Re: [Gnumed-devel] (no subject)

[Karsten Hilbert]

On Thu, Dec 22, 2005 at 08:33:43AM +0800, Syan Tan wrote:

>     a client could
>     be operating on stale data, send an update based on the stale data, and
>     then later receive a delayed notify message
>     to update it's stale data, and it would not see the same total order as
>     other clients.
> 
>  or the client must check xmin with each update, just in case it is 
> overwriting
> a delayed update from elsewhere.
That's what we do.

On each write (sql-update in save_payload()) we

- "select for update" to check whether someone else is
  writing *right now*

- "update ... where xmin=old_xmin" to see whether someone
  else changed/deleted our row while we were away

In case of the former we retry with a timeout.

In case of the latter - which is likely to happen after the
former occurred - we offer all three versions

 - the original data we saw when reading it
 - the new data we were about to write to the backend
 - the modified data as is currently in the backend written by someone else

to the caller for use in a merge widget.

Karsten
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