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Web Services overview – Design Issues

<br /> Web Services overview – Design Issues<br />

Tim Berners-Lee
Originally created: 2002-10, last change:
$Date: 2009/08/27 21:38:10 $
Status: Rough amalgam of input from generally reliable
sources. Editing status: draft. This is really a set of
pointers, as the web services architecture is being
elaborated by a whole communit, with the WS-Arch group
playing a specific role. Started as notes from W3C Web
Services Workshop [@@].

Up to Design Issues


Program Integration across Application and Organization
boundaries

Introduction

Web Services mean many things to many people. In the end,
there will be a set of standards which allow us to do things
we could not do before, but in the mean time different people
and companies approach them from different positions, and
with different expectations. In 2001-2, Web Services have
also been a buzzword used repeatedly and claimed to be one of
the hot new technologies. The common themes are:-

  • A departure from the web as a quasi-static information
    space to one in which interactions are the primary model
  • A use of HTTP, XML and other standards from the web
    architecture as the building blocks
  • A typical focus on enterprise wide and inter-enterprise
    operations

The Web in Web Services is, from the first point, a misuse:
the term Internet Services would be more appropriate. The Web
comes from the second point – the use of the HTTP and XML is
already in use as a well-understood and well-debugged set of
protocols which support the Web, and so it makes sense to
reuse them in providing remote operations and those things
connected with them. The third point is what makes web
service requirements so different from a local RPC system.
The fact that data is exchanged for business purposes and
between different social entities means that accountability
is required, rather than just reliable transmission.

  • The vendors of software see web services as way to
    repackage existing capability in a way which makes it
    interoperable with other systems.
  • The security requirements for web services are dictated
    by the trust environments, whether it is intranet or b2b or
    b2c, etc
  • For b2b one needs not just reliabilioty but
    accountability.

The architecture of Web Services is the scope of the W3C Web
Service Architecture Working Group.

Philosophy

Other articles have dealt with the fundamental architectural
difference between remote operations and the architecture of
the information space, and the mappings between the two.

  • Axioms of web architecture
    (1990s) talks about the information space concept
  • Paper Trail-
    Discusses the relationships between two patterns:
    read/write state derived from read-only documents in real
    life. Which came first, the journal or the database?
  • Conversations and State
    (1998) discusses the trends in many areas away from shared
    information spaces, from Web Services to Voice browsing.

The Web Services architecture group has produced various
drafts:

The architecture uses the following diagram for the highest
level:

However, the essential part of Web services is the
Interact relationship between a Service provider and
Service requestor. This is the Web Service. Discovery
agencies need not be used – they will in some cases but not
in others. The discovery agencies are well represented as a
cloud, rather than being a well-defined module in the web
services architecture. They will become interface to a huge
world of data and query services which provide data about web
services as well as many other things.The Interact between
between requestor and provider is the essential defining
element to web services. As we shall see, the metadata about
web services

Technologies within the Web Servcies umbrella

There is a mass of different pieces being bolted onto the
foundations of Web Services provided by WSDL and SOAP 1.2 and
th diagram implies things considerably. The management layer
is a supervisory layer allowing the conrol of the many agents
involved in a web services-based operation. The “Application
semantics” layer indicates the necessity, for any useful
interoperability, to have

Stcak based on XML, and HTTP has WSDL and SOAP 1.2 as WS foundation.

Run Time messaging

The design work of web services is divided between the run
time protocols and the descriptions of services.

The W3C work at runtime based on HTTP transport of
XML-encoded messages, using the SOAP protocol. (Here by SOAP
we mean SOAP 1.2, previous versions including early
proprietory submissions which are not standards or guaranteed
to interoperate) . There is a bifurcation in the design at
this point, as SOAP operates basically in two modes.

In one, the XML message is used to encode the parameters to a
remote operation in much the same way as remote method
invokation in for example, Corba, DCOM, or RMI. In this mode,
XML is used as the marshalling style, but the system is a
distributed using remote procedure call in a fairly
traditional way.

  • There is a standard marshalling syntax

    Interfaces between software modules have well-defined
    functions, which in turn have well-defined and typed
    input and output parameters

    Stubs (dummy routines which similate the remote procedure
    by a local one which communcates with the remote one) can
    be generated directly from the WSDl definition

  • The remoteness can be transparent, making the design of a
    distributed system similar to the design of a program.

In the other mode, SOAP carries an XML document, and the task
of the receiver is seen more as a document processing
operation. This is less rigid than the RPC style.

  • The interface a service provides is defined just by the
    XML schema. This defines the acceptable document types, which
    can allwo extension in many ways, using XML namespaces.

