- readers & writers

From: Bob Walkden <"Bob>
Date: Sun, 3 Jun 2007 10:04:12 +0100

----boundary-LibPST-iamunique-426916899_-_-
Content-type: text/plain

It seems to me that the current model for files is a reasonable
compromise between simplicity, usefulness and safety. As soon as you
introduce concurrency you are no longer dealing with a 'simple file
application', and you have to respond accordingly.

If you change the model so that other processes can read while one
process can write, then you run the risk of some of the problems which
database transaction management eliminates, such as non-repeatable
reads and dirty reads. To eliminate these you would need a more
finely-grained locking strategy, such as block and record locking.
Even these on their own don't fully eliminate the aforementioned
problems (and others which arise), and you start to need transactions.
This takes you into the realm of DBMSs.

So the answer, in my opinion, is to recognise that you no longer have
a 'simple file application', and use the right tool for the job, in
this case the SQL module with a suitable DBMS which handles
concurrency well enough.

Another possibility might be to look at the design of your
application, and identify those parts which require the files to be
updated. Separate them from the rest of the application and make sure
that they hold their locks for the shortest possible time by opening
the files only when you need to, and closing them as soon as you have
finished writing.

--
Regards,
 Bob 
> -----Original Message-----
> From: blackbox{([at]})nowhere.xy> Behalf Of René Dorta
> Sent: 03 June 2007 05:31
> To: BlackBox Mailing List
> Subject: [BlackBox] - readers & writers
> 
> I have been using for a long time a simple file application. 
> Due to the 
> small size of my data I let alone SQL and use instead modules 
> Files and 
> Stores directly. Every thing worked fine till now, when I 
> need to access 
> my data from different computers in a LAN. How to handle this?: If I
> open my files in "shared" mode ("which is the rule in BlackBox"), I 
> cannot update them; on the other hand, if I use "exclusive" (which
is 
> what I had been doing) I cannot open those files from more 
> than one site.
> 
> What I need is a way to write my files from a master site, but to be
> able to read them from any site in the LAN.
> 
> Thanks folks:
> René Dorta.
> 
> --- BlackBox
> --- send subject HELP or UNSUBSCRIBE to blackbox{([at]})nowhere.xy> 
> 
> 
--- BlackBox
--- send subject HELP or UNSUBSCRIBE to blackbox{([at]})nowhere.xy----boundary-LibPST-iamunique-426916899_-_-
Content-type: application/rtf
Content-transfer-encoding: base64
Content-Disposition: attachment; filename="rtf-body.rtf"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----boundary-LibPST-iamunique-426916899_-_---

Received on Sun Jun 03 2007 - 11:04:12 UTC