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Re: Rough text for State finding

From: <noah_mendelsohn@us.ibm.com>
Date: Mon, 17 Oct 2005 10:50:19 -0400
To: "David Orchard" <dorchard@bea.com>
Cc: www-tag@w3.org
Message-ID: <OF1C06362F.7E0411D7-ON8525709D.004EF778-8525709D.005182E1@lotus.com>
Dave,

Thanks for all the hard work on this.  I've only done an initial 
readthrough, and want to give some more thought before commenting on how 
this stacks up overall as a TAG finding.  At the very least, I think the 
examples in the middle do an excellent job of explaining the motivations 
of and tradeoffs faced by web site designers and web services users.  As a 
start, I have a few detailed comments:

* You introduce "state" by saying:  "What is State:  State is the data 
that pertains to an entity at a particular point in time."  I wonder if it 
would be helpful to go on to say something like: "Most interesting 
resources have state of one sort or another, which is what allows them to 
provide interesting information when interacting with user agents on the 
Web.  This finding concerns itself especially with two particular kinds of 
state that can be problematic: (1) per user or per session state, which 
can cause a resource to interact differently according to the user making 
the access or the network connection on which the request is received; and 
(2) state representing dependent or sub-resources that are not (and 
arguably should have been) themselves separately identified by a URI...for 
example, a bank account that is identified by a bank account number stored 
in a cookie, rather than in a separate URI."  As it stands, the finding 
seems to suggest that all state is problematic, and I don't think that's 
true;  I believe it's these two particular kinds of state that are at 
issue.

* I think it would help to give more motivation and discussion for the 
"EPRs on the Web" section.  Is this something we're recommending? 
Something that seems tempting but that nobody has a robust way to do?  I 
think a paragraph or two on mapping the QNames of RefParms into 
hierarchical URIs and/or query parameters (or whatever it is you're 
recommending) would be helpful.

* Possible bug:  at one point in the Fabram example you say: 
"Alternatively, the EPR could insert the CustomerKey in the EPR:" I 
suspect you meant either "Alternatively,  the CustomerKey could be encoded 
in the [address] property of the EPR" or maybe "Alternatively,  service 
could obtain the CustomerKey from URI", or some such.  I don't think an 
EPR can perform the operation of insertion, and in any case the 
CustomerKey has been in the EPR all along. 

* I think you should say: "Some of the key considerations are scalability, 
reliability, network and application performance, security, and ease of 
design, >>and promoting network effects on the World Wide Web, I.e. 
leveraging and contributing to a single, global information space.<<"

So, take the above as neutral on whether we should do a finding, what its 
key points should be, and how close this draft comes as a start.  I hope 
the above suggestions are helpful in any case.  Thanks.

--------------------------------------
Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
1-617-693-4036
--------------------------------------








"David Orchard" <dorchard@bea.com>
Sent by: www-tag-request@w3.org
10/15/2005 02:54 PM
 
        To:     <www-tag@w3.org>
        cc:     (bcc: Noah Mendelsohn/Cambridge/IBM)
        Subject:        Rough text for State finding


I've written up rough text for the state finding primarily for the EPR-47 
discussion.  If the direction is roughly correct, based on TAG and other 
feedback, I'll do the conversion to xmlspec, fill in the missing pieces, 
add refs, etc.  for more formal publication.
 
State and Applications
 
This is a draft TAG finding on State.  The purpose of the finding is to 
provide guidance to application developers on the use of Stateful or 
Stateless applications in a Web context.  It examines a variety of designs 
for a canonical example application to illustrate the complex trade-offs 
in the designs.  It uses HTML browser based and Web service based examples 
to show the similarities between the design decisions.   The finding 
concludes with an analysis of the architectural property trade-offs 
between stateful and stateless applications.
 
What is State
State is the data that pertains to an entity at a particular point in 
time.  A variety of software entities have state, ranging from 
applications to operating systems to network layers.  The state of an 
entity changes over time triggered by some kind of event. The event could 
be a network message, a timer expiring or an application message. Entities 
that do not have state, that is there is no trigger that causes a 
transition, are called stateless. 
 
