IMG seminar by Michael Parkin at the University of Manchester, January 2006.

1. If Web Services are the Answer,
What’s the Question?
Michael Parkin
Supervisor: John Brooke
IMG Seminar
17 January 2007

2. Acknowledgments & Sources
• Thanks to Dean Kuo, Mark McKeown, Bruno Harbulot, Donal Fellows
• Lots of stuff taken from blogs and web articles
• All quotes are acknowledged and linked to where possible
IMG Seminar
17 January 2007

3. Overview
• Grid computing & it’s requirements
• Introduction of architectural styles:
• Web Services Architecture (WSA) and WS-*
• Representational State Transfer (REST)
• Instant Messaging (IM)
• Evaluation of Web Services, REST and IM
• Using the requirements of Grid computing
• The question is?
IMG Seminar
17 January 2007

4. Grid Computing
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17 January 2007

5. Grid Computing is...
... a new infrastructure that builds on today’s Internet and
Web to enable and exploit large-scale sharing of resources within
distributed groups.
“The Grid”
Ian Foster, ClusterWorld Magazine (January 2004)
... coordinated resource sharing among dynamic collections of
individuals, institutions, and resources.
“The Anatomy of the Grid”
Ian Foster, et al (2001)
... internet-scale distributed computing.
“Distributed Computing Economics”
Jim Gray [Microsoft Research]
IMG Seminar
17 January 2007

6. Non-functional Requirements
• Scalability and interoperation
• Large/internet-scale dynamic sharing of resources
• We need to connect lots of diverse/heterogeneous entities together
• Pervasiveness
• ... through simple clients and consistent implementations
• Network efficiency
• ... this is a distributed system, after all
• Return to these later ...
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17 January 2007

7. Web Services Architecture & WS-*
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17 January 2007

8. Web Services Architecture
The only game in town
Anon (though attributed to many)
A Web service is a software system designed to support interoperable
machine-to-machine interaction over a network. It has an interface
described in a machine-processable format (WSDL). Other systems
interact with the Web service ... using SOAP messages.
Web Services Architecture, W3C
• WSA is just SOAP (structured XML) + WSDL
IMG Seminar
17 January 2007

9. WSA Constraints
[An Architecture’s] properties are created by the application of constraints
Roy Fielding’s PhD. Thesis
• A constraint is a condition that a solution must meet in order to be
acceptable - a confinement of all possible solutions
• Lack of constraints on the WSA other than
• Use of SOAP + WSDL
• Service specific interfaces
• i.e. no restriction on interfaces or message representations
• Sender / receiver pattern
• “Effectively unconstrained” - all possible solutions are acceptable
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17 January 2007

10. No constraints...
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“The Sugar House” Richard Greaves, 2001 17 January 2007

11. Web Services Standards
Web services standards compose together to provide interoperable
protocols ... in loosely coupled systems. The specifications build
on top of the core XML and SOAP standards.
“Web Service Specifications” Microsoft
• Collectively known as Web Service stack or WS-*
• 50+ specifing aspects of reliability, security, transactions, etc.
• WS-* are constraints on the WSA
• Defined though standards bodies (OGF, W3C, OASIS, EMCA?) before
being implemented
• No reference implementations
IMG Seminar
17 January 2007

12. Grid & WS-*
• Since 2002 Grid computing is officially implemented using WS-*
• First through Open Grid Services Infrastructure ...
• ... then through the WS-ResourceFramework (WS-RF) family (2004)
• WS-RF describes the relationship between WSA & WS-*
• As Web Services are stateless WS-Resource describes a stateful
resource
• “Each message carries some information that identifies the [stateful]
resource behind the service which should be the logical receiver of
the message” Savas Parastatidis [Microsoft]
IMG Seminar
17 January 2007

