Teaching Web Services and XML with the Water Language

1
Teaching Web Services and XML with the Water
Language

• Christopher Fry
• Clear Methods, Inc.
• Ed Gehringer and Matt Kendall
• North Carolina State University

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Methods, Inc.
2
Agenda

• Teaching Web services
• Introduction to Water
• ConciseXML
• Things
• Servers
• Steam IDE
• The Water object model

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Methods, Inc.
3
Introduction Web-services example

• A new airline, Web Air, would like to add car
  rental services to their on-line ticket sales
  system.
• The Web Air reservation system operates off of a
  database of flight information no knowledge of
  rental cars.
• Web Airs ticket system must obtain rental data
  from rental car agencies.
• What problems could Web Air encounter in gaining
  this information?

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4
Introduction Web Air Problems

• Rental companies may operate on different
  platforms. Example Mainframes vs. Linux
  clusters.
• Individual rental companies may format rental car
  information differently. Example Using RGB
  values to indicate the color of a car, vs. using
  a string value.
• Rental companies may use proprietary formats to
  store data. Example A binary format created for
  an individual company.
• Access to the data may occur over antiquated or
  proprietary protocols. Example Using Solaris
  remote procedure calls to access the data.

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5
Introduction Web Services

• One way to simplify data sharing is to apply
  standards to interactions between the systems
  involved.
• Web services use a set of standards to simplify
  machine-to-machine communications.
• XML for both input and output.
• Web Airs ticket system only needs to understand
  XML.
• Gets XML data from vendors.

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Web Services Interfaces

• Standards require that the interface for each Web
  service be published.
• The interface is a contract that specifies the
  available inputs, the formatting of outputs, and
  the protocols that the Web service employs to
  transmit data.
• The protocols employed by Web services are
  Internet standards such as HTTP, FTP, and SMTP.

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Web Services Interfaces (2)

• Since inputs and outputs are well defined, Web
  Airs ticket system can
• Query rental car Web services using an XML query
  generated based on the published interface.
• Receive the XML response from the rental car Web
  service.
• Use the known interface to translate the data
  from each rental car agency into a format that it
  can use internally.

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Teaching Web services

• Web services are hard to teach. Why?
• Implementation is complex
• SOAP
• WSDL
• Which platform? J2EE? .NET?
• Syntax-to-concept mapping
• The good (human language capacity)
• The bad (lousy, non-uniform syntaxes)
• The ugly (multiple incompatible languages)
• How do we educate students about Web services,
• Rather than train them in a specific technology?

9
Why Teach with Water?

• Supports Web services natively.
• So we are teaching the concepts.
• Can return responses in XML, HTML, or SOAP
• So we dont have to spend time on libraries.

10
Agenda

• Teaching Web services
• Introduction to Water
• ConciseXML
• Things
• Servers
• Steam IDE
• The Water object model

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Methods, Inc.
11
Introduction to Water

• Water (http//www.waterlanguage.org) is an open,
  all-purpose language designed for XML Web
  services.
• It provides a simple yet powerful way of writing
  Web services by supporting XML within its core.

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Introduction (2)

• Without Water, XML is used on the
  "outside"programs are needed to convert into and
  out of XML.
• Water is a language specifically designed for
  XML. It allows the option of using XML for
  everything presentation, logic, content,
  modeling, etc.

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13
ConciseXML

• XML is, by default, a very verbose method of
  representing data.
• Water uses an extension of XML, ConciseXML, to
  reduce the amount of text required to represent
  data in XML.
• All XML is valid ConciseXML, because ConciseXML
  is an extension.
• Not all ConciseXML is valid XML.

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ConciseXML Tag Shorthand

• Closing tags can be notated as .
• XML 1.0
• Hello!
• ConciseXML
• Hello!

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ConciseXML Positional Arguments

• Water uses XML tags to make method calls with XML
  attributes as parameters.
• Methods are predefined, there is no need to
  specify the attributes as keyvalue pairs.

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ConciseXML Positional Arguments (2)

• The key is simply omitted.
• XML 1.0
• ConciseXML

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ConciseXML Path Notation

• Representing nested logic is complex in XML.
• ConciseXML simplifies nesting by supporting a dot
  notation.
• "abe"..bar
• ConciseXML is designed to allow XML to better
  represent data and logic by removing both
  verbosity and ambiguity from the language.

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ConciseXML Path Notation (2)

• XML 1.0
• "abe"
• bar
• ConciseXML
• "abe"..bar

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Things

• Like many object oriented languages, everything
  in Water is an object.
• The most generic class in Water is a thing.
• Objects in water have a set of properties, called
  fields.
• We can create a thing without fields

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Things (2)

• Creates a thing with the fields shape and size.
• ConciseXML does not require parameter names, but
  we have provided them here.
• What happens when we execute the following Water
  code?

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Things (3)

• We can use the path notation provided by
  ConciseXML to select a specific field
• A Point P
  /.z
• What value does this code fragment return?
• Notice that the message field in the above thing
  contains an object.
• Also note that the objects tag has a body,
  i.e. it is not an empty tag, unlike the thing
  objects we have seen so far.
• This means that Water can support seemingly
  unstructured content in the body of object tags.

-4
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Things (4)

• Water applies a structure to the content of each
  tag.
• A large object!.0
• A large object!.1
• A large object!.2
• What will each statement return?

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23
Servers

• Methods are easy to define in Water.
• This simple method returns a thing with the
  specified fields.

• "bar"
• Foo Bar!
• /

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Servers Executing a Method

• We can create a server that will execute our
  method
• We can execute this method by visiting
  http//localhost8000/?.
• The question mark (?) is included because we want
  to execute .
• What happens if we visit http//localhost8000
  without the question mark?

