Collaborative Network Applications

1
Collaborative Network Applications

• Stephen T. Mohr
• smohr_at_answerthink.com

2
The Challenge of Distributed Web Applications

• Loosely coupled networks (connectivity and
  technology)
• Loosely managed - no central control, little
  common agreement
• Desire for reuse
• Need to cope with rapid change
• Need to deal with noise in data

3
Five Principles of Distributed Development

• Applications will be built from coarse-grained
  services that implement a unit of processing
  larger than components
• Services will be discovered by querying
  directories
• Services will be provided as self-describing data

4
Five Principles (continued)

• Services will be enlisted on a transient basis
• Services must support extension and degrade
  gracefully

5
Services

• Why not components?
• Convenient granularity for Web applications
• ASP, CGI, etc. for implementation
• Small enough to promote reuse, large enough to be
  broadly useful

6
Finding Services through Directories

• Directories are a central repository describing
  the resources and organization of the enterprise
• Describe services in terms of the vocabularies
  they speak and their URL
• Choose scope wisely

7
Communicating through Self-Describing Data

• Cant count on distributed object technology in a
  loosely coupled network
• Write out the state of an object on the server
  and reconstitute it on the client
• XML is a (nearly) universally accepted tagged
  language
• Tagging clearly delimits and describes data -
  less confusion in processing!

8
Use Services and Release Them

• Service providers may not always be available
• Each HTTP round-trip should work like a
  transaction
• Works well with HTTPs stateless nature
• Service consumers maintain the state of the
  computation

9
Promote Extension, Tolerate Degraded Data

• People can find common ground Web apps should
  too
• Views of objects evolve and diverge over time
• Backward and forward compatibility
• Small bugs can put noise in data - cope with it!

10
Building Business Services in ASP

• ASPs that return XML documents
• Each service has one or more pairs of request and
  response vocabularies
• Encapsulate the data and processing of a service
• Use components to implement services

11
Typical ASP Structure

• lt_at_ Language"Javascript" gt
• ltHTMLgt
• ltHEADgt
• lt/HEADgt
• ltBODYgt
• lt
• // Some code here
• Response.Write(xxx)
• gt
• lt/BODYgt
• lt/HTMLgt

12
XML only ASP Structure

• lt_at_ LanguageJavascript gt
• lt
• Response.ContentType"text/xml"
• var parser Server.CreateObject("microsoft.xmldom
  ")
• / Add code to parse a request document, perform
  processing, and create a reply document. /
• Response.Write(parser.xml)
• parser null
• gt

13
Architecture of a Service
14
Directory Services

• Hierarchical store describing network objects
• An API to the data
• Optimized for read access
• Microsoft Active Directory, Netscape Directory
  Server
• Schemas - permissible classes and their properties

15
What is LDAP?

• Lightweight Directory Access Protocol
• Open standard for directory access
• Distinguished names for objects, e.g.,
  CNPrintSrv, OUSales, DCWidgets
• Query syntax
• Language bindings for protocol

16
LDAP Queries

• Syntax
• ltbase DNgt(filter)attributeListscopepref
  erences
• Example
• ltLDAP//sales/DCwidgetsgt((objectClassUser)(mai
  ls))cn, mailSubTree

17
COM Components

• Active Directory Services Interface
• Providers for each directory service, including
  LDAP
• Bind to a known object and query its properties
• ADO Data Source Object for ADS
• ad hoc queries using SQL or LDAP syntax
• read-only in ADSI 2.5 beta

18
Some Conventions

• Use a well known class to represent services
• Have a naming convention to identify service
  objects
• List all the vocabularies the service can
  understand
• List the URL of the service
• Find a service by querying by vocabulary

19
ActiveDirectory Implementation

• serviceConnectionPoint class
• serviceClassName xml
• serviceBindingInformation holds a list of
  vocabulary names
• url property locates an ASP implementing the
  service
• ltLDAP//Salesgt((objectClassserviceConnectionPoi
  nt)
• (serviceClassNamexml))url,serviceBindingInforma
  tionSubTree

20
Collaborating on Data

• Collections - Where do the objects begin and how
  are they ordered?
• Specialization - General properties and multiple
  specialized views of an object
• Versions - Vocabularies change, so provide a
  means of reconciling data in both directions