    The communication is more apparent to the application
    writer, who deals with the document object model (DOM) of
    the recived message, rather than having parameters
    unmkarshalled automatically.

    XML tools such as XSLT and XML-Query, and XML encryption
    and so on can be used.

    It is simpler to use message exchange patterns other than
    the request/response.

The document mode of SOAP seems to be getting the most
traction in the ecommerce stack. This is not an accident. The
XML mode is more flexible than the RPC mode. It is easier in
principle to extend an XML-based message system to include
more information as a system grows. In fact, RDF is
especially powerful in this area, as new information can be
parsed into an entity-relationship form by old agents, and it
becomes logically clear which parts can be ignored by those
who do not understand them.

Functionality which has been mentioned as required above the
basic layer at runtime includes:

  • Routing. Routine data within message for processing bu
    different agents; defining workflow path of message. Black
    box or white box patterns of design.
  • Security. Prolfiling existing security technologies for
    use in ebusiness applications using web sevices.
    Authentication and key management.
  • Packaging of attachments to messages. XML Packaging.
  • Reliable messaging (delivery, non-duplication, ordering)
    for the case in which the transport layer (such as TCP under
    the HTTP) doesnot provide this. (TCP does provide this
    reliability but (a) systems are not designed to keep TCP
    connections open for the weeks or years over which a web
    service may run, and (b) TCP does not provide accountability
    so you can show the tax man the acknowledgement of receipt 7
    years later.)

Description

The descriptions of services are made at various different
models and different levels of abstraction in different specs
proposed as part of the stack, though there is agreement on
WSDL as the modelling of the lowest level, the message or
request/response interaction, and the binding to the specific
HTTP (say) port at which it happens.

Lots of concepts interconnected

Higher layers in the description above WSDL are known
variously as coordination, orchestration, choreography,
composition.

They involve (compared with basic WSDL), for example:

  • Protocols involving more than two messages
  • Protocols having a common shared state over a long period
  • Protocols having more than two parties involved; web
    service workflow

    • structured version (ws-*, damls process model)
    • precondition-postcondition style (DAMLS)
  • The protocols as business protocols, in terms of common
    business functions
  • The relationship between allowed transitions in the
    protocol and the content of messages. For example, the
    requirement for a transaction ID to match across a
    transaction, or for possible responses to be a function of a
    request code.

Composability and Choreography

Composability of web services refers to the building, from a
set of web services, of something at a higher level,
typically itself exposed as a larger web service.

Choreography refers more abstractly the part of the
description of web services which defines a way, or the ways,
in which a acyual invokations to various web services work
together. (Peltz uses Choregraphy when it involves
multiple parties, and Orchestration when it is
internal to one party. Thus the former crosses application
boundaries, the latter also crosses organization boundaries.
here we use Choregorahy in general for both.)

There is so small amount overlap here, which has led to some
confusion. To be general, one might say, for example, that a
flight confirmation must involve an already reserved flight.
This the actual constraint. One can describe a particular
choregraphy (a particular dance, if you like) in which a
flight query service is called, and produced a list of
flights, and then a reservation service is called to reserve
the flight, that is successful, and the resulting reservation
is passed to the confirmation service. It may be that there
are other ways — other choreographies — in which one could
have achived a reserved flight. The engineer has the choice
of modelling the many possible ways all in one choreogrpahy,
or of making several choreographies.

Web services can be combined in such as way that
messages are passed around in a very random fashion. However,
a particular design techniqe is for a master process to
delegate to other services in a recusive tree-like manner, as
has been de rigueur in programming languages since Pascal.
For example, if the consumer asks the travel agent and the
the travel agent books a hotel, the hotel will reply to the
travel agent, not to the consumer. This makes everything
orderly.

WSCI, BPML and BEPL take this approach to choreography. This
is a programming language approach with

  • Sequential, Parallel and Exception execution, loops &
    conditionals
  • Message-passing rendezvous between processes
  • Calls: Web Services
  • Data: bits of XML
  • Assignment to variables
  • Expressions: XPath 1.0 plugable in BPEL
  • Does not handle actual calculations, rules etc.

WSCI has the empahsis on description, and BPEL on being able
to compile to an executable agent. As neither is intended to
do the actual calculations or business rules, it would be
closer to compare themm with scripting shells such as bash
which handle concurrency and synchronization but actually
call programs (or rather web services) to do the real work.

