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Objects in XML: The SOAP Data Model

As you saw in Chapter 2, XML has an extremely rich structure—and the possible contents of an XML data model, which include mixed content, substitution groups, and many other concepts, are a lot more complex than the data/objects in most modern programming languages. This means that there isn't always an easy way to map any given XML Schema into familiar structures such as classes in Java. The SOAP authors recognized this problem, so (knowing that programmers would like to send Java/C++/VB objects in SOAP envelopes) they introduced two concepts: the SOAP data model and the SOAP encoding. The data model is an abstract representation of data structures such as you might find in Java or C#, and the encoding is a set of rules to map that data model into XML so you can send it in SOAP messages.

Object Graphs

The SOAP data model g is about representing graphs of nodes, each of which may be connected via directional edges to other nodes. The nodes are values, and the edges are labels. Figure 3.6 shows a simple example: the data model for a Product in SkatesTown's database, which you saw earlier.

Figure 3.6Figure 3.6 An example SOAP data model






In Java, the object representing this structure might look like this:

class Product {
  String description;
  String sku;
  double unitPrice;
  String name;
  String type;
  int numInStock;
}

Nodes may have outgoing edges, in which case they're known as compound values, or only incoming edges, in which case they're simple values. All the nodes around the edge of the example are simple values. The one in the middle is a compound value.

When the edges coming out of a compound value node have names, we say the node represents a structure. The edge names (also known as accessors) are the equivalent of field names in Java, each one pointing to another node which contains the value of the field. The node in the middle is our Product reference, and it has an outgoing edge for each field of the structure.

When a node has outgoing edges that are only distinguished by position (the first edge, the second edge, and so on), the node represents an array. A given compound value node may represent either a structure or an array, but not both.

Sometimes it's important for a data model to refer to the same value more than once—in that case, you'll see a node with more than one incoming edge (see Figure 3.7). These values are called multireference values, or multirefs.

Figure 3.7Figure 3.7 Multireference values

The model in this example shows that someone named Joe has a sister named Cheryl, and they both share a pet named Fido. Because the two pet edges both point at the same node, we know it's exactly the same dog, not two different dogs who happen to share the name Fido.

With this simple set of concepts, you can represent most common programming language constructs in languages like C#, JavaScript, Perl, or Java. Of course, the data model isn't very useful until you can read and write it in SOAP messages.

The SOAP Encoding

When you want to take a SOAP data model and write it out as XML (typically in a SOAP message), you use the SOAP encoding g. Like most things in the Web services world, the SOAP encoding has a URI to identify it, which for SOAP 1.2 is http://www.w3.org/2003/05/soap-encoding. When serializing XML using the encoding rules, it's strongly recommended that processors use the special encodingStyle attribute (in the SOAP envelope namespace) to indicate that SOAP encoding is in use, by using this URI as the value for the attribute. This attribute can appear on headers or their children, bodies or their children, and any child of the Detail element in a fault. When a processor sees this attribute on an element, it knows that the element and all its children follow the encoding rules.

SOAP 1.1 Difference: encodingStyle

In SOAP 1.1, the encodingStyle attribute could appear anywhere in the message, including on the SOAP envelope elements (Body, Header, Envelope). In SOAP 1.2, it may only appear in the three places mentioned in the text.

The encoding is straightforward: it says when writing out a data model, each outgoing edge becomes an XML element, which contains either a text value (if the edge points to a terminal node) or further subelements (if the edge points to a node which itself has outgoing edges). The earlier product example would look something like this:

<product soapenv:encodingStyle="http://www.w3.org/2003/05/soap-encoding">
  <sku>947-TI</sku>
  <name>Titanium Glider</name>
  <type>skateboard</type>
  <desc>Street-style titanium skateboard.</desc>
  <price>129.00</price>
  <inStock>36</inStock>
</product>

If you want to encode a graph of objects that might contain multirefs, you can't write the data in the straightforward way we've been using, since you'll have one of two problems: Either you'll lose the information that two or more encoded nodes are identical, or (in the case of circular references) you'll get into an infinite regress. Here's an example: If the structure from Figure 3.7 included an edge called owner back from the pet to the person, we might see a structure like the one in Figure 3.8.