Abstract example
Dirk decides to build an online banking application.  Customers will be 
able to view their account balances and make transfers.   The first step 
is logging on to the application.  When the customer selects accounts 
view, the banking application will ask them for their username and 
password.  If they have already entered their username and password, they 
will not be asked for it again.  The system will automatically log the 
customer out if they haven?t done any activity for 10 minutes. 
 
We see a prototypical stateful application from the client perspective. 
The application has 2 states: logged-in and not-logged-in.    This state 
may be realized by storing state on the client or on the server.
 
Decisions
 
The first decision is whether data or state is persisted.  It?s either the 
data used to recreate the state, such as username and password, or it is 
the state itself.  If the data is stored, then it must be stored in the 
client and then sent to the service for each request.
 
Given a decision of storing the state rather than data used to create the 
state, another decision is whether the state is stored on the client or 
the server.  Applications where the client stores the state are typically 
called stateless applications, even though there is state on the client. 
 
Given a decision of storing the state on the server, how is the state 
identified and transmitted by the client.  Web applications will typically 
use URIs for identifying entities aka resources.  Where does either the 
state identifier exist in the message to the server: in the URI, the 
message body, a particular HTTP header?
 
Example using HTTP Authentication
Dirk decides that the banking application will be stateless on the server 
and the client will resend the data for each request.  The application has 
a URI for the entry page to the banking application and a link to the 
account balances.  When any banking URI is requested, the 
username/password features of HTTP are used, usually implemented as a 
pop-up window asking for username and password.
 
There are very few web sites that are built this way, perhaps the largest 
is the W3C web site.  Most web sites use alternative technologies for 
logging on and they store the state using HTTP cookies or using URL 
rewriting. 
 
The primary reasons for customized security are security concerns, that is 
wanting greater control over the security timing out, and ease of use 
concerns, particularly wanting direct control over the look and feel of 
the screens including helpful tips and links to forgotten passwords. 
 
Example using URL Rewriting with client-side state
Dirk decides that a customized security screen is needed.  A new page with 
the entry of username and password is inserted in the application, after 
the ?show item? page in the state flow.  Upon successful completion, the 
URL is rewritten to contain the state that the user has logged on.  At 
first, Dirk was going to have the URL contain the username and password, 
but that was rejected for obvious security reasons.  After the security 
page, any URLs in pages returned are rewritten to contain the state and 
the state is encrypted to prevent tampering and guessing. 
 
This approach has a significant downside of the URL rewriting.  In 
general, it is unlikely that URLs with a particular users login state need 
to be exchanged or bookmarked.  From a modeling perspective, the resources 
that would likely be identified are accounts and particular transactions, 
not login state.  Also, it is difficult for the application to have full 
control over the URL and do the rewriting, it is difficult for the 
application to parse the URL to extra the state. 
 
HTTP Cookies offer the benefit of a well-defined place, the HTTP Cookie 
header, for storing and retrieving data without rewriting the URL.
 
Example using Cookies with client-side state
Nadia decides to change the banking application to store the application 
state in a cookie.  The application still has URIs for the banking 
application page.  The application stores the state in a cookie that is 
sent to the browser upon successful completion of the page, and sent back 
to the service on every request. 
 
Yet still, very few web applications are built this way.  Most secured web 
sites use cookies where the state is stored on the server, rather than 
encapsulated in the client.  The motivations are primarily about 
performance, particularly giving the serviced application the control over 
whether to keep the state in memory or passivate to disk.  The state could 
get quite large or it may be difficult to serialize and so serialization 
to the client could be difficult.  Another motivation is concerns about 
network performance if the state gets quite large and security of the 
state.  A final motivation is the visibility of the state id
 
Example using Cookies with session ids.
Nadia further updates the banking application to store the log-in state in 
a server side component.  The server-side component is identified with an 
id, commonly called a session id.  This session id is stored in the 
cookie.
 
