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RE: Proposed text on reliability in the web services architecture

From: Hao He <Hao.He@thomson.com.au>
Date: Fri, 17 Jan 2003 10:47:25 +1100
Message-ID: <686B9E7C8AA57A45AE8DDCC5A81596AB046AE269@sydthqems01.INT.TISA.COM.AU>
To: "'Assaf Arkin'" <arkin@intalio.com>, "'Walden Mathews'" <waldenm@optonline.net>, Peter Furniss <peter.furniss@choreology.com>, "Champion, Mike" <Mike.Champion@SoftwareAG-USA.com>, www-ws-arch@w3.org
Agreed. RM only helps in efficiency, as also pointed out in the paper by
Saltzer et al.  

I also like your idea of giving an application the ability of selecting
different level of RM.

The remaining question is: how do we do ene-to-end coordination between
client and server? Or, has this
basically become a choreography issue?


-----Original Message-----
From: Assaf Arkin [mailto:arkin@intalio.com]
Sent: Friday, January 17, 2003 10:17 AM
To: Hao He; 'Walden Mathews'; Peter Furniss; Champion, Mike;
Subject: RE: Proposed text on reliability in the web services

I think we care about both.

It's a mistake to assume that RM solves your coordination problem. It
doesn't attempt to do that, so it shouldn't be viewed as a solution. So if
anyone positions RM as a solve-all, I would like to refer them to the
comments made by Walden which articulate exactly what RM does not do.

But RM is still necessary, it's orthogonal to the coordination problem.
Which is why I am interested in RM, but also interested in coordination
protocols and also interested in choreography. A solution may require any
combination of these mechanisms to get the level of guarantee it demands.

At the minimum you can think of TCP as being an RM mechanism. But if you are
doing asynchronous messaging either using TCP or using other protocols, then
TCP doesn't given you the level of RM you need. In this situations you will
find the need to have something else.

Ideally Web services could be structured such that there are certain levels
of RM that the application can request and it's up to the WS stack to figure
out how to make it work. That way, whether you're getting your RM needs met
by TCP, or you are doing something else which requires a different
mechanism, you write the application in exactly the same way.

That in my opinion improves decoupling and allows a variety of delivery
mechanisms to be used.