If we tried to encode this with a naïve system that simply followed edges and turned them into elements, we might get something like this:

<person soapenv:encodingStyle="http://www.w3.org/2003/05/soap-encoding">
 <name>Joe</name>
 <pet>
  <name>Fido</name>
  <owner>
   <name>Joe</name>
   <pet>
    --uh oh! stack overflow on the way!--

Figure 3.8Figure 3.8 An object graph with a loop

Luckily the SOAP encoding has a way to deal with this situation: multiref encoding. When you encode an object that you want to refer to elsewhere, you use an ID attribute to give it an anchor. Then, instead of directly encoding the data for a second reference to that object, you can encode a reference to the already-serialized object using the ref attribute. Here's the previous example using multirefs:

<person id="1" soapenv:encodingStyle="http://www.w3.org/2003/05/soap-encoding">
 <name>Joe</name>
 <pet id="2">
  <name>Fido</name>
  <owner ref="#1"/> <!-- refer to the person -->
 </pet>
</person>

Much nicer. Notice that in this example you see an id of 2 on Fido, even though nothing in this serialization refers to him. This is a common pattern that saves time on processors while they serialize object graphs. If they only put IDs on objects that were referred to multiple times, they would need to walk the entire graph of objects before writing any XML in order to figure that out. Instead, many serializers always put an ID on any object (any nonsimple value) that might potentially be referenced later. If there is no further reference, then you've serialized an extra few bytes—no big deal. If there is, you can notice that the object has been written before and write out a ref attribute instead of reserializing it.

SOAP 1.1 Differences: Multirefs

The href attribute that was used to point to the data in SOAP 1.1 has changed to ref in SOAP 1.2.

Multirefs in SOAP 1.1 must be serialized as independent elements, which means as immediate children of the SOAP:Body element. This means that when you receive a SOAP body, it may have multiref serializations either before or after the real body element (the one you care about). Here's an example:

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope"
xmlns:soapenc="http://schemas.xmlsoap.org/soap/encoding">
 <soap:Body>
 <!-- Here is the multiref -->
 <multiRef id="obj0" soapenc:root="0" xsi:type="myNS:Part"
soapenv:encodingStyle="http://www.w3.org/2003/05/soap-encoding"> <sku>SJ-47</sku> </multiRef> <!-- Here is the method element --> <myMultirefMethod soapenc:root="1" soapenv:encodingStyle= "http://www.w3.org/2003/05/soap-encoding"> <arg href="#obj0"/> </myMultirefMethod> <!-- The multiref could also have appeared here --> </soap:Body> </soap:Envelope>

This is the reason for the SOAP 1.1 root attribute (which you can see in the example). Multiref serializations typically have the root attribute set to 0; the real body element has a root="1" attribute, meaning it's the root of the serialization tree of the SOAP data model. When serializing a SOAP message 1.1, most processors place the multiref serializations after the main body element; this makes it much easier for the serialization code to do its work. Each time they encounter a new object to serialize, they automatically encode a forward reference instead (keeping track of which IDs go with which objects), just in case the object was referred to again later in the serialization. Then, after the end of the main body element, they write out all the object serializations in a row. This means that all objects are written as multirefs whenever multirefs are enabled, which can be expensive (especially if there aren't many multiple references). SOAP 1.2 fixes this problem by allowing inline multirefs. When serializing a data model, a SOAP 1.2 engine is allowed to put an ID attribute on an inline serialization, like this:

<SOAP:Body>
  <method>
   <arg1 id="1" xsi:type="xsd:string">Foo</arg1>
   <arg2 href="#1"/>
 </method>
</SOAP:Body>

Now, making a serialized object available for multireferencing is as easy as dropping an id attribute on it. Also, this approach removes the need for the root attribute, which is no longer present in SOAP 1.2.

