RE: [provision] The modification problem gets worse...

[Steve Anderson <sanderson@opennetwork.com>]

I've been following this thread, and it think Gerry's example here actually illustrates Jeff's point.  I'll take a stab at explaining ...

In Gerry's example, after initial creation, two different RA's want to change Item B.  The first, RA1, wants to replace both 'values' (3,4) with two new values (5,6).  The second RA, RA2, only wants to replace value 4 with value 5.  RA1 executes first, followed by RA2.

In Gerry's example, it appears that the modifications can only be done by supplying the whole set (3,5), so the resultant set is just that -- (3,5).  The problem is that value 6 has been inadvertently replaced with value 3.

In Jeff's approach, the RA2 can perform the following:
    - Modify B: remove value 4
        - result: Value not found -- interpreted by RA2 to be fine, since
          this is the desired result anyway
    - Modify B: add value 5
        - result: Value already exists -- interpreted by RA2 to be fine, since
          this is the desired result anyway
The final value set for B is (5,6).  No corruption.

Hope this helps.
--
Steve


-----Original Message-----
From: Gearard Woods [mailto:gewoods@us.ibm.com]
Sent: Thursday, March 13, 2003 12:56 PM
To: Jeff Bohren
Cc: provision@lists.oasis-open.org
Subject: Re: [provision] The modification problem gets worse...






Jeff,
Your argument here and in your previous message is predicated on the
assumption that the RAs in my scenario hold the state of the data while in
yours they do not.  In yours they simply request a modification that is
independent of the current state of the data.  You argue that this is not a
race condition problem and that your scenario will not suffer from it.
Let's go through it again using your implementation:

RA 1                Entry                    RA2
create
                          A: 1,2
                          B: 3,4
                                                       read
                                                       A:1,2
                                                       B:3,4

replace
    B: 5,6
                           A:1,2
                           B:5,6
                                                       replace
                                                       B:3,5

                           A:1,2
                           B:3,5


This is how the current specification works and according to you this is
fine.  I'm not sure that I see much of a difference in the end result.
Please explain.
Gerry



|---------+---------------------------->
|         |           Jeff Bohren      |
|         |           <jbohren@opennetw|
|         |           ork.com>         |
|         |                            |
|         |           03/13/2003 05:22 |
|         |           AM               |
|         |                            |
|---------+---------------------------->
  >------------------------------------------------------------------------------------------------------------------------------|
  |                                                                                                                              |
  |       To:       provision@lists.oasis-open.org                                                                               |
  |       cc:                                                                                                                    |
  |       Subject:  [provision] The modification problem gets worse...                                                           |
  |                                                                                                                              |
  >------------------------------------------------------------------------------------------------------------------------------|







In a previous posting I have shown how the current proposal by Gerry Woods
will cause data corruption when doing modifications. I have shown this
using a very simple case. Gerry has suggested that this could be overcome
by a modification mechanism that uses XPath in combination with ldap like
modification semantics. Although this alternative has not been fully
fleshed out, it will not work either. Even theoretically it will fail. Let
me explain why.





Suppose a record has data that looks like:





<A>


            <B>


                        <C>foo1</C>


                        <C>foo2</C>


                        <C>foo3</C>


            </B>


            <B>


                        <C>foo4</C>


                        <C>foo5</C>


                        <C>foo6</C>


            <B>


</A>





No suppose RA1 wants to change the first B component. It would have to
issues a modification request with an XPath that refers to B where B has
foo1, foo2, and foo3. Now suppose the modification changes the record so
that it looks like:





<A>


            <B>


                        <C>foo1</C>


                        <C>bar8</C>


                        <C>foo3</C>


            </B>


            <B>


                        <C>foo4</C>


                        <C>foo5</C>


                        <C>foo6</C>


            <B>


</A>





Now suppose at a later time RA2 wants to remove the first B component from
A. It would have to issues a modification request with an XPath that refers
to B where B has foo1, foo2, and foo3. This request would now fail. There
is no automatic way for RA2 to reconcile the differences.





Jeff Bohren


Product Architect


OpenNetwork Technologies, Inc













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