XML Patterns
In these patterns XML proves to be a smart way to achieve the goal. Through
examples, we show precise solutions that can be used alone or combined. They can be
especially useful when developers need to get data that belong to opaque systems, when the
separation of data from processing is a must, and when data have to cross platform
boundaries.
XML Patterns
XML Patterns
María Laura Ponisio* and Gustavo Rossi*
(*)LIFIA, Facultad de Informática, UNLP
La Plata, Buenos Aires, Argentina
Mail: {lponisio, gustavo }@sol.info.unlp.edu.ar
Acknowledgments
Thanks to our shepherd, Paul Asman for his dedication and willingness to get this work
right.
Abstract
In this paper we deal with the problem of getting distributed data onto a Web site.
We present four patterns that can be utilized to achieve a successful solution in that
endeavor. Each individual pattern is a way of solving part of this general problem.
In these patterns XML proves to be a smart way to achieve the goal. Through
examples, we show precise solutions that can be used alone or combined. They can be
especially useful when developers need to get data that belong to opaque systems, when the
separation of data from processing is a must, and when data have to cross platform
boundaries.
The patterns use the power of XML to share data between distributed sources as
well as to transform XML data on behalf of the user view.
Keywords: XML; XSL; XSLT; Design patterns; Web design
Introduction
We present four patterns dealing with a general problem, getting distributed data
onto a Web site. Each pattern uses XML, the eXtensible Markup Language, to form part of
the solution. The XML specification offers a way of organizing data so that the data can be
shared. XML makes easy to transfer data between platforms and separates data from data
transformation.
The first pattern, XML In Out Tray, solves the problem of getting, processing and
showing data. In this pattern, XML holds the place of the in and out trays that a worker uses
to receive petitions. This pattern solves the problem of getting and giving data from and to
applications where the internal processes are hidden.
The second, External Assistant, adds outsourcing to the process of generating an
HTML page from an XML document and XSL stylesheet. With a model that transforms
XML data while keeping it completely separated from the processing instructions, External
Assistant solves the problem of how to add external computation.
Here XML data on one side and the instructions in the XSL stylesheet on the other
feed a transformation process that generates output in the form of an HTML file. In this
context, External Assistant explains how to call an external process whose results are
1
Copyright © 1999, María Laura Ponisio and Gustavo Rossi. Permission is granted to copy for the PLoP
2001 conference. All other rights reserved.
XML Patterns
incorporated to the output, always keeping high modularity and clear distinction of the
responsibilities of each component.
The third, Information Grouping, solves the problem of grouping and presenting
XML data in an HTML page. Any opaque application running on any platform can to
provide XML input. An XSL stylesheet then takes this and generates HTML. This process
resembles what SQL’ Group By does on database tables.
s
Finally we present XML Mediator, a pattern that solves the problem of transferring
data between foreign applications by automatically collecting data from those applications
on behalf of a working client collector. It uses a process that collects XML data from data
providers. This process takes the different feeds and builds a compilation of the data in an
XML document, even when foreign applications run on different platforms. Through this
pattern, a thin client receives distributed information that it requires. Furthermore, the
process of collecting information is transparent to the user (usually a person at a browser,
but potentially an automated process) except when he declares what data he requires.
The complete code for the implementation of the examples present in all the
patterns can be found in [Lifia xml].
1. XML In Out Tray
Intent
Organize the activity of components involved in the process of getting and showing
data.
Motivation
Let’ assume we have to get data from a source, following some criteria. We don’
s t
need all the data held by the source, but some of it. The criteria by which we will retrieve
the data comes from some data entry, for instance a user-filled form submitted through a
browser. Once we’ collected and processed data, our final task is to present it.
ve
We wish the design to be flexible enough so that two distinct computational
components are capable of interchanging data without coming together into a single mass
of code. For instance, we wish to get data from a source generated by foreign systems. The
foreign system is opaque and generates some output that acts as our input. But in the
interchanging process only data are interchanged.
Our goal is to develop connections between components through which data can
travel while keeping high cohesion and low coupling.
The naive solution is to have just one component to receive the request, fetch data
from the sources, process what it has found, and generate the output. This is messy, and
could lead to too many entrances in the data source looking for the data. This in turn makes
it difficult to optimize the system and lowers performance.
