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RE: whenToUseGet-7 Making SOAP Restful

From: Williams, Stuart <skw@hplb.hpl.hp.com>
Date: Fri, 26 Apr 2002 09:09:46 +0100
Message-ID: <5E13A1874524D411A876006008CD059F044B89F1@0-mail-1.hpl.hp.com>
To: noah_mendelsohn@us.ibm.com
Cc: www-tag@w3.org
Hi Noah,

Haven't thought about this in detail... the bit I find a bit ugly is the
'magical restful" REST:GET. However, it occurred to me that simply
'overloading' a null request-message (ie. a requesting node that requests an
exchange and presents no outbound infoset from which to from the entity body
of an HTTP request) to use HTTP GET would yield something of very similar
spirit to your proposal.

I think it would give the HTTP binding (and thence SOAP applications) a
clean, natural, way to exploit HTTP GET. The request-message (on the-wire)
as with your proposal would be completely empty - no HTTP entity body (so no
SOAP headers either).

Just a thought anyway... a 'null' SOAP request message as the 'trigger' to
use HTTP GET rather than some other 'magical' incantation. What do you
think? Others? Mark B?

Best regards

Stuart
---

> -----Original Message-----
> From: noah_mendelsohn@us.ibm.com [mailto:noah_mendelsohn@us.ibm.com]
> Sent: 24 April 2002 17:31
> To: www-tag@w3.org
> Subject: whenToUseGet-7 Making SOAP Restful
> 
> 
> I'm delighted to see the tone of this discussion improving.  
> The purpose of this note is to share some (half-baked) ideas on how SOAP
and REST 
> might work well together.  These thoughts are mine, and don't represent 
> positions of either the protocols workgroup or my employer. 
> 
> BAD NEWS:  Long Note Warning
> POSSIBLY GOOD NEWS:  It's long because it contains a specific (if very 
> tentative) proposal
> 
> By the way, I really like Larry M.'s rewording of Dan's proposal.  I think

