As the subject mentions, is fwrite method atomic in PHP? What I mean by that is, does fwrite function acquire an implicit lock while writing or do I need to explicitly acquire an EXCLUSIVE lock on the file before I call fwrite?
That depends on your mode. If you opened the file in append mode, then yes, it is atomic. On some operating systems that may or may not be true over network filesystems like NFS if you are writing more than a block size, but if it is a local fs and you are in append mode, you can count on it being atomic. If you are not in append mode, then it is not atomic at all.
The problem I am having is that, I want to track user requests by logging simple request parameters such as user IP, request URI, etc. instead of inserting into the db for avoiding overheads. And I will have a cronjob read this data around midnight and write it into the database (when there is little load on the server).
For straight logging like this you should be able to simply append to the log file and thus your fwrite will be atomic.
2. While I am inserting this data into the database, I will have to lock the file, dump them into the database and empty it. So, this might make some users wait, which is something I am trying to avoid. But, in anycase I will need to have a secondary lock object which controls access to this log I guess, i.e. acquire secondary lock, rename the file to .tmp or something, release the lock, and in the meantime read from .tmp, parse it and put it in the db.
Have your cronjob rename the file to a temporary filename which is also an atomic operation. Then read that temp file into your database. The only slight wrinkle here is that a process could be writing to the file as you rename it. This in itself is fine. The write will survive the rename and still complete writing to your renamed file since at that level it is simply an open file descriptor and it doesn't care what its name is. To make sure any outstanding writes complete, simply add a little delay between your rename and your read in your cron job.
The next log write that comes in after the rename will automatically recreate the file since you are opening it in create+append mode.
This way you avoid all locking and your application will actually work. Locks are evil!
-Rasmus
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