13. The WS-RF Family
• WS-Resource
• WS-ResourceProperty
• WS-ResourceLifetime
• WS-BaseFaults
• WS-RenewableReferences
• WS-ServiceGroup
• WS-Notification
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14. REST Architecture
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15. REST
• REpresentational State Transfer
• The architecture of the Web introduced in Roy Fielding’s PhD. thesis
Fetch representation
Client Resource
Representation
• Not a standard, but it uses common standards
• Best known implementation of REST is HTTP
• Resource - not service - orientation
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17 January 2007

16. Service vs Resource Orientation
/jobs
GET - list all jobs
PUT - unused
• The resource is the central abstraction - POST - add new job
DELETE - unused
not the service
/job/[id]
GET - get job details
PUT - update job
JobManagementService POST - unused
+ ID submitJob (u_id: ID, j:Job) DELETE - cancel job
+ void cancelJob (j_id: ID)
+ Job getJobDetail (j_id: ID)
+ Job[] getJobs(u_id: ID) <<interface>>
/users
Resource
GET - list all users
GET
PUT - unused
PUT
POST - create user
POST
DELETE - unused
DELETE
UserManagementService
+ ID createUser (u: User)
/user/[id]
+ User getUser (u_id: ID)
GET - return user
+ User[] getUsers ()
PUT - update user
POST - unused
DELETE - unused
/user/[id]/jobs
GET - return user's jobs
PUT -
IMG Seminar
POST - create new job
Adapted from: “REST - The Better Web Services Model”, Stefan Tilkov DELETE17 January 2007
-

17. REST Constraints
• Highly constrained architecture (cf. WSA)
• Self-descriptive messages
• ... stateless interaction
• ... resource representation
• “Hypermedia as engine of client state”
• Resource identification
• Name things with unique URIs
• Uniform interface
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18. Architecture with Constraints
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19. REST and the Grid
• REST style is not used by the Grid
• Why can’t a Grid Virtual Organisation (VO) be a REST resource?
• Find VO member:
• Fetch /myVO/people/fred
• Returns representation of Fred in format requested
• If no representation returned, Fred’s not a member
• Used for role-based access control as part of my work
• Very lightweight and interoperable ...
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17 January 2007

20. Instant Messaging Architecture
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21. Instant Messaging
• Hub and spoke/email-type infrastructure
• Not just text: Google’s Jingle library can stream VOIP and Video
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17 January 2007

22. IM Constraints
• Asynchronous, routed messaging
• Open connection to server is used as bi-directional message pipe
• Document-style interaction
• Complexity is in the message - not the interface (cf. REST self-
describing message)
• Unique naming of servers and clients
• Number (ICQ) or email style (name@domain)
• Presence of clients can be determined
• Clients may have multiple sessions with one or more other clients and
multi-user chats
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17 January 2007

23. Wider Constraints
IMG Seminar
Eixample, Barcelona 17 January 2007

24. Just for Multi-User Chat?
• Multi User Chat can be used
as a messaging ‘bus’
• ‘Plug-in’ services across
domains
• Each client listens for a
message it’s interested in
• Ignores other messages
• A lightweight collaborative
environment
• Implemented using XMPP
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25. Grid Computing Requirements
• Scalability
• Interoperation
• Pervasiveness / simple clients
• Network efficiency
• How does each architecture do?
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26. Scalability / Interoperability
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27. WSA Scalability
• Remember - no constraints
• ... therefore an infinite number
ways to do things...
• ... which promotes coupling
• “Web services are effectively
unconstrained, and therefore
much more tightly coupled.”
Mark Baker [W3C Working
Groups]
• Loose coupling is better than tight
coupling for scalability
IMG Seminar
17 January 2007

28. WS-* Scalability / Interoperability
• Requires each service to be predictable and implement the same set of
specifications in the same way
• Specifications MUST be unambiguous
• Interoperability through consistency of implementations
• Most WS-* specifications aren’t unambiguous ...
• ... and sometimes they’re deliberately ambiguous - e.g. WS-BA
• There’s no reference implementation to code to
• No consistency, therefore no reliability
IMG Seminar
17 January 2007