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Servers Parameters

• We can also provide required parameters in the
  request.
• This Water program defines a method that
  has a required parameter x. Foo creates a thing
  with a field value that equals x.
• What will happen if we execute http//localhost80
  00/?x42?

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Servers Omitting a Parameter

• What if we execute http//localhost8000/? now?
• What about the message returned by the server
  reminds us of our previous studies of Web
  services?

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Servers Calling from Programs

• So far, all the services we have called have been
  invoked from a browser. It is also possible to
  call a Web service from a program.
• This code snippet will execute the method
  with x"Hello World!

• "http//localhost8000/"/
• /

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Servers Local Invocation

• Of course, for local methods such as , we
  can also execute

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Steam IDE

• Water source files can be edited in any text
  editor, but the Steam IDE is designed to write
  Water programs.
• Code is executed by highlighting a statement and
  pressing the execute button.
• Once code is executed, its effects remain in the
  environment until they are changed by a later
  execution.

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Steam IDE (2)

• Execution One /
• The variable myvar is assigned the object.
• If at a later time you execute
• The value of myvar is retained from the previous
  execution.
• This retention of the environment allows for
  rapid, iterative development.

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Steam IDE (3)

• Objects can be inspected by double-clicking on
  their name in the editing pane.

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Agenda

• Teaching Web services
• Introduction to Water
• ConciseXML
• Things
• Servers
• Steam IDE
• The Water object model

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Methods, Inc.
33
The Water object model

• The Water object model is simple, yet extremely
  powerful. It can be described in four
  statements.
• Everything is an object.
• An object has fields.
• Each field has a key and a value.
• The key and value can be any object.

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An Object

• Consider this object
• 10 20/ /
• How many fields does it have?
• What are the keys?
• What are the values?

4
a, s, d
2.7, 343, "This is a string",

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35
Classes Methods

• Recall that Web services must publish a well
  defined interface, or contract, that defines
  their inputs and outputs.
• Water contracts are based on class and method
  definitions.
• Classes are created using and methods
  are created using .

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Classes Methods Classes

• We can easily create a class in Water
• This expression defines a class called
  that has two fields one and two.

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Classes Methods Classes (2)

• We can create new instances of .
• What is the only requirement of this myclass
  contract?

• 1/
• null /

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Classes Methods Classes (3)

• We can further define the contract by supplying
  each field with a type.
• now expects to receive a number for
  field one and a string for field two.

• tworequiredstring/

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Classes Methods Classes (5)

• What are the values of one and two for each of
  the following statements?

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Classes Methods Classes (6)

• So far, we have created instances of classes but
  we have not saved them.
• You can store an instance into a variable
• /

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Classes Methods Methods

• Method definitions are similar to class
  definitions.
• "bar"
• Method parameters use all of the same contracts
  as class parameters.
• "bar"

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42
Classes Methods Methods (2)

• The value of the last statement executed in a
  method is the value returned by the method.
• Methods can specify their return type
• _return_typestring
• x.
• This method concatenates the string supplied for
  x with the string literal "-bar".

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Classes Methods Methods (3)

• Methods can be declared as members of a class.
• What is the purpose of _subject in this code?

• _subject.

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Classes Methods Methods (4)

• It is also possible to assign methods outside of
  the class definition.
• If a method is defined outside of a class, we
  have the option of assigning it to a class.
• What class is the method accessible from if we do
  not explicitly assign it to a class?

• person.
• _subject.

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Classes Methods Methods (5)

• For example, from what classes could the
  following method be invoked from?
• _subject.._subject.. /
• What does this method do?

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Classes Methods Invoking 1

• In Water, a call is either a method call or a
  constructor call. The syntax is the same.
• In the following example, the first line creates
  a method named foo_method.
• The second line calls the method with the
  argument x"test". The method call executes the
  method implementation and returns the result.
• x
• What is the result of the above execution?

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Classes Methods Invoking (2)

• You can also invoke a method on a subject
• _subject.
• thing.
• thing.tag

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Classes Methods Invoking (3)

• Or, you can declare a method as a member of a
  specific class (instead of thing) and invoke it
  on the owning class and its subclasses
• _subject.
• /
• a_game.
• a_game.tag

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Classes Methods Creating

• Now consider the following example of creating an
  object instance
• Notice the identical structure between this
  instance creation and invoking a method without a
  target. In this example
• The first line creates a class object named
  foo_object.
• The second line calls the default initialization
  constructor (the init method) for foo_object with
  an argument x"test".
• The constructor call executes the init method,
  which returns the new instance.

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Classes Methods Creating (2)

• The init method is Waters version of a
  constructor. We can override the init method to
  perform tasks during object creation.
• What does _new_object contain?
• The student class maintains a field called peers.
  student.peers is a that is initially
  empty, it has no fields.
• When a new student is created, it is added to the
  peers object as a new field. The key of the
  field is the value of student.id. The value is
  the student object itself.
• Browse to http//localhost to see the results

• student. /
• student.peers._new_object/
• _new_object

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Classes Methods Creating (3)

• We have created new instances using local Water
  expressions, but remember that we can also create
  new objects based on URLs. For example,
• http//localhost/student?id400nameTom
• is identical to

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52
Is Water Easy to Learn?
Overall score Java 58, Water 33, Smalltalk 28
53
Is Water Perceived to Be Useful?

• Overall score Java 58, Water 40, Smalltalk 25

54
Summary

• Web services are rapidly growing in importance.
• Water is a good way to learn Web services.
• It is concise.
• Small Web services can be written in a few lines
  of code.
• One can concentrate on the concepts, not the
  implementation.