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Re: Proposal for LC73/LC75n (multiple interfaces for a service)

From: Sanjiva Weerawarana <sanjiva@watson.ibm.com>
Date: Wed, 24 Nov 2004 21:55:09 +0600
Message-ID: <0ed801c4d23d$faf191b0$bcffff0a@LANKABOOK>
To: "Roberto Chinnici" <Roberto.Chinnici@Sun.COM>, "WS Description List" <www-ws-desc@w3.org>

Hi Roberto,

You do realize that the cleaner syntax you proposed is isomorphic
to the serviceGroup idea syntax right?

I gotta think about your proposal (and consult the other 186K
technical employees of IBM) before I can respond.

I am however a bit disappointed that one person can make a well-
publicized critique of WSDL and that that gives sufficient ground
to re-open an issue we belaboured over for so long. Sigh.

Also, we'd be going back to WD (I would absolutely insist on that;
this is a fundamental change) ==> at least 6 more months? Sigh sigh.

Need to grow my hair again so I can pull it out. Sigh sigh sigh.

Sanjiva.

----- Original Message ----- 
From: "Roberto Chinnici" <Roberto.Chinnici@Sun.COM>
To: "WS Description List" <www-ws-desc@w3.org>
Sent: Wednesday, November 24, 2004 5:33 AM
Subject: Proposal for LC73/LC75n (multiple interfaces for a service)