29. WS-* Scalability / Interoperability
This document does a disservice to the community; it is
ambiguous in many places, incomplete in others, and riddled with
errors throughout. Indeed any company that implemented it would
end up with an unreliable system.
Werner Vogels [CTO, Amazon] on WS-Reliability
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30. ?
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“Untitled” Richard Greaves, 2001 17 January 2007

31. WS-Interoperability
Conformance to the Web services Addressing Test Suite does not
by itself enable a party to claim conformance with the Web
Services Addressing specification.
Disclaimer from the WS-Addressing test suite
IMG Seminar
17 January 2007

32. WS-Incoherence
• WS-Reliability or WS-ReliableMessaging ?
• WS-Management or WS-DistributedManagement ?
• WS-Eventing or WS-Notification ?
• WS-Context or WS-Session ?
• WS-RF or WS-Transfer ?
• Multiple ways of doing things
• e.g. SOAP vs Literal encoding, Document vs RPC style
• e.g. “4 ways to send SOAP attachments (DIME, MTOM, SOAP w.
Attachments, WS-I Attachments)” Dan Diephouse [XFire author]
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17 January 2007

33. WS-* Stack Criticism
No matter how hard I try, I still think the WS-* stack is bloated, opaque,
and insanely complex. I think it’s going to be hard to understand, hard to
implement, hard to interoperate, and hard to secure.
Tim Bray [Director of Web Technologies, Sun Microsystems]
Much of [WS-* is] recently invented, untested and unproven
in the real world.
Sean McGrath [CTO, Propylon]
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17 January 2007

34. “If you don’t like it, don’t use it”
Are there too many specs? If you think so, then there are.
So don't use them all.
You don't have to use any of them.
Matt Powell [Web Platform and Tools Marketing Team, Microsoft]
• OK, if within the same administrative domain - you are ‘God’
• Sixth fallacy of distributed computing is that “there is a single
administrator” Peter Deutsch [author of the definitive Smalltalk
implementation] via Mark Baker
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17 January 2007

35. Across Administrative Domains
• Grid computing is about creating virtual organisations across
organizational boundaries
• “[Grid computing ] is distributed computing across multiple
administrative domains” Dave Snelling (via Mark McKeown)
• You can’t control who implements what WS-* specifications ...
• ... or how they interpret and implement it ...
• ... or which version they implement
• Tighter - not looser - coupling is required to solve these problems
IMG Seminar
17 January 2007

36. WS-Tooling Support
Tools are being created by people everywhere to make it so you can just
indicate the capabilities you need and the rest will be done for you.
Matt Powell
Each toolkit vendor is likely to interpret the specification somewhat
differently.
Werner Vogels [CTO, Amazon]
IMG Seminar
17 January 2007

37. REST Scalability & Interoperability
• The best known implementation of REST is HTTP
• The Web:
I look at Google and Amazon and EBay and Salesforce and see them doing
tens of millions of transactions a day involving pumping XML back and
forth over HTTP, and I can’t help noticing that they don’t seem to need
much WS-apparatus.
Tim Bray
• All clients can immediately interact with a resource once we know it’s
address because of uniform interface and operation semantics
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17 January 2007

38. Even Looser Coupling
• Loose coupling in time is also possible
• HTTP has asynchronous communications built in ...
• GET empty job representation
• POST completed job representation to asynchronous job submission
resource (URL contained in empty job)
• Server returns a ‘202 Accepted’ message and either:
1. With “Reply-To” request header: a notification is sent to that
URL when the job is complete
2. Without “Reply-To” request header: a “Location” response
header is returned with a URL with the status/result
• ... in a standard way
IMG Seminar
17 January 2007

39. IM Scalability & Interoperability
• Considering XMPP, an open XML message based IM implementation...
• Scales very well
• Jabber has 40-50 million users (source: Wikipedia)
• Each XMPP server can support ~30-50,000 concurrent users
• Interoperability seems to be no problem
• Lots of heterogeneous clients connecting to lots of heterogeneous
servers
• Gateways to ‘closed’ IM implementations
• AIM, ICQ, etc.
• Well-defined and accepted message semantics
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17 January 2007

