What happens when many requests are received to read & write to a file in PHP? Do the requests get queued? Or is only one accepted and the rest are discarded?
I'm planning to use a text based hit counter.
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1Take a look at my answer on flock usage: here. Might be helpful.Nemoden– Nemoden2011-06-05 13:30:10 +00:00Commented Jun 5, 2011 at 13:30
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3 Answers
You can encounter the problem of race condition
To avoid this if you only need simple append data you can use
file_put_contents(,,FILE_APPEND|LOCK_EX);
and don't worry about your data integrity.
If you need more complex operation you can use flock (used for simple reader/writer problem)
For your PHP script counter I suggest you to do with something like this:
//> Register this impression
file_put_contents( $file, "\n", FILE_APPEND|LOCK_EX );
//> Read the total number of impression
echo count(file($file));
This way you don't have to implement a blocking mechanism and you can keep the system and your code script lighter
Addendum
To avoid to have to count the array file() you can keep the system even lighter with this:
//> Register this impression
file_put_contents( $file, '1', FILE_APPEND|LOCK_EX );
//> Read the total number of impression
echo filesize($file);
Basically to read the number of your counter you just need to read its filesize considering each impression add 1 byte to it
9 Comments
Sourav
yeah i know you can lock a file for r/w operation, but if then other 2 request get received to read or write are those request thrown or get queued ?
dynamic
with file_append queued. Also there aren't any blocking for read request if you use this FILE_APPEND | LOCK_EX
Sourav
"with file_append queued" what does it mean ? you mean only if i open the file in append mode ?
BenMorel
Maybe he doesn't want to append only. With a write lock, you can do any modification you want on the file, the other threads will be queued (not thrown) until the lock is released.
dynamic
@sourav: if it becomes facebook maybe you will start using something like google analytics ^^
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You can use flock() to get a lock on the file prior to read/write to it. If other threads are holding a lock on the file, flock() will wait until the other locks are released.
2 Comments
Dalibor Filus
Correct me if I'm wrong, but flock will not guarantee you the exclusivity of the lock, when there are 3 writers at once... which can happen (and happened many times in my cases) in multithreaded environment, such as apache (in fork mode or in multithread mode, doesn't matter), FCGI, whatever... Request #1: flock(); fwrite(); .. #2 waits for the lock, fwrite ... #3 waits too for the lock, at the same time. fwrite... BAM, #2 and #3 writes at the same time... Am I right?
BenMorel
In my understanding,
flock() works at the OS level, and should be thread-safe. Maybe someone with better OS knowledge can confirm this?