> it's quite complementary to what I've written below.
> 
> Background
> ----------
> 
> The proposal will make more sense if you have reviewed the 
> latest drafts 
> of SOAP 1.2 [1,2] (I am pointing to the editors drafts.  They have no 
> official standing but are public and representative of the 
> workgroup's 
> latest thinking.)  In case you don't read them, here are the 
> pertinent 
> highlights.  Skip ahead to the proposal if you know all this:
> 
> * SOAP used to be mainly RPC.  Now it is primarily a 
> messaging framework. 
> An RPC convention is described, but optional.  This proposal 
> therefore 
> focuses first on making SOAP messaging RESTful.  RPC is 
> discussed built on 
> that substrate.
> 
> * SOAP distinguishes the so-called request/response message 
> pattern from 
> RPC.  RPC is a specific use of request/response, optimized to the 
> representation of method invocations (parameters, return 
> values, etc). 
> Non-RPC request/response is a very important use case ("send purchase 
> order document, get response document" -or of more concern to 
> the rest 
> community - "get price list document").
> 
> * SOAP is designed to work over protocols other than HTTP, 
> and to support 
> scenarios in which a request arrives (probably from the 
> public network) 
> through HTTP, and is then further routed through other 
> protocols (perhaps 
> MQSeries).  You write new "bindings" [3] to describe the 
> mapping of SOAP 
> to a protocol. 
> 
> *** SOAP envelopes are Infosets, not angle brackets.  The 
> binding can move 
> the envelope information in any way it wants.  This is key to 
> the proposal 
> below.
> 
> * One of SOAP's great strengths in comparison to HTTP is the 
> richness of 
> its header and extensibility architecture.  In addition to use of XML 
> syntax, it provides mechanisms (mustUnderstand) to facilitate 
> the safe use 
> of extensions in combination.  Capabilities such as digital 
> signatures, 
> authentication, transaction control, etc. can be encoded using these 
> facilities, and MAY be implemented end-to-end when going 
> multiple hops 
> (e.g. MQSeries + HTTP).
> 
> * FWIW:  I believe it's fair to say that something like 90 
> percent of the 
> SOAP working drafts are not at issue in this discussion.  The REST 
> question relates primarily to the specific HTTP binding that has been 
> proposed, and to the implications for the (optional) RPC 
> facilities.  That 
> said, the proposal below attempts to point a direction for 
> making SOAP 
> more RESTful, not just on HTTP, but with any other RESTful underlying 
> protocol.
> 
> 
> A Proposal for RESTful SOAP Information Retrieval
> -------------------------------------------------
> 
> This proposal is different from some earlier ones, in that it 
> makes no 
> attempt to encode SOAP bodies in a URI.  On the contrary, it 
> makes the 
> conservative assumption that if the envelope contains any useful 
> information (digital signatures, transaction control, 
> information in the 
> body) other than a signal that a GET is being done, then the 
> operation is 
> presumed unsafe, in which case POST is used.  Also: here we deal with 
> non-RPC SOAP requests; RPC is discussed below.
> 
> This proposal therefore provides that there is exactly one 
> form of a SOAP 
> request that will result in an HTTP GET.  It looks like this:
> 
> <soap:Envelope xmlns:soap="http://www.w3.org/2001/12/soap-envelope>
>         <soap:Body>
>                 <REST:GET 
> xmlns:REST="http://www.fieldingsthesis.org/rest4soap"/>
>         </soap:Body>
> </soap:Envelope
> 
> Let's call this the "magical restful GET request".  It must 
> appear exactly 
> as shown, with no additional body information, and no SOAP 
> headers.  It is 
> an error to use the REST:GET element in any other context (if 
> you study 
> the SOAP feature architecture, you'll find that it gives you away to 
> loosen this restriction in a controlled way later [4]). 
> 
> To retrieve information in a RESTful manner, a SOAP node 
> sends the magical 
> get request envelope to the resource in question.  Here's where the 
> infoset trick comes in.  >>> The HTTP binding represents that 
> envelope as 
> an HTTP GET, with no body.<<  The current HTTP binding uses 
> POST of the 
> XML 1.0 serializations of the envelope infoset.  The 
> "improved" binding 
> would say:
> 
> * The magical restful GET request is represented as an HTTP 
> GET, with no 
> body. 
> * All other envelopes are transmitted as POST, using XML 1.0 
> serialization 
> (I.e. as today)
> 
> Receipt of an HTTP GET is a signal to the receiving node that 
> the envelope 
> transmitted was in fact the magical get request envelope.  
> That envelope 
> is therefore processed per the normal SOAP processing rules [5].  The 
> semantics of the REST:GET element at the receiver are:  retrieve a 
> representation of the resource to which the request was sent. 
>  That will 
> result in an SOAP HTTP response with the (representation of) 
> the request 
> resource.  That representation SHOULD be a SOAP document (typically 
> application/xml+soap).
> 
> Some characteristics of this proposal
> -------------------------------------
> 
> * It applies REST:GET in a uniform way across multiple underlying 
> protocols.  The HTTP to MQSeries example above can in principle be 
> supported.
> 
> * It correctly separates the identification of the resource (the URI, 
> about which we need say nothing) from the operation (GET).
> 
> * It correctly labels the operation as a RESTful get, and not 
> some other 
> vaguely safe operation.
> 
> * It is appropriately conservative in presuming that any 
> additional in the 
> information in the envelope might represent loss of safety. 
> 
> * By representing the request as a SOAP envelope, we allow 
> receiving nodes 
> to apply the full SOAP processing model, albeit to a very stylized 
> request.
> 
> * It's relatively simple.
> 
> What about RPC?
> ---------------
> 
> I believe the REST community is correct that the famous getStockQuote 
> example is not RESTful. 
> 
>         stockprice = getStockQuote("ibm","1/1/2002)
> 
> We can do this RPC in a restful way by applying something 
> similar to Tim 
> Bray's trick [6] >> at the endpoint << and sending the 
> magical RESTful get 
> to retrieve the information.  So, the URI would look like: 
> 
>         
> http://www.stockprices.com/prices?company="ibm"+date="1/1/2002"
> 
> The body would be as above, and the binding would turn it 
> into a vanilla 
> HTTP GET.
> 
> What makes this proposal different from Tim's is that we 
> don't start with 
> a SOAP RPC envelope.  The mapping of method arguments into the URI is 
> purely the business of the endpoints...the company name and 
> date never 
> exist in a SOAP body.  One way to do this would be with a 
> WSDL convention: 
>  mark the parameters that identify the resource.  The 
> endpoint then knows 
> to map them to the URI.  I'm not 100 percent sure this is a 
> good proposal, 
> but I think it's worth considering.
> 
> Now, what's more complicated is maintaining symmetry in the following 
> cases:
> 
>         bal = getBankBalance(NoahsAccount);
>         updateBankBalance(NoahsAccount, "10000000.00");
> 
> The getBankBalance would be handled exactly as for 
> getStockQuote above. 
> 
>         URI:  
> http://www.megabank.com/accounts?accountname="NoahsAccount"
>         SOAP Envelope:  magical get request
>         HTTP operation: GET
>         HTTP body:  none
> 
> The WSDL-based description of the RPC for update would have 
> to distinguish 
> parameters used to identify the resource, from those to be 
> included in the 
> operation on wire.  In this case, NoahsAccount identifies the 
> resource, 
> and the 10000000.00 second parameter is actually sent in the 
> RPC in the SOAP body.  Once 
> again, this is correct REST, I believe.
> 
>         URI:  
http://www.megabank.com/accounts?accountname="NoahsAccount"
        SOAP Envelope:  SOAP 1.2 RPC for updateBankBalance (not sure if we 
send only
                        2nd parm or both)
        HTTP operation: POST
        HTTP body:  XML 1.0 serialization of the Envelope.