>
>
> After giving more consideration to LC73 [1] and LC75n [2], following
> conversations with some of our folks and stimulated by Rich Salz's article
> and comments [3], we'd like to propose to (re-)introduce the ability for
> a service to implement multiple interfaces.
>
>
>
> A first motivation is to allow the definition of management endpoints on a
service.
>
> With the present state of the spec, WSDL authors would either be forced to
define
> a completely separate service and express (currently in a non-standard
way) its
> relationship to the original one, or define an interface that extends the
normal
> interface of the service plus the management one.
>
> Here's an example of the former approach:
>
>   <wsdl:service name="FooService"
>                 interface="myns:foo">
>
>       <!-- a SOAP endpoint -->
>       <wsdl:endpoint name="SoapEndpoint"
>                      binding="myns:MyGenericSoapBinding"
>                      address="..."/>
>
>       <!-- an alternate endpoint, this time using XML/HTTP -->
>       <wsdl:endpoint name="XmlHttpEndpoint"
>                      binding="myns:MySpecializedXmlHttpBinding"
>                      address="..."/>
>   </wsdl:service>
>
>   <wsdl:service name="FooManagementService"
>                 interface="wsmg:ServiceManager"
>                 somens:relatesTo="myns:FooService">
>
>       <!-- a management endpoint -->
>       <wsdl:endpoint name="ManagementEndpoint"
>                      binding="wsmg:SoapManagementBinding"
>                      address="..."/>
>   </wsdl:service>
>
> Although it would be possible to define a new component in WSDL with the
desired
> semantics (a "service group" maybe?), we believe that the common case is
one
> where a service is exposing different interfaces to different classes of
users.
> Consequently, it is desirable to use the term "service" to denote such an
entity.
>
> The other approach, i.e. interface inheritance, is not the right solution
in this case.
>
> WSDL authors shouldn't be forced to bind all the operations in a giant
"Foo+ServiceManager"
> interface for every endpoint that they expose, especially considering that
> management operations are likely to require different
protocols/features/modules
> than ordinary ones. Also, this complexity might well leak onto the client,
> affecting client developers even if they are not in the least interested
in
> using the management functionality.
>
> As mentioned in [1], other WS-specifications define a number of additional
> interfaces that a service might want to implement (e.g.
WS-MetadataExchange,
> WSRP). They raise issues similar to those described above wrt to Web
services
> management, and none of them is more amenable to the use of interface
inheritance.
>
>
>
> A different motivation is given by versioning.
>
> Although it is theoretically possible to create a new version an interface
by
> extending an old one, such an approach turns out to be severely
impractical.
>
> First of all, it's impossible to redefine an operation in a derived
interface,
> e.g. to expand the set of allowed incoming messages. Thus, fine-grained,
> schema-based approaches to provide versioning are killed right from the
start.
>
> Furthermore, interfaces are not the only consideration. Existing clients
of
> a web service will be using one or more endpoints and their bindings.
>
> Extending an old endpoint E implementing base interface I1 to support a
new
> operation defined in a derived interface I2 requires extending the binding
> for I1 used by E so as to cover all of I2. In turn, this may require some
> serious tinkering to the binding itself. E.g. the WSDL author would like
> to use different SOAP modules, or features, for the new operations but
> not the old ones
>
> But clearly forcing a WSDL document to be heavily modified even in
relatively
> simple versioning scenarios is unnecessarily complicated and even
dangerous,
> as it may expose latent bugs in the WSDL processing library used by the
clients.
>
>
>
> Our argument then is that there are at least two important scenarios which
> are pretty poorly served by the current WSDL 2.0 specification. We claim
> then that (re-)introducing the ability for a service to implement multiple
> interfaces would go a long way in addressing them.
>
> Additionally, motivations based on toolability and backward compatibility
> with WSDL 1.1 are hinted at in [2]. We won't elaborate on them further
> here, but we believe that indeed they do provide additional arguments
> for supporting multiple interfaces on a service.
>
> We should also point out that, unlike [3], we are not proposing to remove
> interface inheritance. Inheritance has its uses and it's the correct
approach
> in many cases. Furthermore, removing inheritance and replacing it with
> copy-and-paste of faults and/or operations would obliterate the "extends"
> relationship between interfaces, resulting in a net loss to WSDL users.
>
>
>
> After this lengthy introduction, here is the detailed proposal.
>
> Service changes:
>
>   (1) remove the {interface} property from the Service component;
>
>   (2) remove the @interface attribute from the wsdl:service element;
>
>   (3) remove the constraint that all endpoint components in the
{endpoints}
>       property of a service component contain only bindings with an
unspecified
>       interface or bindings with the same interface as the service;
((note:
>       for some reason, this constraint appears in section 2.14.1 instead
of
>        2.13.1 where it belongs))
>
>   (4) change the first paragraph of 2.13.1 to say that "endpoints that
>       implement the same interface are in effect alternate places at which
>       the service provides the functionality described by that interface";
>       ((note: I haven't found any stronger statements in the spec of the
fact
>       that different endpoints in a service are "equivalent". Is this a
bug?))
>
> Endpoint changes:
>