See:

  • WS Choreography Group
  • IBM, Microsoft and BEA, under OASIS, BPL4WS (not
    W3C, not RF).
  • BPMI, Business Process Modelling Language BPML
  • Sun et al: Web Services Choreography Interface
    (WSCI),
    W3C Note
  • IBM specs ws-coordination, ws-transaction,
    ws-orchestration

    Chis Peltz, Hewlett Packard,
    Web Services Orchestration – a review of emerging
    technologies, tools, and standards.

@@ Different attitudes – top down program design, or
bottom-up agent design, bottom up document design.

Message-oriented choregraphy

The Paper Trail concept is that
the state of a mult-agent multi-process system can be looked
at, sometimes rather effectively, as a function of the
documents which have been transmitted.

The process-oriented attitude to a bank-customer relationship
may be “In parallel, the customer writes checks, merchants
pay in checks, credit card transactions happen, all month.
Then, the charges, interest are assessed and a bank statement
sent from the bank to the customer”. The document-, or
message-oriented one is more like “Every month a bank balance
lists valid transaction dated that month. A cleared incoming
check in a valid transaction. A cleared outgoing check is a
valid transaction. A validated credit card debit is a valid
transaction. A check is cleared if it is incoming and there
is a matching transfer from the payee bank”, and so on. This
builds the relationships up in a bottom-up, weblike way. The
process-oriented attitude suggests the bank be written as a
procedure in a top-down way using for example WSCI and BPL.
The document-oriented attitude suggests the use of business
rules systems triggered by the receipt of new information —
new documents, in this case new web services messages.

(Web service messages are of course documents just like
documents sent in email. Messages are particular in that they
have a particular time of transmission, and their document
content sdo not change. They do of course generally have
identifiers, and even though they can only be accessed by
sender and explicit receivers, they can still be regarded as
part of the web by those parties.)

Whether the design process is a top-down process-oriented one
or a bottom-up document-oriented one, the design will have to
be translated into a set of agents and their responses to
incoming messages. This manipulation can of course be done
automatically.

A concern in all this frantic design is it evolution with
time. A BPEL script sets out to be a description of a a
business process at a high level. The critical values which
decide on conditional execution, or which correlate a
particular process with a given transaction, are expressed as
parts of the structure of the XML messages. This may lead to
what has been called “DTD fragility”. What happens which you
change the DTD? The design of the message types with XML
schema is the sort of thing which is difficult to get
everyone in a company to agree on, and tents to change with
time. There are many arbitrary choices made as to how the
knowledge in the message is serialized as XML. Moving to RDF
may, by removing a layer of arbitrary design, reduce that
fragility and allow web sevice choregraphy to evolve with
time within and outside a company.

Process modeling

When considering a business system with multiple agents and
multiple concurrent processes, one would like to have an
automated way of checking some fundamental questions.

  • Will the process necessarily terminate?
  • Will the service respond within a given time?
  • Will the net gain from a sale always be positive?
  • Will we ever promise to ship something we do not have in
    stock?

and so on.

The pi calculus and other calculi derived from it are formal
ways of modeling systems with multiple agents and multiple
processes. They can do some way to answering these questions.
Rule-based systems can also be designed so that proofs can be
found of these sort of conjecture. This is a good reason for
keeping the languages involved as simple as possible It may
be the design reason for the limitations on computational
power in WSCI and BPEL.

See petri nets (IBM; and stadford), Pi calculus @@ refs.

Much of the functionality is seen in terms of tying web
services down to well-known functionality such as exiting
transaction processing systems, PKI trust infrastructure, and
so on.

Discovery

In a large number of applications, web servcies will be
provided by on one hand and used by on the other peers who
have established relationships. Indeed, until a trust
infrastructure is fairly developed it is not reasonable to
expect computers to do automatic comparison shopping for very
many services. Web services will probably (like the web in
1993, and the Semantic Web in 2003) spread first within the
corporate firewall, where security problems are minor and
mistakes less embarassing than inter-enterprise or
publically. However, the goal is that so many web services
should be available that it will be important to be able to
find them in all kinds of ways.

The UDDI project and the related work on description and
query systems is aimed at this. A positive aspect of UDDI is
the definition of an ontology for web services. Problems with
it are that it is centralized by design, both in the
single-tree ontology, and in the design based fundamentally
on a central registry, with inter-registry operation as a
secondary thing.

From the semantic web point of view, web services are simply
one aspect of the many things which will be searched for.
Indeed, the fact that a web service is provided may in fact
be rather incidental to the essential nature of the business
item which is discovered — a trader in stocks, a seller of
lawnmowers, and so on. The semantic web aims to describe any
aspect of anything, including the catalogs, parts, materials,
services organization, relationships and contracts. A query
system which addresses web services only makes sense when
smoothly integrated with the rest of the web of enterprise
knowledge.