Stateful resource identifers
The previous examples explored the issues and designs related to session 
identification and transmission.  As described in the URL rewriting 
example, the session information is probably not a stateful resource that 
requires an identifier.  However, a particular user?s account view, 
particular bank account or particular transaction is intuitively a 
stateful resource where the identifier could include the particular 
account or transaction identifier.
 
In the banking application, there are 2 different account balance URI 
designs: one URI for all users or URI per user.  The first design does not 
have distinct URIs for each of the user account balances.  Rather, there 
is a ?dispatch? URI and the particular user account requested is encoded 
in the request message or headers.  For example, after logging in, the 
http cookie contains the user id.  When the user requests the generic 
page, the particular user id is sent in the HTTP POST data. 
 
The second design has a distinct URI for each of the user ids.  The user 
clicks on the login, and this redirects them to a unique URI for their 
account.. 
 
The URI per account design, sometimes called ?deep-linking?, has all the 
network effect advantages that the web has to offer: the users account is 
bookmarkable, exchangeable, etc. 
 
It does suffer from potential increased complexity as it may be easier to 
populate and parse the FORM POST data for the account id rather than the 
URI.  Another problem with clicking on a URL that takes them to say 
'cleared checks for my savings account' then if the website is redesigned 
(a frequent event, at least on the back end) then that URI will break. 
Either that or the website has to maintain complex mapping tables to 
handle versioning URIs across multiple versions of the website.   Hence 
many websites would rather just force users to come in through a well 
defined home page and then focus on making navigation as easy as possible 
to get them quickly to where they want to be.
 
It is worth noting that the application has 2 different types of state 
information that are being identified: the account balance and the session 
id.  By putting the account id in the URI and keeping the session id 
separate, the application has achieved a separation and the different 
benefits achievable from the transient session information and the network 
effect of re-usable URIs.
 
Web service example
Dirk is tasked with making the banking application available as a Web 
service rather than HTML pages.  He uses XML, SOAP, WSDL, and 
WS-Addressing to do this.  The banking application is a service with an 
interface containing two operations:log-in and getBalance.   The first 
operation is a log-in operation.  If successful, it returns a 
WS-Addressing ?ReplyTo? containing an EPR that client should use for 
requesting the account information.  The EPR contains a reference 
parameter that contains the session id and a reference parameter that 
contains the account id.
 
An example, slightly modified from the WS-Addressing specification is
 
<wsa:ReplyTo>
  <wsa:EndpointReference
     xmlns:wsa="http://www.w3.org/2005/08/addressing"
     xmlns:fabrikam=http://example.com/fabrikam>
   <wsa:Address>http://example.com/fabrikam/acct</wsa:Address> 
   <wsa:ReferenceParameters>
    <fabrikam:CustomerKey>123456789</fabrikam:CustomerKey>
    <fabrikam:SessionID>ABCDEFG</fabrikam:SessionID>
   </wsa:ReferenceParameters>
  </wsa:EndpointReference>
</wsa:ReplyTo>
 
A request to the service, such as ?GetBalance?, might have a fragment 
like:
 
<S:Envelope xmlns:S="http://www.w3.org/2003/05/soap-envelope"
         xmlns:wsa="http://www.w3.org/2005/08/addressing"
         xmlns:fabrikam="http://example.com/fabrikam">
   <S:Header>
     ...
    <wsa:To>http://example.com/fabrikam/acct</wsa:To>
    <wsa:Action>http://example.com/fabrikam/GetBalance</wsa:Action>
    <fabrikam:CustomerKey 
wsa:IsReferenceParameter='true'>123456789</fabrikam:CustomerKey>
    <fabrikam:ShoppingCart 
wsa:IsReferenceParameter='true'>ABCDEFG</fabrikam:ShoppingCart>
     ...
   </S:Header>
   <S:Body>
     ...
   </S:Body>
</S:Envelope>
 