40. Network Efficiency
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17 January 2007

41. Network Efficiency - A Note
• In a network-based application it is often the communications which are
the bottleneck
• Generally increased through the use of network intermediaries
• Caches, proxy servers, etc.
• Reduce the ‘distance’ a request has to go
• Relieves stress on the server/application
• An intermediary can only make a decisions when a client’s request
shows, in the clear, the targeted resource, and the method requested
is understood
IMG Seminar
Adapted from: “REST”, Roger L. Costello 17 January 2007

42. WSA Network Efficiency
• Most (all?) Web Services use SOAP/HTTP
• Can they re-use Web caches to increase efficiency?
• A SOAP message is always a HTTP POST operation
• A cache cannot know from the HTTP method whether the client is
doing a read, write, update or delete
• I don't know what operation is being
A SOAP URI is always to the SOAP server, not to the actual requested.
target
• I don't know which resource is being
Cache server cannot know from the URI what resource is being requested.
requested. I must forward the request
Message
Web
Client
Cache
EPR
operation = highFive() IMG Seminar
17 January 2007

43. Reuse or Abuse?
[Web Services are] on the web but they aren’t of the web. They
don’t use any of the web’s features, or interact with anything else
on the web. They just use HTTP as a transport protocol. They
could just as easily run over TCP and get better performance.
Leonard Richardson [co-author “REST Web Services”]
IMG Seminar
17 January 2007

44. REST Network Efficiency
• Uniform, clear semantics of the interface’s operations
• GET always means “retrieve the resource identified”
• PUT always means “replace resource identified with this new one”
• POST always means “submit this data for processing”
• DELETE always means “delete the resource identified”
• URI’s can be compared
• Allows optimization of the network (i.e. caching, comparison of
metainformation)
IMG Seminar
17 January 2007

45. IM Network Efficiency
• Connection overheads removed (client only connects once to server)
• Login to server once
• Open pipe is used as an asynchronous bi-directional messaging bus
• Only send messages once
• Stored on server if destination is unavailable
• Guaranteed in-oder delivery when client reconnects
• No need to poll - response comes asynchronously when ready
• But, all messages are routed via server - not the most efficient route
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46. Pervasiveness
pervade, v. “To spread, extend, diffuse; to
be present and apparent throughout.”
(Source OED)
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47. WS-* Pervasiveness
• Complexity of WS-* limits overall uptake of technology
• Clients are service specific
• Clients must be ‘active’
• ... no information about the possible state transitions for the client in
SOAP messages
• ... must process each message even if content has not changed
• ... therefore client more complex
• Stack overhead unsuitable for restricted/lightweight clients
IMG Seminar
17 January 2007

48. REST / IM Pervasiveness
• Simpler clients than WS clients as all possible state transitions are
contained in the returned message through hypermedia
• “REST is particularly useful for limited-profile devices such as PDAs
and mobile phones” Sun Microsystems
• More complex clients than REST, but simpler than WS-*
• New York Times: “... instant messaging is used in more than 80 percent
of corporations”
• No of IM accounts in 2006: 995 million (Source: Radichi Group)
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49. Evaluation
Network
Client Simplicity Scalability Interoperation
Efficiency
- - -- -
WS-*
++ ++ ++ +
REST
+ +/++ +/++ +
IM
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50. Finally ...What is the question?
• If Web Services are the answer:
What foundations do we choose for
limited-scale, tightly-coupled,
implementation dependent Grid?
• The question should be:
What foundations do we choose for
massively-scalable, loosely-coupled,
implementation independent, consistent Grid?
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17 January 2007

51. Or...
vs
No architectural constraints? Architectural constraints?

52. Summary
• Grid computing is based on WS-*
• WS-* is widely acknowledged to be complex
• Across administrative domains you can’t control who implements
what and how they implement it
• Other architectures and technologies that meet the Grid’s requirements
should be considered
• These have been proven to be interoperable and scalable - the
ultimate goal of the Grid
IMG Seminar
17 January 2007