Web Services and Semantic
Web

The question of the relationship between these two activities
is constantly in the air.

  • The whole description side is a clear semantic web
    application, and so long as XML languages are defined which
    introduced with english language specs but no RDF mapping,
    there is a potential ambiguity which will have to be resolved
    later in making that mapping, there is an inability to use
    common semnatic web tools, and there is cost down the road
    assuming semantci web tools will eventually be used.
    Essentailly, web serv ices become instant legacy technoplogy
    for the semantic web.
  • The DAML-services collalition of researchers is tackling
    the job of service description at a higher level.
  • Many things which are described as web services can in
    fact be described as the publication of a series of semantic
    web documents, just as the billing of a peer company is in
    reality effected by the issuance of an invoice.
  • When Semantic Web agents query each other, they could use
    SOAP (though a direct encoding into an HTTP URI may also be
    effective).
  • When Semantic Web agents update each other, they should
    use SOAP, running typically over HTTP POST.

The argument against integration of the technologies is
mainly social. It is costly to coordinate very large groups.
It is much more effcient to develop WS and SW independently.
Neither side has a great incentive to take on the learning
required to absorb the needs and potentials fo the other.
Using technology in preparation by another group takes a
great leap of faith, and does really add to the development
time. These are real issues. So while it may take more effort
in the long run, it is a better parallelization of the design
task to allow web services and semantic web to proceed in
together without a mandated link. (This was the apparent
consesnsus of the W3C AC meeting in Nice, 2000/11)

That said, wherever overlap of expertise between the
technologies occurs, those who form a bridge should do their
best to make the conceptual differences as small as possble.
There is a Semantic Web Services group, connected to the
______

Service design tools

Most modern software design differentiates strongly between
the design of an interface, and the design of the software
which implements it.

Web services are required to be composable – you should be
able to make a web service implmentation by building it out
of component web services. At the low level, think of making
a latittude/longitude to state code converter composed from a
latittude/longitude to postal code converter and a postal
code to state code converter. At a high level, think of a
making a vacation being composed of resrvations of flights,
hotels and entertainment.

Runtime System management

Real web services have multiple agents running commerical
environments, in which downtime is expensive, and incorrect
operation could be disasterous. The running, monitoring,
provisioning and upgrading of such systems clearly requires
tools, but their design is out of scope for this overview.


http://www.bpmi.org/

web ser vices orchestrationa review of emerging technologies,
tools, andstandards,
http://devresource.hp.com/drc/technical_white_papers/WSOrch/WSOrchestration.pdf

Related activites elsewhere

  • Rosettanet, UDDI, various Oasis activties.

RPC history

  • Sun/RPC, Apollo domain, DCOM, OMG’s Corba, XML/RPC

See: @@@ Web Services
Specs

From W3c Web Services Workshop

IBM, Microsoft: “Web
Services Framework”

http://www.w3.org/2001/03/WSWS-popa/paper51

Other white papers

@@ HP’s ESpeak papers – Web Services roots

From IBM:

http://www-106.ibm.com/developerworks/library/w-ovr/

From Microsoft:

http://msdn.microsoft.com/webservices/default.aspx?pull=/library/en-us/dnglo
bspec/html/ws-securitypolicy.asp

http://msdn.microsoft.com/library/en-us/dnglobspec/html/ws-trust.asp?frame=true

http://msdn.microsoft.com/webservices/understanding/whatsnext/default.aspx

Fodder

@@@@@

BPSS From FAQ: Q. What is the relationship between WSCI and
ebXML BPSS? A. There is no direct relationship between BPSS
and WSCI. They are used for different purposes and have
different design centers. BPSS is used for defining the
semantics of commercial collaboration between businesses. Its
design center is the commercial transactions between two
business partners, and as such it provides full commercial
semantics, and is designed to work in with conjunction with
the ebXML Collaboration Partner Profile/Agreement (CPP/CPA).
In contrast, WSCI is used for describing a Web service and
the operations performed by that Web service. Its design
center is the WSDL service definition, and it describes the
relationship between multiple WSDL operations that are
performed by a given Web service. Because WSCI is generic to
all Web services, it does not provide any explicit commercial
semantics, nor does it have any notion of a trading
collaboration partner agreement. ]]] — http://wwws.sun.com/software/xml/developers/wsci/faq.html

@@ concept diagram

Other areas one can contemplate include:

  • Constraints on the semantics of the messages as opposed
    to merely their syntax
  • Trust structures
  • What sort of a service this is
    • classification of things bought and sold etc

Up to Design Issues

Tim BL

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