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php - MYSQL Making sure two cronjobs don't process the same request

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Solution:

you can try to make a lock/unlock file so cronjobs can check to "know" if a job is/isn't already running. also process pid could be used for identifying status of process in case something failed, not responding etc..

Undefined asked

2023-06-8

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41

votes

Answer

Solution:

Set the status to a unique value, and then process everything with that unique value. Have a timeout or some sort of fallback if your processing fails half-way through (but you need that with your current situation anyway).

Something like (and I'm just making up PHP, since I don't particularly know the language):

 $guid = new_guid();

 $this->db->query(
    'UPDATE x__mx_request SET status = ? WHERE status = "pending";', 
    $guid
 );
 $requests = db_all(
    $this->db->query('SELECT * FROM x__mx_request WHERE status = ?;', $guid)
 );
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Another option is transactions - but I think you'd need SERIALIZABLE, which means you're basically stuck with only 1 job processing anyway. If you want to do that, a lock file for your cron job makes that easy to do without changing code.

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333

votes

Answer

Solution:

transactions are what you want. So you have a single transaction that reads the data, changes the pending status to some intermediate value such as inprocess, and returns the data.

This prevents two queries from returning the same data.

Roughly you want something like this pseudo sql

begin transaction 

select into tmp table * from blah where status = "pending"

update status = "being processed" where id in tmp 

end transaction 

return select * from tmp table
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Date the issue was resolved:

2023-06-8

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