Conclusions
-----------

I'm not convinced this proposal is a good one.  If nothing else, I hope it 
goes some way toward highlighting where the issues and opportunities are. 

I believe this shows that SOAP is not deeply broken, but that modest work 
can get us where we need to be.  There is, of course, the question of 
whether a capability like this should be in the initial SOAP 1.2 
recommendation.  The advantage would be having a normative, interoperable 
way of doing GET on day 1.  On the other hand, nothing in the 
recommendation will break if someone publishes an updated binding a little 
later.  The risk would be that early implementations would not understand 
the GET idiom as representing a true SOAP request.  I personally am more 
or less neutral as to whether it goes in the REC or not, but I wouldn't 
mind (presuming the proposal holds up).

Also:  I think it's worth pointing out that the added capabilities in SOAP 
(vs. HTTP)  are not gratuitous.  The reason for structured headers and the 
associated processing model is that many requests will be end-to-end 
signed, authenticated, transaction controlled etc.  I am not an HTTP 
expert, but I suspect that an information retrieval request accompanied by 
such higher level controls is rightfully a POST (I.e. not safe), at least 
in many cases.  The feature architecture of SOAP would allow us to later 
identify specific situations in which an authenticated GET (for example) 
could indeed map to a RESTful retrieval.

Again, apologies for the long note.  I hope this is useful.

Noah

[1] http://www.w3.org/2000/xp/Group/2/04/11/soap12-part1-1.86.html
[2] http://www.w3.org/2000/xp/Group/2/04/11/soap12-part2-1.55.html
[3] 
http://www.w3.org/2000/xp/Group/2/04/11/soap12-part1-1.86.html#transpbindfra
mew
[4] 
http://www.w3.org/2000/xp/Group/2/04/11/soap12-part1-1.86.html#extensibility
[5]
http://www.w3.org/2000/xp/Group/2/04/11/soap12-part1-1.86.html#msgexchngmdl
[6] http://lists.w3.org/Archives/Public/www-tag/2002Apr/0211.html

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Noah Mendelsohn                              Voice: 1-617-693-4036
IBM Corporation                                Fax: 1-617-693-8676
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Received on Friday, 26 April 2002 04:10:53 GMT

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