> -----Original Message-----
> From: Hao He [mailto:Hao.He@thomson.com.au]
> Sent: Wednesday, January 15, 2003 7:34 PM
> To: 'Walden Mathews'; Assaf Arkin; Peter Furniss; Champion, Mike;
> www-ws-arch@w3.org
> Subject: RE: Proposed text on reliability in the web services
> architecture
> I think Walden has made a good point. We don't really care about RM, which
> itself has been solved in the TCP/IP layer or other messaging
> layer already.
> The whole RM thing is misleading within Web Services context.
> What we really
> care is a reliable way of coordinating a client and its server,
> although RM
> might be helpful.
> Making an operation idempotent seems to be a simple and effective
> solution.
> What we need to distinguish
> is the difference between logical operations and physical operations.
> In the bank account example, depositing money into an account is a logical
> operation while sending
> a request to the bank's Web Services to achieve such an operation is
> physical. A logical operation can
> be idempotent or not, but we do want all physical operations to be
> idempotent.  More precisely, we mean:
> 1. r=f(u,x,t), where f is a physical operation on a Web Services
> identified
> by u, x is a request, and t is time, and r is the results returned by the
> WS.
> 2. r'=f(u,x',t')
> 3. if x eq. x', and r eq. r', then we say that f is idempotent, where eq.
> stands for "is equivalent to".
> This idempotent def is different than the one in math.
> Is this summary good enough?
> Hao
> -----Original Message-----
> From: Walden Mathews [mailto:waldenm@optonline.net]
> Sent: Thursday, January 16, 2003 2:01 PM
> To: Assaf Arkin; Peter Furniss; Champion, Mike; www-ws-arch@w3.org
> Subject: Re: Proposed text on reliability in the web services
> architecture
> > Consider depositing funds. You can do that by:
> >
> > 1. Sending the end-state
> > 2. Sending the pre-state+part end-state
> > 3. Sending a delta (the amount to deposit)
> >
> > In order to perform a deposit all you need is to send the
> delta, you don't
> > need to know what the pre-state is.
> Deposit is, by definition, a non-idempotent operation.  Now, where
> do you want to go with that?
> If the deposit itself is a resource of interest, then you can use
> idempotence to set the amount of the deposit.  This is one way
> to reconcile the account.
> If the deposit has no lasting value to you, and all you care about
> is setting the account balance to the "right" figure (Quicken has a
> feature like this for people who like to "try" to balance their accounts,
> but don't want to put too much into the effort), then you could
> idempotently set the balance, overriding deposit-based calculations.
> In either case, an alternate but unattractive strategy would be to
> issue increments against the amount in question until the right
> value was converged upon.  This is the problem RM is focused
> on, and that's the reason RM is the hard way to do what the
> application really needs.
> >
> > In order to send the end-state you need to know more than the
> balance. In
> > fact, when you make a deposit two states are changed: 1) the balance, 2)
> the
> > journal (where every change is recorded).
> This is a matter of scope.
> >
> > So the end-state is a composition of the new balance and the
> journal which
> > now contains a new record. It's not practical to send the full
> end-state.
> No, and it shouldn't be necessary. The point is that if you care about the
> entries in the journal, then you give them identity and acknowledge the
> sovereignty of their state, which gives you a platform from which to use
> an idempotent operation in that scope.
> >
> > However, you can send a partial end-state, just the balance
> plus a version
> > number of the balance. That would allow the server to determine whether
> the
> > change has been done (based on the version number) and if not change the
> > end-state by also adding a record in the journal.
> Version numbers are like RM.  They're not what you really care about,
> and they carry the potential to screw you despite your good intentions.
> What happens to your versioned account when a deposit that's been lost
> in the network for a week finally comes home?
> > Here is my understanding so far.
> >
> > When you can use idempotent operations you should do so. But in
> some cases
> > they are impractical and you are better off not doing idempotent
> operations.
> This overstates the case.  Your application should behave in a way
> that makes sense.  You should not be forcing idempotent patterns on a
> domain where they do not make sense.  A "deposit" is such a case.
>  It means
> "here's some money to add to my account, whatever the balance was and
> whatever it will become."
> >
> > If you are not doing an idempotent operation then you must perform the
> > operation at most once. You actually plan for exactly once, but
> that's not
> > practical either, you need to account for at most once possibility.
> > Otherwise, I would not be balancing my checks at the end of the
> month, but
> > even big banks are known to make mistakes.
> If the bank erroneously introduced a withdrawal of $100 on your
> account, transaction number 29707421, and you and the bank agreed
> it was posted in error, then the operation "remove 29707421" fixes
> the problem, and is idempotent.
> >
> > If you are sending a message and know whether the message has been sent
> with
> > 99.99% success rate (i.e. only in 0.01% of cases you need to somehow
> resolve
> > the situation) then you are fine.
> Maybe yes; maybe no.  If that was the deposit of my paycheck and it got
> lost, then I'm not fine.  Not all problems are modeled well by statistics.
> >
> > If you are sending a message and do not immediately know whether the
> message
> > has been sent, and the success ration is 90% (e.g. when you use IP
> multicast
> > or UDP or SMTP) then you need to resolve 10% of the time.
> Yes, but WHICH 10%?  Which messages were received and which
> were not?  How do we find out?
> >
> > You can reduce that to 0.01% (or something like that, my numbers are all
> > made up) if you have some messaging layer that makes the messages
> idempotent
> > and keep resending until it gets close to 100% of success rate.
> You're dreaming.
> >
> > So reliable messaging buys you:
> >
> > - Operation is idempotent or not idempotent, it works the same way
> > - You can use a high latency non-reliable messaging exchange
> > - The success rate gets as close to 100% as possible
> > - The layer is reusable and works the same way for all applications
> without
> > complicating the application
> You are approaching 100% fallacy here; perhaps some retries will
> improve the odds.  ;-)
> Walden

Received on Thursday, 16 January 2003 18:53:47 UTC

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