lock. Exclusive use of code

lock. Exclusive use of code

^file:lock[file_to_be_locked]{code}

Code is not simultaneously executed by multiple visitors. File_to_be_locked is used to ensure exclusive use.

Example
^file:lock[/counter.lock]{
    $file[^file::load[text;/counter.txt]]
    $string[^eval($file.text+1)]
    ^string.save[/counter.txt]
}
Number of visitors: $string<br />

If locking is not used, two simultaneous requests can increase the counter's value... by 1, not by 2:

·	first visitor comes;

·	second visitor comes;

·	first visitor reads counter's value-value equals 0;

·	second visitor reads counter's value-value equals 0;

·	first visitor increases counter's value-value now equals 1;

·	second visitor increases counter's value-value now equals 1;

·	first visitor writes new value-1;

·	second visitor writes new value immediately after the first visitor, the value is 1, not 2.

Note: you should always keep in mind simultaneous requests. If you work with databases, SQL-servers usually have built-in means that provide correct processing for simultaneously incoming requests.

Note: when there are more then one lock, always analize there mutual relations to avoid "A waits B, B waits A", so called deadlock situation.

Last updated: 21.09.2007