Re: Two-way mirroring

[Larry Bolhuis <lbolhui@xxxxxxx>]

Terry,

  Yup it's complex at best.  In DataMirror you assign exit programs to do the
updates in this case.  DM still handles all the communications and journal
processing but you create the program that compares the before and after images
and 'do the right thing'.
  An example would be a quantity field.  Suppose that 'simultaneously' system A
changes the value from 3 to 5 and system B changes it from 3 to 2.  When system 
B
sees system A's change it needs to compare the before and after, calculate that
the change is +2 and change the value in it's DB from 2 to 4 (remember the
previous update had set it to 2).  When system A sees system B's change it also
'does the math' and calculates a -1 and set's it value from 5 to 4. Shazaam!

  That's one scenario, lets conside 'simultaneous' updates to the address 
field. 
Now system A........   (Just kidding)  You get the point.

  - Larry

> It would be interesting to know how these products handle multiple
> updates to the same data.  For example, if system A updates a field in
> a record with value "X", and system B updates that same field and
> record with value "Y", what value are you going to use when you go to
> resynch the databases later?  Are you going to compare the journal
> entries from both systems to determine which update happened later,
> and then use that value?  Or what if system A deletes a record that
> was updated by system B?  Do we just delete that record and not bother
> with why system B was doing an update to it?  Whatever choices are
> made in the above examples, assumptions will be made according to a
> set of rules.  How am I to know those rules are the way I want my data
> to behave?  This sounds like it would be a great thing to have, but to
> me it opens up a whole Pandora's box of issues.

-- 
Larry Bolhuis         | What do You want to Reload today?
Arbor Solutions, Inc  | Don't throw your PC out the window,
(616) 451-2500        |  throw WINDOWS out of your PC.
(616) 451-2571 -fax   | Two rules to success in life:
lbolhui@ibm.net       | 1. Never tell people everything you know.
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