The naive solution doesn’ have a clean data interchange with foreign systems. It
t
does not even have a clean data interchange between internal components. The solution
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XML Patterns
therefore forces the developer to understand the data model of the source, and also
increases programming efforts and coupling between the internal components. This design
also lacks modularity, increasing coupling. It doesn’ use the virtues of abstraction, and so
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suffers from low flexibility and reusability. As a consequence a small change could affect
the whole system.
We need to find a useful ‘glue’between application components to allow language
independence on one hand and different data structure coexistence on the other.
Solution
We have three computational components and an XML file. The first component
(IN) receives input data containing the search criteria.
The second component (WORKER) receives the criteria from the IN component
and fetches the needed data from the XML file. So the only requirements are that the
WORKER must understand the request and the format (structure) of the XML file. The
WORKER then processes the data and adds a business rule if needed. For instance, it could
contain a business rule that applies a specific discount to items bought by some client, or
write an entry to a log. Finally this WORKER sends the unformatted results to a third
component named OUT.
WORKER processes data and in this process can format and transform it.
WORKER also is responsible of performing whatever function is needed for business logic.
It can record an activity in a log table for ISO conformance, for instance. But once
WORKER finds out the results, it transfers them to OUT, and OUT performs the
formatting and transformation (in the XSLT sense) of the results.
OUT gets the data and builds the output, formatting, transforming and rendering
them. OUT is designed upon the output device, in the same way as IN is designed upon the
input device.
The XML files store the data. Since it is XML it doesn’ matter how different the
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foreign system is that generated the data, as long as the data have some basic and known
structure.
If we need to receive data from different input devices and output it in different
output devices, we could use several IN and OUT components, one for each proper final
device (browser, handheld, wireless phone, etc.). Figure 1.1 shows component connections.
HTML
IN
XML
WORKER
HTML
OUT
Figure 1.1 Relation between components present in the solution
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XML Patterns
Example: Web product catalog display
This pattern has been used successfully for a catalog of products. We wished to
show our products one at a time at customer request. So we built a web application that lets
our customers enter some criteria that identify a product, and then goes and fetches the data
for the product. In order to fulfill this requirement, we designed a solution based on an
XML document that stores our catalog, a component IN that gets the user criteria, a
component WORKER that performs the search in our catalog for the right product, and a
component OUT that presents it to the customer. Here we included in the patterns only part
of the code. Nevertheless the complete code can be seen and downloaded from [Lifia xml].
First of all we used the XML document to store the data in our catalog. To do that
we defined the structure of our data in terms of XML elements, which together form the
XML document. Figure 1.2 shows part of the XML file and the data format we’ chosen.
ve
Here we have a element for each product and a , and
containing information about the item. The list of products present in the catalog is
composed of a list of . The XML document is not the ultimate storage of data, but
an intermediary between a total different database and our application.
Then we used a component to let the user enter the criteria by which we will search
Figure 1.2 Part of a catalog in a
XML document storage.xml Figure 1.3 IN component where user
enters search criteria
the product in our catalog. This is accomplished by the IN component, so we built it as a
form element in an ASP file, but it could also be a form element in html to get user input.
In this example, the IN component is performed by a file called in.asp. Figure 1.3 shows
the (simple) form that our clients see. Again, the code can be found at [Lifia xml].
At this point we needed something to search the catalog (in fact to search store.xml),
looking for the product that meets the criteria entered and to present it to the client. So we
built worker.asp, a program to do the work of the component WORKER and we also built
out.asp, a program to do the work of the OUT component. Figure 1.4 shows part of the
worker.asp’ code and Figure 1.5 shows the output presented to the client.
s
4
XML Patterns
'Search xml data from the hints the user entered
Dim xmlDoc
Dim nodeList
Set xmlDoc=createObject("Microsoft.XMLDOM")
Dim str
' Load from a local XML file
xmlDoc.Load "C:\Inetpub\wwwroot\XML\InOutTray\storage.xml"
'Load XML tree node matching user criteria
Set nodeList=
xmlDoc.selectNodes("results/item[name='"strName"']//price")
price=nodeList.item(0).text
listl =nodeList.length
session ("sprice")=price
session ("sname")=strName
session ("sresult") =listl
response.redirect "out.asp"
%>
Figure 1.4 A sample of worker.asp
Using the file worker.asp, we look for the data we need in the XML document
(storage.xml) and show it.
