Crond script execution concurrency conflicts,

Crond script execution concurrency conflicts,

In a scheduled task, you will occasionally see repeated executions:

For example, an example of a scheduled task in our company is as follows:

*/2 * * * * root cd /opt/xxxx/test_S1/html/xxxx/admin; php index.php task testOne >/dev/null 2>&1*/2 * * * * root cd /opt/xxxx/test_S1/html/xxxx/admin; php index.php task testTwo >/dev/null 2>&1

This is a task executed once every two minutes. It cannot be guaranteed that the process that is enabled can be completely executed and shut down within two minutes. If the process has been accumulated, the system resources may be exhausted, this causes system downtime.

Example:

Create the test. php file with the following code:

<?phpsleep(70);?>

Add a scheduled task:

*/1 * * * * root cd /home/ganjincheng;php test.php

Wait for execution, Accumulation

root     26722  0.0  0.0   9232  1064 ?        Ss   12:05   0:00 /bin/sh -c cd /home/ganjincheng;php test.phproot     26744  0.0  0.0 112304  8840 ?        S    12:05   0:00 php test.phproot     29102  0.0  0.0   9232  1060 ?        Ss   12:06   0:00 /bin/sh -c cd /home/ganjincheng;php test.phproot     29116  0.1  0.0 112304  8840 ?        S    12:06   0:00 php test.phproot     29906  0.0  0.0 103320   904 pts/3    S+   12:06   0:00 grep test.php

Solution 1: Control concurrency in code

This method is to transform the code. Determines whether a process is executed. The following code:

<?php  $lockfile = '/tmp/mytest.lock';     if(file_exists($lockfile)){      exit();  }file_put_contents($lockfile, date("Y-m-d H:i:s"));   sleep(70); unlink($lockfile);  ?>

This method of determining whether a file does not exist poses a problem. That is, the program may not be executed until the end, that is, the mytest. lock file created earlier has not been deleted. This will cause the program to fail to run normally.

Second, database Concurrency Control

You can transfer the first scheme to redis and memache for key value judgment.

If the scheduled task is to access the database, you can lock the table. Occasionally, we can use the uniqueness of the unique index and the joint index to avoid repeated inserts.

Third, determine whether a process exists

Example:

$ Fp = popen ("ps aux | grep 'test. php' | wc-l "," r "); $ proc_num = fgets ($ fp); if ($ proc_num> 3) {// note the reason why the number of processes is greater than 3. After you perform the operation, you will understand exit;} sleep (70 );

The disadvantage of this method is that the ps command must be accurate. Avoid counting the processes that are not executing the test. php script. For example:
Run vim to open the test. php file. The preceding command may be incorrectly used for statistics. So when we accidentally open the test. php file in vim, it cannot be executed.

Fourth, use the linux flock command

Let linux help us determine. The flock command provides the file lock function. The command parameters are as follows:

[root@qkzj_Multi-Purpose_1A_113.107.248.124 ganjincheng]# flock -hflock (util-linux-ng 2.17.2)Usage: flock [-sxun][-w #] fd#       flock [-sxon][-w #] file [-c] command...       flock [-sxon][-w #] directory [-c] command...  -s  --shared     Get a shared lock  -x  --exclusive  Get an exclusive lock  -u  --unlock     Remove a lock  -n  --nonblock   Fail rather than wait  -w  --timeout    Wait for a limited amount of time  -o  --close      Close file descriptor before running command  -c  --command    Run a single command string through the shell  -h  --help       Display this text  -V  --version    Display version

Configuration example:

*/1 * * * * root flock -xn /tmp/mytest.lock -c 'php ./test.php'