Translating web service applications with XSL

A common problem with web service applications is that the actual backend software does not support xml (or at least not the standardized XML used by your web service). To solve this problem, many architectures implement a translation phase that interprets the received XML and converts it into a more appropriate format that the backend system can recognize. Although there are many ways to translate XML into different formats, the extensible stylesheet language (eXtensibleStylesheet Language, XSL) provides a robust, standard, and XML-friendly solution.

Some common issues

There are many ways that translation may be needed between an XML message and the application that will process it. However, there are also many common problems, including:

Lookup
Mapping
Aggregation
Splitting
Formulas )
Reordering
Query processing will accept the received value and map it to a different value for the target system. For example, your XML has a value of 309, but the value your application requires is "Uber Widget".

Mapping essentially reassigns a value from one field to another value. For example, in XML, you might have an AccountNumber element that needs to be reassigned into a new element called CustomerAccountNumber.

Aggregation processing is the combination of two or more items from XML into a single item for use in back-end systems. A common example is combining the last name and first name fields into a single name (field). Splitting is the inverse of aggregation, which is dividing an XML value into two or more separate components.

Formula processing generally involves the calculation of one or more XML values ​​to obtain a value for a new application. One example is using XML sub-orders to calculate the total amount of an order.

Finally, reordering is the process of changing the sequence or structure of items in XML so that they conform to the sequence or structure expected by the target system.

Mapping Example
Now let’s take a closer look at this process using an example. We assume that the XML received is like the one in Listing 1:

Listing 1: webserviceorder.xml

<?xml version="1.0" ?>
<Order>
  <OrderNumber>8100</OrderNumber>
  <AccountNumber>99213</AccountNumber>
  <Item>
    <SKU>2388</SKU>
    <Description>Uber Widget</Description>
    <Quantity>15</Quantity>
    <PRicePer>10.95</PricePer>
  </Item>
  <Item>
    <SKU>6273</SKU>
    <Description>Flangeoid</Description>
    <Quantity>10</Quantity>
    <PricePer>52.00</PricePer>
  </Item>    
</Order>

Now, our ordering system requires a slightly different format. All we need to do is convert the order received from the web service into the format in Listing 2.

Listing 2: applicationorder.xml

<?xml version="1.0" encoding="UTF-8" ?>
<NeWorder>
  <CustomerAccountNumber>99213</CustomerAccountNumber>
  <CustomerOrderNumber>8100</CustomerOrderNumber>
  <OrderItems>
    <OrderItem>
      <SKU>2388</SKU>
      <CustomerPrice>10.95</CustomerPrice>
      <Quantity>15</Quantity>
      <Subtotal>164.25</Subtotal>
    </OrderItem>
    <OrderItem>
      <SKU>6273</SKU>
      <CustomerPrice>52.00</CustomerPrice>
      <Quantity>10</Quantity>
      <Subtotal>520</Subtotal>
    </OrderItem>
  </OrderItems>
</NewOrder>

Translation

Since this is a simplified example, we are converting the format There are only a few things you need to do with your XSL template. The first thing we need to pay attention to is: we have to map some elements:

Map Order to NewOrder
Map AccountNumber to CustomerAccountNumber
Map OrderNumber to CustomerOrderNumber
Map Item to OrderItem
Map PricePer to CustomerPrice
Then, we need to rearrange the OrderItem element under a new element called OrderItems. Finally, we add a new element called Subtotal, which is calculated based on the unit price and quantity.

Mapping translation is the simplest, as you simply define the new element in the template and specify that the new element has the value from the element in the received XML document. Reordering Items is accomplished by placing the Item subtemplate call into a new element called OrderItems. Finally, use a simple XPath expression to perform the calculation. Listing 3 shows the XSL document used to transform the received XML into application-specific XML.

Listing 3: translate.xsl

<?xml version="1.0"  ?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:apply-templates select="Order" />
  </xsl:template>
  <xsl:template match="Order">
    <NewOrder>
      <CustomerAccountNumber><xsl:value-of select="AccountNumber" /></CustomerAccountNumber>
      <CustomerOrderNumber><xsl:value-of select="OrderNumber" /></CustomerOrderNumber>
      <OrderItems>
        <xsl:apply-templates select="Item" />
      </OrderItems>
    </NewOrder>
  </xsl:template>
  <xsl:template match="Item">
    <OrderItem>
      <SKU><xsl:value-of select="SKU" /></SKU>
      <CustomerPrice><xsl:value-of select="PricePer" /></CustomerPrice>
      <Quantity><xsl:value-of select="Quantity" /></Quantity>
      <Subtotal><xsl:value-of select="PricePer * Quantity" /></Subtotal>
    </OrderItem>
  </xsl:template>
    
</xsl:stylesheet>

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