating interfaces between Web services and existing components – you simply wrap existing components with RPC-style Web services interfaces. RPC-style messaging is also an excellent component standard in situations where you are using multiple technologies, such as J2EE and .NET, and want to develop sharable components. So, there is clear justification for adopting an RPC style as a standard in these roles.
Document-style messaging's strengths are in situations where an XML document is part of the data being passed across the interface, where you want to leverage the full power of XML and XSL, and in instances where you want to minimize coupling between services forming an interface, such as in application-to-application and systemto- system interfaces. So, there is clear precedent here as well.
Neither style is a panacea. You must consider the relative strengths and weaknesses of each against your requirements. With these guidelines in mind, however, it is safe to adopt either based on your specific needs.
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