Avoiding File Update Anomalies (Collisions) on a Server

[Gregory Lypny]

Hello everyone,

I would appreciate your thoughts on how to avoid collisions on a server on 
those presumably rare occasions when two or more visitors attempt to write to a 
file at exactly the same time.  In my case, it’s students writing via FTP to a 
text log file, but the same situation can occur if visitors to a web site, for 
example, submitting an entry to a guest book, do it at the same time.  I don’t 
know what the consequences of this are.

My crude and imperfect attempt to minimize this possibility is to have a Busy 
file that acts as a flag when the log file is being written to.  It contains 
just one line of data:

False, the seconds

If the first item is False, it means that the log file is not currently being 
written to.  A user about to write to the file invokes a handler that changes 
this first item to True, and inserts a timestamp in the form of the seconds.  
When they are finished writing to the file, the same handler sets the Busy file 
to False and inserts a new timestamp.  The purpose of the timestamp is to cover 
for the possibility of the write process or the handler being aborted, 
therefore failing to set the Busy file back to False.  If the timestamp is more 
than, say, four minutes old, the True flag can be overridden.  This method is 
obviously imperfect because it is possible that two people might be writing to 
the Busy file at the same time!  I’m curious to know of other handler-based 
approaches that do not involve setting up separate databases.



Regards,

Gregory
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Re: Avoiding File Update Anomalies (Collisions) on a Server

[Pierre Sahores]

Would recommend to use a PHP or LC server-side front-end script to a MySQL or 
PostgreSQL SQL insert/update instruction instead. The SQL servers would manage 
this for you in a cleanest way than what can be done in using direct writes to 
flat files.

HTH,

Le 22 août 2011 à 17:05, Gregory Lypny a écrit :

 Hello everyone,
 
 I would appreciate your thoughts on how to avoid collisions on a server on 
 those presumably rare occasions when two or more visitors attempt to write to 
 a file at exactly the same time.  In my case, it’s students writing via FTP 
 to a text log file, but the same situation can occur if visitors to a web 
 site, for example, submitting an entry to a guest book, do it at the same 
 time.  I don’t know what the consequences of this are.
 
 My crude and imperfect attempt to minimize this possibility is to have a Busy 
 file that acts as a flag when the log file is being written to.  It contains 
 just one line of data:
 
   False, the seconds
 
 If the first item is False, it means that the log file is not currently being 
 written to.  A user about to write to the file invokes a handler that changes 
 this first item to True, and inserts a timestamp in the form of the seconds.  
 When they are finished writing to the file, the same handler sets the Busy 
 file to False and inserts a new timestamp.  The purpose of the timestamp is 
 to cover for the possibility of the write process or the handler being 
 aborted, therefore failing to set the Busy file back to False.  If the 
 timestamp is more than, say, four minutes old, the True flag can be 
 overridden.  This method is obviously imperfect because it is possible that 
 two people might be writing to the Busy file at the same time!  I’m curious 
 to know of other handler-based approaches that do not involve setting up 
 separate databases.
 
 
 
 Regards,
 
 Gregory
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Pierre Sahores
mobile : 06 03 95 77 70
www.sahores-conseil.com




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Re: Avoiding File Update Anomalies (Collisions) on a Server

[Gregory Lypny]

Hi Pierre Sahores and Bob Sneider,

Thanks for the tips.  I’m getting the message that an external database (PHP or 
mySQL) is the way to go.

Regards,

Gregory
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