Alternatively, the EPR could insert the CustomerKey in the EPR:
<wsa:ReplyTo>
  <wsa:EndpointReference
     xmlns:wsa="http://www.w3.org/2005/08/addressing"
     xmlns:fabrikam=http://example.com/fabrikam>
   <wsa:Address>http://example.com/fabrikam/acct/123456789</wsa:Address>   
 
   <wsa:ReferenceParameters>
    <fabrikam:SessionID>ABCDEFG</fabrikam:SessionID>
   </wsa:ReferenceParameters>
  </wsa:EndpointReference>
</wsa:ReplyTo>
 
EPRs ?on the Web?
For the purposes of EndpointReferences-47 discussions, there is no binding 
of an WSA Message Addressing Properties, including EPRs, into an HTTP 
request.  Some hypothetical instances of the above EPR into an HTTP GET 
request:
 
GET /fabrikam/acct?CustomerKey=123456789&SessionID=ABCDEFG
 
GET /fabrikam/acct/CustomerKey/123456789?SessionID=ABCDEFG
 
GET /fabrikam/acct/123456789?SessionID=ABCDEFG
 
GET /fabrikam/acct/123456789
Cookie: $Version=?1?; SessionID=?ABCDEFG?; $Path=?/fabrikam?
 
GET /fabrikam/acct/
Cookie: $Version=?1?; SessionID=?ABCDEFG?; 
        CustomerKey=?123456789?; $Path=?/fabrikam?
 
 
<<Insert WSDL samples?  This would show the application structure and 
messages, but might also overly complicate what is already a moderately 
lengthy write-up.  >>
 
State decision factors
The decision on where to place the state in the distributed application 
and how to identify the state are affected by numerous factors.  Some of 
the key considerations are scalability, reliability, network and 
application performance, security, and ease of design.
 
Roy Fielding argues in his REST dissertation [1] that stateless server has 
the benefits of increasing reliability, scalability, visibility and while 
potentially decreasing network performance.  However, I believe the 
trade-offs from an application developers perspective are somewhat 
different, and need to be examined from a holistic perspective.
 
Ease of Application construction
 
There are two primary types of designers that are relevant: the network 
administrator that controls the deployment of applications and publication 
of URIs, and the application developer that controls the contents of 
messages including http headers.  The application developer can develop 
the application without affecting the URI with the state id information 
and so avoid a potential conflict with the administrator.
 
Many, if not most, applications are built to exchange state information 
that is not ?identifying? information, such as session ids.  This is 
evidenced by the widespread use of HTTP Cookies.  In the cases where these 
applications are also exchanging identifying information, the application 
development is simpler when the same mechanisms are used for exchanging 
both types.  Examining the Web example, the application developer can 
easily insert and parse information in the cookie header, rather than 
rewriting the URI that is sent. 
 
In the Web services example, it is very easy to do dispatch based upon a 
soap header block, which is an XML QName.   The tree-like structure of XML 
and use of SOAP and SOAP Header blocks means that an application developer 
can use widely available tooling, such as JAX-RPC handler chains, that 
makes it easy to use the XML QNames.  On the converse, there are no 
standards available for inserting or parsing a QName(s) into or from a 
URI. 
 
It is worth explicitly noting that there is a trade-off between the 
control over the URI versus other parts of the message body, and a 
trade-off between the ease of updating/parsing URIs and the other parts of 
the message body. 
 
Scalability
Scalability is directly related to the availability of important resources 
for requested load.  The resources can be processes, threads, memory, cpu 
cycles, database connections, network connections.  Allocation and re-use 
of the resources happens on a per-resources basis.  For example, most 
applications use database connection pooling but they typically gain the 
functionality from middleware of some kind.  The scalability trade-off is 
whether the cost of acquiring the necessary resources for a request is 
best served with the state on the client or on the server, and that 
completely depends upon how the resources are freed up and then 
re-allocated. 
 