Consequences
• We don’ get involved with the source data representation.
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• Our solution uses abstract components only.
• We can mix the components to better fit the problem context.
• The final design is flexible enough to be used under different circumstances.
• The solution is modular, so it’ easier to use, develop and optimize.
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Further comments
If we want to keep implementation simple, we can mix components as long as we
keep in mind the role that each component plays. But if the activities are really simple or
the data are generated within the application (instead of being transferred from another
Figure 1.5 Output presented
to the client
5
XML Patterns
application), then the use of this pattern is nor recommended.
This pattern wouldn’ apply when the components depend heavily on each other in
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the sense that one component uses code elements that belong to another.
When rendering the data with the OUT component, or after retrieving the data from
the XML file with the WORKER, we can group selected data using Information Grouping,
a pattern we describe below.
There are no language specific issues. As a consequence, this pattern doesn’ restrict
t
the language used for implementation. Furthermore, a key benefit in the design is the use of
XML to assure platform independence. This should work even with foreign systems acting
as data sources. In the implementation XML acts as a gateway connecting the system that
uses this design and neighbours with which the system has to interact.
The XML document plus the programming of the part of WORKER that gets the
data from a foreign system provides a placeholder to control access to the data. In this
aspect XML In Out Tray resembles the GoF’ Proxy pattern [Gamma+95].
s
2. External Assistant
Intent
Show how to call external computation from an XML-XSL model.
Allow a stylesheet to ask for a task performed by an external assistant.
Motivation
It is a common problem to break a simple architecture to add some kind of
computation that performs work needed as part of the output, for instance the task to
calculate some value upon some of the data received. This leads to complex and expensive
code both in writing and in maintenance, and this in turn ends up raising the cost of the
whole system.
So we need to add new functionality but at the same time avoid code complexity. In
calculating some value based upon some of the data, for example, we need to figure or
fetch some information by some kind of computation that we do not wish to include in the
stylesheet, even if the expression relies upon the XML data stored in an XML document.
The inclusion could strain the simpler architecture to a breaking point.
If we are working in a context where we need to keep data separate from
instructions by using XML and XSLT, adding tasks can increase the complexity of the
code. So the designer ends up with one of two naïve solutions: breaking the separation
between data and transformation, or forcing the underlying XML+XSLT+parser
architecture to handle more that it should.
For instance, we could receive more data than we need to show. We could then
select some data and perform certain specific calculations before submitting it for output.
The naïve solution would be to overload the stylesheet past breaking.
Solution
6
XML Patterns
Make the stylesheet call Java class methods.
On one side we have an XML document containing data to process. On the other we
XSL JAVA
Stylesheet classes
XML Transformation HTML
Figure 2.1 Stylesheet calling an external assistant in a transformation process
have a XSL stylesheet with precise instructions about what to do with the data. Both of
them feed a transformation process that renders complex output. But the process is aided by
an external function that adds functionality and then improves the behavior defined in the
stylesheet. Figure 2.1 shows the sketch of the solution.
Example 1: Calling a Java aid from a stylesheet
In this example we call an external Java function and show how to call a Java class
method, which returns the number 1 for demonstration purposes. It symbolizes the call of a
program that performs some task.
First, we have an XML document performing as a dummy file called dummy.xml. In
Figure 2.2 we can see the notepad showing dummy.xml.
Then we have a stylesheet call a Java class method that returns something (the
number 1, in this example). Figure 2.3 shows part of the code of the stylesheet.
Figure 2.2 XML document dummy.xml
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XML Patterns
Figure 2.3 Part of the stylesheet external.xsl calling a method
In addition to the XML document and the stylesheet, we need a Java class method
to perform the task of returning the number 1. This Java class method is called by the
stylesheet and appears in Figure 2.4.
Finally, a parser performs the transformation and leaves the result of its work in an
HTML file called external.html, which we show in Figure 2.5.
public class javaclass
{ public static int method()
{ int n = 1;
return n;
}
}
Figure 2.5 HTML file generated by
Figure 2.4 Method called by the stylesheet the parser
Example 2: Inserting current date in a page
We use this solution when we are building a site and we want to add the current date
to a page before submitting it to the user. We accomplish this task by applying our solution
together with the Java standard class library.
