RE: SELECT FOR UPDATE

You should handle this at the application level as well.
User 1 and 2 open and Work on page
User 1 commits changes
User 2 goes to commits change and gets error message pages updated during
editing.
    User 2 could be     Prompted overwrite current values, user1's
update would be lost.
                Save page under a different value
                Merge data ...



--
Ian Willis

-----Original Message-----
From: Oliver Elphick [mailto:olly@lfix.co.uk]
Sent: Friday, 24 August 2001 2:06 AM
To: Mike Castle
Cc: Postgres
Subject: Re: [GENERAL] SELECT FOR UPDATE


Mike Castle wrote:
  >On Thu, Aug 23, 2001 at 10:09:19AM -0400, Jan Wieck wrote:
  >> Oliver Elphick wrote:
  >> > I can see arguments to support this view, but consider this classic
  >> > scenario:
  >> >
  >> > User1: Read data into an interactive program
  >> > User1: Start to make changes
  >> > User2: Read data into an interactive program
  >> > User2: Start to make changes
  >> > User1: Save changes
  >> > User2: Save changes
  >
  >Consider replacing "Save changes" with:
  >
  >User1: Lock record, compare original with current record, save if same,
unlo
      >ck
  >User2: Lock record, compare original with current record, notice
difference,
      > abort.

Yes, but if User2 has done substantial editing changes to a field (after all
we could store whole books in a SQL field now), his changes will be rejected
and the program will have to throw them away or else try to integrate them
with the new field contents - in either case there is substantial wasted
effort.

I prefer Jan's solution: on first attempt to change, acquire a user-level
lock by creating a lock record; if you can't get the lock, don't allow
any change.

However, it would be convenient if the database would do this for me.  I
still don't understand why people think it undesirable for it to do so,
since
it is a problem universal to multi-user databases and the effort is
therefore more economically spent at the database rather than at the
application level.

--
Oliver Elphick                                Oliver.Elphick@lfix.co.uk
Isle of Wight                              http://www.lfix.co.uk/oliver
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