>   (5) add a REQUIRED {interface} property to the Endpoint component;
>
>   (6) introduce an optional @interface attribute on the wsdl:endpoint
element;
>
>   (7) change the definition of the {address} property of the Endpoint
component
>       to refer to the {interface} property on the same component
(introduced in (5));
>
>   (8) add the following XML mapping rules:
>
>        - if the @interface AII is present, then either the binding doesn't
>          specify an interface or it specifies the same one; in either
case, the
>          value of the {interface} property of the endpoint component is
the
>          Interface component referenced to by the (value of the)
@interface AII;
>
>        - if no @interface AII is present, then the binding must specify an
>          interface; that interface becomes the value of the {interface}
property
>          of the endpoint component;
>
> Here's the updated pseudo-schema for the wsdl:service element:
>
>   <service name="xs:NCName">
>     <documentation />?
>
>     <endpoint name="xs:NCName" binding="xs:QName" interface="xs:QName"?
address="xs:anyURI"? >
>       <documentation />?
>
>       <feature ... />*
>
>       <property ... />*
>     </endpoint>*
>
>     <feature ... />*
>
>     <property ... />*
>   </service>*
>
>
>
> As an example, here's a WSDL snippet using the new syntax:
>
>   <wsdl:service name="FooService">
>
>       <!-- a SOAP endpoint for the Foo interface -->
>       <wsdl:endpoint name="SoapEndpoint"
>                      interface="myns:Foo"
>                      binding="myns:MyGenericSoapBinding"
>                      address="..."/>
>
>       <!-- an alternate endpoint for the Foo interface, this time using
XML/HTTP -->
>       <wsdl:endpoint name="XmlHttpEndpoint"
>                      binding="myns:MySpecializedXmlHttpBinding"
>                      address="..."/>
>
>       <!-- a management endpoint for the service -->
>       <wsdl:endpoint name="ManagementEndpoint"
>                      interface="wsmg:ServiceManager"
>                      binding="wsmg:SoapManagementBinding"
>                      address="..."/>
>
>       <!-- a new version of the Foo interface, labeled "Foo2" -->
>       <wsdl:endpoint name="SoapEndpoint"
>                      interface="myns:Foo2"
>                      binding="myns:MyGenericSoapBinding"
>                      address="..."/>
>
>   </wsdl:service>
>
> where the "myns:MySpecializedXmlHttpBinding" binding is defined as
follows:
>
>   <wsdl:binding name="MySpecializedXmlHttpBinding"
>                 interface="myns:Foo"
>                 type="...">
>       ...
>   </wsdl:binding>
>
> The "FooService" above exposes two endpoints supporting the "Foo"
interface.
> It also offers a "ServiceManager" interface for management purposes.
Finally,
> it offers an endpoint for a new interface, "Foo2", which extends "Foo".
>
> Although "Foo2" is a new version of "Foo", it's valuable to be able to
keep
> it separate so as to minimize the impact on clients. It's worth noting
that,
> although in this example we use the "myns:myGenericSoapBinding" for the
endpoints
> for both versions, the only requirement is that the binding for the
endpoint
> corresponding to the new version be compatible with the one used by the
old one.
> In more advanced versioning scenarios, "Foo2" could differ more
substantially
> from "Foo", even in ways that would prevent using interface inheritance
altogether.
> Still, we claim that "Foo2" has every right to be seen as an interface
offered
> by "FooService" as "Foo" itself, and it shouldn't be relegated to a
separate
> service.
>
>
>
> The changes listed above are pretty much the  minimal set of changes that
> would provide the desired result, i.e. allowing a service to implement
multiple
> interfaces all while making it clear that endpoints that implement the
same
> interface are simply different access paths to the same service.
>
> If this proposal is accepted, then we'd like the working group to
entertain the
> possibility of changing the syntax to be more readable.
>
> In particular, consider that a service would be offering a number of
interfaces
> in parallel ("I expose Interface1 AND Interface2 AND Interface3"), and for
> each interface it'd offer a number of equivalent endpoints ("You can reach
me
> at EndpointA OR EndpointB OR EndpointC").
>
> The following syntax might be more appropriate then:
>
>   <wsdl:service name="FooService">
>
>       <wsdl:interface ref="myns:Foo">
>         <wsdl:endpoint name="SoapEndpoint"
>                        binding="myns:MyGenericSoapBinding"/>
>         <wsdl:endpoint name="XmlHttpEndpoint"
>                        binding="myns:MySpecializedXmlHttpBinding"/>
>       </wsdl:interface>
>
>       <wsdl:interface ref="wsmg:ServiceManager">
>         <wsdl:endpoint name="ManagementEndpoint"
>                        binding="wsmg:SoapManagementBinding"/>
>       </wsdl:interface>
>
>       <wsdl:interface ref="myns:Foo2">
>         <wsdl:endpoint name="SoapEndpoint"
>                        binding="myns:MyGenericSoapBinding"/>
>       </wsdl:interface>
>
>   </wsdl:service>
>
> This syntax would make it easier to figure out at a glance if a service
> implements a given interface and which endpoints it offers, but it has
> the drawback of requiring more extensive changes to the specification.
>
> In the interest of not bogging down the discussion with syntactic issues,
> we'd like to defer syntax enhancements to a later time, and focus for
> the time being on the proposal as detailed above in points (1)-(8).
>
> [1] http://www.w3.org/2002/ws/desc/4/lc-issues/#LC73
> [2] http://www.w3.org/2002/ws/desc/4/lc-issues/#LC75n
> [3]
http://lists.w3.org/Archives/Public/public-ws-desc-comments/2004Nov/0018.html
>
> Thanks,
> Roberto
>
> -- 
> Roberto Chinnici
> Java Web Services
> Sun Microsystems, Inc.
> roberto.chinnici@sun.com
Received on Wednesday, 24 November 2004 15:55:46 GMT

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