8
XML Patterns
Here we show how to call a method of the Java standard class library. To do that we
Figure 2.6 date.xsl inserting the current date in a page
start with the same dummy.xml file used in the previous example. We also have a stylesheet
that calls java.util.Date, creates a Date object and initializes it with the current date. This
stylesheet is date.xsl shown in Figure 2.6. Finally, Figure 2.7 shows the result of running a
parser on the XML dummy document, for which the stylesheet date.xsl builds a page
Figure 2.7 Page with the system date generated
by the parser
containing the current system date after calling a Java standard class method.
Consequences
• We keep high modularity because we add external computations
maintaining the abstraction of the behavior. We avoid mixing external code
with the code needed to show the XML data, which is in the stylesheet.
• We lower stylesheet complexity while increasing computation. Today, many
parsers allow calling Java class methods. Note how the benefits of this
model increase as we need to add computations to the stylesheet to show
data processed according to certain criteria.
• This approach also makes it easier to personalize output.
• We have found a simple way to add computation when received data aren’ t
sufficient for what we want to show, and intermediate calculations are
needed. [Nielsen99]
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XML Patterns
Further comments
The mechanism used here isn’ defined in XSLT or XPATH specifications.
t
However, some parsers support its implementation. It will probably become a standard in
the future [Kay00].
We can use this pattern when the parser that will perform the transformation has a
mechanism for binding to external functions written in Java. Since at the time of writing it
is not defined in the XSLT or XPath specification, care must be taken, because using this
mechanism lowers the portability of the stylesheet considerably. If the developers call a
Java class, since it is not still written in the spec, the way they call it depends on the
processor. So the fact that a stylesheet works with one XSLT processor doesn’ mean that it
t
will work with other. Once that subject is solved, we can use External Assistant with
patterns whose solution uses the XML+XSLT+parser model, like the Information Grouping
pattern below.
It’ a delicate matter to decide whether to use this pattern or Information Grouping.
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In Information Grouping there’ an example of when the decision process reveals that is
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better not to call an External Assistant. That is because the XSLT spec includes sorting,
which is prima facie evidence that this sort of computation should be handled in the
stylesheet. But when the process that needs to be performed isn’ included in the XSLT
t
specification, then External Assistant can help.
3. Information Grouping
Intent
Model a transformation in XML using XSLT. Given an XML document, the pattern
shows the way to format the XML data and generate an HTML file containing the data of
the XML document, but grouped by certain criteria.
Motivation
We need to show elements grouped by some criteria, but we have those elements
stored without the desired order. In other words, starting from an XML document storing
elements, we wish to group the elements on the basis of some subelement. Therefore we
build a way of presenting (showing) the resulting ordered list. We use a stylesheet to
generate the transformation that we want. For instance, we could build a web mail
application to let web clients to get their mail. While doing this, we should offer our user
the choice of how to see his mail messages. He should decide if he wants to see the list of
messages grouped by date, sender or subject, in ascending or descending order.
Furthermore, the approach followed in this pattern is useful when we need to keep
the presentation logic separated from the business logic. See Further comments in XML In
Out Tray and Further comments and Consequences in External Assistant.
Solution
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XML Patterns
Have a stylesheet group the elements. We have an XML file with the data, and we
have to create a file with the transformation. This file is the stylesheet. The stylesheet
instructs a program (a parser) what to do with the data stored in the XML file. It specifies a
transformation (in the XML sense, since data in the original file remains the same) that
traverses the XML tree implicit in the XML file that has the data, and makes a new list
which has all the groups existing in the tree. The groups in this list are distinct.
In a second step, the transformation loops through the distinct group list generated
previously in such a way that for each group in this list, it inserts a row in a table that will
be shown in the output. Then it walks again through the complete XML document tree
collecting all the elements belonging to the current group and showing them.
It renders the desired part of each element in the corresponding element of the
HTML file. For instance it prints part of the element in the second column of the
corresponding row of a table. The group to which the element belongs is the deciding factor
of the proper row. As a result the parser builds an HTML file. Figure 3.1 shows the basic
solution where an XML file and a stylesheet feed a transformation process that produces an
HTML page containing data properly grouped.