21
Collections

• An XML convention for collecting object
  representations within an XML document
• Indicates object boundaries
• Attributes
• ObjectType - top level tag names for contained
  objects
• Order - objects sorted by tag name
• Name - collection name

22
Sample Collection

• ltCollection ObjectType"Person" Order"LName,
  FName ascending" Name"Customers"gt
• ltPersongt
• ltNamegt
• ltLNamegtAmanlt/LNamegt
• ltFNamegtAlbertlt/FNamegt
• ltMIgtAlt/MIgt
• lt/Namegt
• . . .
• lt/Persongt
• ltPersongtlt/Persongt
• ltPersongtlt/Persongt
• lt/Collectiongt

23
Specialization

• Different users have different views of an object
• Reuse a general representation by adding
  specialized properties
• Our XML convention models this by making the root
  of the general representation a peer property of
  the specialized representation

24
Specialization in Theory
General
Specialized
25
Specialization in Practice

• ltOrderCustomergt
• ltCustomergt
• ltCustomerIDgt13lt/CustomerIDgt
• ltFirstNamegtBucklt/FirstNamegt
• ltMIgtQlt/MIgt
• ltLastNamegtRogerslt/LastNamegt
• ltStreet1gt10E3 Galactic Lanelt/Street1gt
• . . .
• ltTelephonegt999-999-9999lt/Telephonegt
• lt/Customergt
• ltPaymentOptionsgt
• ltCreditCardgt
• ltTypegtMasterCardlt/Typegt
• ltNumbergt4444444444444444lt/Numbergt
• ltExpirationDategt12/1/2000lt/ExpirationDate
  gt
• lt/CreditCardgt
• lt/PaymentOptionsgt
• lt/OrderCustomergt

26
Versions

• Data schemas evolve
• Loosely coupled apps may use different versions
  of the schema
• Objective communicate several versions of an
  objects representation in one document
• Document recipient can faithfully recreate the
  object in a version it understands

27
Versioning Conventions

• Version, EarliestVersion attributes
• Each root child is a version
• List properties by exception
• Receiving software reconciliation rules
• Add new properties
• Replace changed properties
• Ignore dropped properties

28
Sample Version-Controlled Representation

• ltPerson Version"v19981019
• EarliestVersion"v19980418"gt
• ltv19980418gt
• ltNamegtJohn Doelt/Namegt
• ltAddressgt1234 Nowhere Lane, Philadelphia,
  PA
• 19102lt/Addressgt
• lt/v19980418gt
• ltv19980521gt
• ltNamegt
• ltFirstgtJohnlt/FirstgtltLastgtDoelt/Lastgt
• lt/Namegt
• lt/v19980521gt
• ltv19981019gt
• ltEmployeeTypegtFull-timelt/EmployeeTypegt
• lt/v19981019gt
• lt/Persongt

29
Sample After Reconciliation

• ltPersongt
• ltNamegt
• ltFirstgtJohnlt/Firstgt
• ltLastgtDoelt/Lastgt
• lt/Namegt
• ltAddressgt1234 Nowhere Lane, Philadelphia, PA
• 19102lt/Addressgt
• ltEmployeeTypegtFull-timelt/EmployeeTypegt
• lt/Persongt

30
Avoiding Shared State

• State consumes resources
• Shared state makes scaling hard
• Extra complexity to keep data consistent
• Resources come and go - might not be able to
  process some portion of the data later
• Let the client worry about state, if necessary

31
Handling Change

• Use directory scope wisely to promote good
  implementations
• Circulate data schemas
• Write XML code to be tag driven
• Use versioning as needed
• Write services to do as much as possible with the
  data at hand

32
Issues of Scale

• ASP-based services reduce the problem to that of
  scaling a Web site
• Server farms
• Scalable application tier
• Data scaling
• Asynchronous messaging

33
A Scalable Architecture
34
Summary

• Future belongs to loose, dynamic networks
• Adopt the Five Principles throughout the
  enterprise
• Use best tools for the job within service
  boundaries
• Use open standards between services

35
Further Resources

• Designing Distributed Applications (Wrox)
• Understanding and Deploying LDAP Directory
  Services (MacMillan)
• Microsoft XML MSXML reference-
  http//msdn.microsoft.com/xml
• Microsoft Millennium - http//www.research.microso
  ft.com/sn/Millennium
• Java Jini - http//www.sun.com/jini/