Encoding Arrays

The XML encoding for an array in the SOAP object model looks like this:

<myArray soapenc:itemType="xsd:string"
     soapenc:arraySize="3">
 <item>Huey</item>
 <item>Duey</item>
 <item>Louie</item>
</myArray>

This represents an array of three strings. The itemType attribute on the array element tells us what kind of things are inside, and the arraySize attribute tells us how many of them to expect. The name of the elements inside the array (item in this example) doesn't matter to SOAP processors, since the items in an array are only distinguishable by position. This means that the ordering of items in the XML encoding is important.

The arraySize attribute defaults to "*," a special value indicating an unbounded array (just like [] in Java—an int[] is an unbounded array of ints).

Multidimensional arrays are supported by listing each dimension in the arraySize attribute, separated by spaces. So, a 2x2 array has an arraySize of "2 x 2." You can use the special "*" value to make one dimension of a multidimensional array unbounded, but it may only be the first dimension. In other words, arraySize="* 3 4" is OK, but arraySize="3 * 4" isn't.

Multidimensional arrays are serialized as a single list of items, in row-major order (across each row and then down). For this two-dimensional array of size 2x2

    	0			1
    	Northwest	Northeast
    	Southwest	Southeast

the serialization would look like this:

<myArray soapenc:itemType="xsd:string"
     soapenc:arraySize="2 2">
 <item>Northwest</item>
 <item>Northeast</item>
 <item>Southwest</item>
 <item>Southeast</item>
</myArray>

SOAP 1.1 Differences: Arrays

One big difference between the SOAP 1.1 and SOAP 1.2 array encodings is that in SOAP 1.1, the dimensionality and the type of the array are conflated into a single value (arrayType), which the processor needs to parse into component pieces. Here are some 1.1 examples:

arrayType Value

Description

xsd:int[5]

An array of five integers

xsd:int[][5]

An array of five integer arrays

xsd:int[,][5]

An array of five two-dimensional arrays of integers

p:Person[5]

An array of five people

xsd:string[2,3]

A 2x3, two-dimensional array of strings

In SOAP 1.2, the itemType attribute contains only the types of the array elements. The dimensions are now in a separate arraySize attribute, and multidimensionality has been simplified.

SOAP 1.1 also supports sparse arrays (arrays with missing values, mostly used for certain kinds of database updates) and partially transmitted arrays (arrays that are encoded starting at an offset from the beginning of the array). To support sparse arrays, each item within an array encoding can optionally have a position attribute, which indicates the item's position in the array, counting from zero. Here's an example:

<myArray soapenc:arrayType="xsd:string[3]">
 <item soapenc:position="[1]">I'm the second element</item>
</myArray>

This would represent an array that has no first value, the passed string as the second element, and no third element. The same value can be encoded as a partially transmitted array by using the offset attribute, which indicates the index at which the encoded array begins:

<myArray soapenc:arrayType="xsd:string[3]" soapenc:offset="[1]">
 <item>I'm the second element</item>
</myArray>

Due to several factors, including not much uptake in usage and interoperability problems when they were used, these complex array encodings were removed from the SOAP 1.2 version.

Encoding-Specific Faults

SOAP 1.2 defines some fault codes specifically for encoding problems. If you use the encoding (which you probably will if you use the RPC conventions, described in the next section), you might run into the faults described in the following list. These all are subcodes to the code env:Sender, since they all relate to problems with the sender's data serialization. These faults aren't guaranteed to be sent—they're recommended, rather than mandated. Since these faults typically indicate problems with the encoding system in a SOAP toolkit, rather than with user code, you likely won't need to deal with them directly unless you're building a SOAP implementation yourself:

  • MissingID—Generated when a ref attribute in the received message doesn't correspond to any of the id attributes in the message

  • DuplicateID—Generated when more than one element in the message has the same id attribute value

  • UntypedValue—Optional; indicates that the type of node in the received message couldn't be determined by the receiver

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