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PO
 

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    <p>I would suggest:</p> <ol> <li><p>Using <code>InnoDB</code>, which supports row-level locking:</p> <pre class="lang-sql prettyprint-override"><code>ALTER TABLE `table` ENGINE=InnoDB; </code></pre></li> <li><p>Using a value of <code>Status</code> to indicate "in progress"; to be a little more explicit, let's use MySQL's <code>ENUM</code> type:</p> <pre class="lang-sql prettyprint-override"><code>ALTER TABLE `table` ADD status_new ENUM('queued','in progress','complete') AFTER status; UPDATE `table` SET status_new = CASE status WHEN 0 THEN 'queued' WHEN 1 THEN 'complete' END; ALTER TABLE `table` DROP status; </code></pre></li> <li><p>Using transactions within the client:</p> <pre class="lang-php prettyprint-override"><code>$dbh-&gt;setAttribute(PDO::ATTR_AUTOCOMMIT, FALSE); // pdo $mysqli-&gt;autocommit(FALSE); // mysqli (OOP) mysqli_autocommit($link, FALSE); // mysqli (procedural) mysql_query('START TRANSACTION'); // ext/mysql * </code></pre></li> <li><p>Using a <a href="http://dev.mysql.com/doc/en/innodb-locking-reads.html" rel="nofollow">locking read</a> to fetch a row to be processed:</p> <pre class="lang-sql prettyprint-override"><code>SELECT * FROM `table` WHERE status_new = 'queued' LIMIT 1 FOR UPDATE; </code></pre></li> <li><p>Setting the <code>status</code> of the fetched row to <code>'in progress'</code>:</p> <pre class="lang-sql prettyprint-override"><code>UPDATE `table` SET status_new = 'in progress' WHERE ID = ? </code></pre></li> <li><p>Committing the transaction:</p> <pre class="lang-php prettyprint-override"><code>$dbh-&gt;commit(); // pdo $mysqli-&gt;commit(); // mysqli (OOP) mysqli_commit($link); // mysqli (procedural) mysql_query('COMMIT'); // ext/mysql * </code></pre></li> <li><p>When processing is complete, <code>UPDATE</code> the record again to mark it as such.</p></li> </ol> <p>Beware that you correctly detect and handle any errors that arise at each stage.</p> <p>* note that <code>ext/mysql</code> has not been recommended for use in new code since June 2011 and is now deprecated in PHP v5.5.</p>
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    1. POBest way to access/update one table from two or more scripts?
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    1. USeggyal
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    1. COThank you for the detailed answer. Very functional! This addresses the first challenge where we can lock a row and mark it as In-Progress. But with the 2nd challenge, when the script loads again to process the next chunk in the record, do I pass a GET query string with the present ID? Or can I carry over the 'lock' to the next instance?
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    2. CO@Reddox: It depends on your security requirements. Your suggestion works okay, provided that there is no risk (or no harm) from an erroneous `GET` string (be it malicious or accidental). Other things you might consider, depending on your requirements, include tracking the extent of progress in the database or in a server-side `$_SESSION` variable; you could also consider including some sort of authentication token, e.g. an HMAC, in the querystring.
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    3. COI'll have to lookup HMAC. Thanks :)
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