Stylesheet that
performs the
grouping
XML
document
with list of Transformation
items Process
HTML showing information
by group
Figure 3.1 Transformation process
Example: Retrieving information from a knowledge recorder
In this example we show how to group XML data according to some criteria and
then present it to a client in a browser with an HTML page. The items we work with
represent knowledge acquired on a subject. So the list of items is a small store of
knowledge kept in an XML document. This list of items is made of subject elements. So
our task is to get an XML document containing a list of subjects with no particular order,
group those subjects and present them, grouped, on a table in an HTML page. Each subject
has its specific format (structure) and belongs to a group in such a way that all the subjects
can be placed under some group they belong to. Figure 3.2 shows part of list.xml, the
document that stores our knowledge data.
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XML Patterns
Select all
Selecting Data
This is the common problem of
selecting every item in a table
...
Creating Tables
Managing Tables
We need to create a Data Base table
...
Tuning SQL Statements
Data Base Tuning
How to write a SQL statement to
minimize data retrieval cost.
Use indexes
Figure 3.2 Part of list.xml
Now we need to work with the data so we can transform the XML tree to get
another XML tree with the same data, but ordered as we want to show it. So we create a
stylesheet that tells the parser to produce an HTML file showing the data in conformance
with our specification. In this example we output the title and the group to which the
subject belongs (the Owner Group). The output is shown in a browser conforming the rule
that titles must be gathered by Owner Group. Figure 3.3 shows a sample stylesheet, list.xsl,
the stylesheet that we used. Finally in Figure 3.4 we present the parser generated output as a
user sees it in a browser.
Grouping items
Group by
Titles gathered by 'group'
GroupTitles
Figure 3.3 list.xsl, the sample stylesheet
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XML Patterns
Figure 3.4 HTML showing data grouped by the value of the element
Consequences
• There’ a clear gap between the data content and representation format. This
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advantage is forced by XML. The content-representation separation leads to easy-to-
try prototypes. For instance, we can test the transformation while not knowing the
actual data yet. Besides, a change in the transformation doesn’ affect data. While
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keeping consistency between XML document and tags used in the transformation, a
change in one doesn’ affect the other.
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• Reusability. This transformation can be used whenever we need to group items by
criteria shown in one of their elements or attributes. Note that there are some
restrictions to the file storing the data, for instance that the data must be an XML
document. But this fact leads to an increase in the number of contexts (situations)
where we can apply Information Grouping with low effort.
• It solves the problem of showing a list of interleaving items.
Further comments
Information Grouping can organize data in a different structure (format) from that in
which it is stored. For instance if the XML document that stores the data have elements
with attributes, then this pattern can be used to group by attributes as well.
This pattern would prove useful grouping an XML format storing mail by sender,
for instance.
Information Grouping should be called if the calculation we want to apply to the
data is included in the XSLT specification. This stands in contrast with External Assistant,
which should be called when the data processing includes some specific calculation that is
not part in the specification or, even if specified, will require code that is hard to write.
Nevertheless, when Information Grouping is used, it could be used with External
Assistant to add processing without breaking the architecture of the Information Grouping’ s
approach.
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XML Patterns
4. XML Mediator
Intent
Organize XML information gathering from a wide range of sources.
Motivation
XML is a good, extensible way to collect information from disparate sources. This
pattern takes advantage of that to answer the problems of manual data integration,
verification of transmission completeness and transparent addition of components.
To collect information from several sources nowadays the user is often forced first
to look for the information and then to copy and paste it to some kind of notepad, learn it
by heart or print it. He must then repeat the process until he gathers all the information he
needs. Note that the information could be spread over several sites. For example, consider a
user planning his or her vacations. He must visit several different sites in order to gather
information about weather, prices, availability of hotel rooms, flights, etc. This is a tedious
task and error prone. In fact, we could argue that Internet acts as a mainframe because the
users can see pages one at a time, which makes it difficult to catch scattered data. In order
to get the wanted data, the user has to scribble them in a notepad.
Here, unrelated applications must share information. We need efficiency in data
transference as well as high performance in handling simultaneous requests. Besides, we
want to carry information from several applications into one in a comprehensible format.
We want to translate the information to a universal format to carry it through
applications so we can use the formatted data generated by a foreign system. The data
coming from a foreign system being formatted is text where markup indicates an inner
structure, for instance the formatted text can be a list of elements, which also are text. The
pattern used in those foreign systems to produce XML doesn’ affect the use of this pattern.