In the simplest case, it may be that not freeing up the resources an 
amount of time is the most scalable.  Keeping the state in memory, with a 
time-out optimized for typical client latency, can scale better than 
release resources when the time-out is set correctly and the resource 
acquisition/freeing is significant. Anecodatally, Jim Gray has observed 
that 5 minutes has been a historically accurate cut-off time for caching 
in memory rather than persisting to disk. 
 
In other configurations, it may be that it is ?cheaper? to free up 
resources by responding with the session id in the response and persisting 
the data to the database rather than responding with the entire state to 
the client because the ?cost? of transmitting to the client is more 
expensive than the cost of sending to the database.  Likewise, it may be 
?cheaper? to reify the session by acquiring it from the database than from 
the client. 
 
However, the cost of freeing up state and recovering state is based on a 
variety of factors, specifically the system architecture and the 
connections to the client and database, the middleware software, the 
database software, and hardware/software platforms used for the system. 
 
Reliability
Reliability of Stateful applications has two distinct aspects: reliability 
of the machines and reliability of the network.  The reliability of the 
network is not typically a factor in the design of the application style, 
as it is typically assumed that the network is unreliable.  The aspect of 
reliability that concerns this writing is machine reliability.  For a 
given client, the two time periods of interest are during a request and in 
between requests.
 
In a stateless application design, a machine can fail between a request 
without affecting the clients view of the system.  They send a request and 
it is dispatched to an available machine.  If a machine has crashed in 
between requests, there is no disruption. 
 
In a stateful application design, the systems can be designed to handle 
failures between a request.  Common techniques are duplicating the state ? 
RAID disks, back-up nodes ? and hardening the system ? UPS, memory 
checksums, etc.  For example, an application server can have a primary and 
backup node.  If a machine fails, then the backup node is used for 
subsequent requests.
 
Stateful and stateless application design must deal with the situation of 
where a machine crashes during a request.  In stateless applications, 
typically the request is lost.  Let us make a simplifying assumption that 
the request is ?atomic? and is either completed or aborted.  This allows 
us to avoid the problem of determining application state where the 
problems of meaning of reliability in a synchronous environment arise. 
Many systems are designed to handle machine failure during processing by 
having a stateful ?dispatcher? that has tracked the request and can replay 
the request to a different machine if one fails during a request. 
 
Related to Reliability is manageability, as systems are often managed for 
reliability.  For example, a component may be starting to behave 
erratically and the administrator wishes to replace the component. 
Stateful and stateless systems would probably be designed for this task by 
letting the requests ?drain? off of the system that is due for 
maintenance.  A stateful system has the downside that the states may be 
long-running and hence take longer to ?drain?.   Advances in application 
server technology provide for managing these by supporting ?transferring? 
state from one machine to another. 
 
The discussions so far have not discussed ?client? reliability.  Web based 
systems typically have a simplifying assumption that browsers are for a 
single user, are unreliable, and responses must be received within about 
30 seconds for good HMI design.  We have made a simplifying, but erroneous 
assumption that systems where the client has the state are ?stateless?. 
The state always exists somewhere, and in many EAI and B2B systems, the 
web based simplifying assumptions are not true.   The system, whether it 
is the client or the server or the network, must contain the state. 
Imagine a system where the client keeps the state for long periods of 
time, it must deal with reliability of the state information.  If a 
machine crashes, the system can?t lose the state. 
 
Stateful systems can deliver virtually the same reliability as stateless 
systems, it is more appropriate described as a matter of cost.  A stateful 
system may require more costly infrastructure in the form of components 
selected to achieve the same reliability.  OTOH, the difference between 
the reliability for a stateless system versus a stateful system may be 
small given the overall reliability desired.
 
<<TBD: Improve text to hit the point that many systems are not like the 
human-centric web and ?stateless? is a complete misnomer>>
 
Network Performance
<<TBD>>
 
 
 
 
 
 
Received on Monday, 17 October 2005 14:50:57 UTC

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