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The XML data are created by the foreign system or are data from a foreign system where
the XML is added outside that system. If the component systems do not provide XML, an
additional process must take the data provided by the components and creates XML from it.
Besides, we need some inner validation for the data carried, in order to verify that we’ ve
received all the data that were sent. For instance, if we were carrying an invoice, we would
need to guarantee that we’ received every item of it in the transaction, and that none was
ve
left out due to problems in the transference.
In addition, we want the design to be modular so we can increase the abstraction.
Sites offering information and sites gathering it should both add the proper abstract
functions for giving and taking information.
Furthermore, there’ a need for easy-to-implement solutions for the interacting sites.
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This is because adding the connection involves adding programming, and the new
component shouldn’ interfere with the design of the site. For instance, it shouldn’ affect
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the navigational layer, except for adding the connection module to let the user download
the data. At this layer we just need to add the component used for providing the
information.
Solution
14
XML Patterns
Each application that wishes to offer information should add a component from
which clients can take formatted information or have an additional process outside of the
foreign system to format the data into an XML valid format for the clients to consume that
data. This component interacts with a user-guided client component. Both of them couple at
the user’ request and allow information transference using XML.
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We suggest that web sites include a connection device to let the user have the data
that belongs to external data sources like foreign, non-related systems. Once the user has
found what he’ looking for, he can order the download of the data avoiding the task of
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copying and pasting data.
We call a server each site that offers information. The mating component,
information collector, acts as a client asking for XML formatted information on behalf of a
user and belongs to a separate system.
So sites must have a specific object for giving information (the connecting
component) in such a way that client mates (information collectors) can get it through
interacting with it.
We then build the client application that makes requests to server applications.
Figure 4.1 Information gathering with XML
Servers connect to the client one at a time.
The information collector component acts under user guidance and plugs into the
connecting component, its mating component on the side of the application that’ offering
s
information.
Using XML, both kinds of components match perfectly and are capable of
transferring information that follow XML standard.
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XML Patterns
Example: Getting XML data from foreign web applications and storing it in a XML
document
In this example we used the solution to build a web application that gets information
from several different servers and then stores the gathered data for the use of a web client.
The sites that offer information are the servers. Each server has a component to eventually
connect to a client. These components are the connecting components.
We also have the client that collects information from servers. In order to
accomplish it’ task, the client has a component that matches and plugs into the server’
s s
connecting components. This component is the information collector.
Once the user has found the information on one of the servers, the client connects its
information collector to the server’ connecting component and starts the XML data
s
transmission. After receiving this server data, the client connects its information collector to
the connecting component of the other server and repeats the process. By doing this, the
client creates an XML data pool with the information obtained in the web sites. For instance
if the client is planning vacations then he goes to the first server to reserve seats on a plane
and to the second to book hotel rooms. The XML data pool is in fact an XML document.
Figure 4.1 presents part of pool.xml as an example.
AcmeAirlines
Destination: Montevideo
Item Subtotal: $59.00
Total Before Tax: $59.00
Tax: $11.95
Total : $70.95
Items
Traveller: 1
Ship: Catalonia
date:08/01/2000
Shipping time:7:40
Availablility: Yes
Telephone: 01147762973
01/02/2001
https://www.acmebus.com/Orders
...
Figure 4.2 Part of pool.xml, the XML document where
data gathered remains
The client leaves collected data in an XML document. The client connects to the
servers one a time and adds the corresponding information to the XML file. The user can
then edit this file with a text editor as well as with any of the XML editor available.
Furthermore, the data obtained can directly feed specific user’ application.
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Consequences
• The pattern enforces user and application integration and collaboration through the web.
16
XML Patterns
• We get a modular solution that allows clean sharing of data between non-related
components, avoiding one’ interference in the other’ affairs. In addition we don’
s s t
interfere with internal data representation in the source, nor in the client.
• Increased flexibility in connections with information providers. As a client application
can get data from foreign systems without interference, it can increase the number of
information servers it can connect to. It does not need to change its code to connect to a
specific server. It even could connect to non-standard devices without a significant
change in the code.
• Wider target of data consumers for a site’ data. Information provider sites wish to
s
widen the target readership for their data. Again it could be possible to connect to other
devices such as handheld, as long as this device has a matching component to connect
to.
• No interference with the navigation design layer [Rossi+98]. We just add a token of the
connection component.
• A flexible design for data format transmission and storing. We reutilize data keeping
the format.
• Storing gathered data in an XML document allows us to add more data to the XML
pool in another session, which lets the user have his data centralized as well as at hand.
Further comments
This pattern is a mediator in the sense of the GoF pattern Mediator [Gamma+95]. It
resembles the mediator in that it serves as an intermediary that helps in interactions of a
group of applications.
In the implementation we suggest the use of XML to share information because
XML is an effective way to transmit structured information between non-related
applications. If needed, the client and server could negotiate an XML data format. The
server would give the client a specific data format. To improve implementation it would be
possible to let the client connect simultaneously to several servers.
Also, we use XML as a standard to facilitate computer integration and
collaboration. In the implementation, servers and clients are responsible for giving context
information (if needed). That means that both kinds of objects have sharable information as
well as context information. This resembles Gang of Four’ Flyweight pattern
s
[Gamma+95] in that the objects have intrinsic and extrinsic state. Intrinsic state is stored in
the objects and it is information that is independent of the object context, and thereby
sharable. Extrinsic state, on the other side varies with the object’ context.
s
Furthermore, the use of XML helps language and platform independence. It brings
flexibility at the moment of choosing the language, because as long as the servers offer data
following the XML standard, the pattern’ users don’ care about the language they use.
s t
Besides, as shown in the example, we can develop an XML structure to store gathered data
and read it from the XML document using a text editor, an XML editor or a custom
application specially designed to process that data.
5. References
[Cagle00] Cagle, K., XML Developer´s Handbook, SYBEX, 2000.
17
XML Patterns
[Gamma+95] Gamma, E., Helm, R., Johnson, R., Vlissides, J., Design Patterns, Addison-
Wesley, 1995.
[Kay00] Kay, M., XSLT Programmer´s Reference, Wrox, 2000.
[Nielsen99] Nielsen, J., Designing Web Usability: The Practice of Simplicity, New Riders
Publishing, 1999.
[Lifia xml]. http://www-lifia.info.unlp.edu.ar/xml/, LIFIA, Facultad de Informática,
UNLP, Argentina.
[Rossi+98] Rossi, G., Schwabe, D., An Object Oriented Approach to Web-Based
Application Design, Theory and Practice on Object Systems. Wiley and Sons, October
1998.
[Rossi+99] Rossi, G., Lyardet, F., Schwabe, D., "Patterns for designing navigable
Information Spaces" Pattern Languages of Programs IV, Addison-Wesley, 1999.
[Rossi+00] Rossi, G., Lyardet, F., Schwabe, D., Patterns for e-commerce applications,
EuroPLoP'2000, 2000.
[Saxon] Kay, M., About Saxon, http://users.iclway.co.uk/mhkay/saxon/instant.html, 1999.
[Schwabe99] Schwabe, ., D Rossi, G., Lyardet, F., Improving Web information systems
with navigational patterns, Computer Networks 31 pp.1667- 1678, 1999.
[Spencer99] Spencer, P., Professional XML Design and Implementation, Wrox, 1999.
[W3CDOM] Document Object Model (DOM) Level 2 Specification Version 1.0 Candidate
Recommendation of the World Wide Web Consortium, http://www.w3.org/TR/REC-DOM-
Level-1, 01/10/98.
[W3CNamespaces] Extensible Markup Language (XML) Specification Version 1.0
Recommendation of the World Wide Web Consortium, http://www.w3.org/TR/REC-xml-
names, 14/01/1999.
[W3CXML] Extensible Markup Language (XML) Specification Version 1.0
Recommendation of the World Wide Web Consortium, http://www.w3.org/TR/REC-xml,
10/02/1998.
[W3CXPATH99] XML Path Language (XPath) Version 1.0 Recommendation of the
World Wide Web Consortium, http://www.w3.org, 16/11/1999.
[W3CXSL] Extensible Stylesheet Language (XSL) Version 1.0 W3C Candidate
Recommendation, http://www.w3.org/TR/xsl/, 21/11/2000.
[W3CXSLT] XSL Transformations (XSLT) Version 1.0 Recommendation of the World
Wide Web Consortium, http://www.w3.org, 16/11/1999.
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