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PODecrease value of attribute by one in Hibernate
 

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    <p>I have an <strong>InnoDB table</strong> in my <strong>MySQL</strong> which stores the current stock of a product, whenever a product is sold (always by the unit, i.e. always -1), the stock has to be updated by doing, let's say, something like:</p> <pre><code>UPDATE product_stock SET current_stock = current_stock -1 WHERE product_id = ?; </code></pre> <p>Now, I'm using <strong>Hibernate</strong>, so I'm updating the object ProductStock by setting the value of the currentStock attribute to "currentValue -1" like this:</p> <pre><code>instance.setCurrentStock(instance.getCurrentStock()-1); ... session.saveOrUpdate(instance); </code></pre> <p>Now, I'm using transactions to commit the changes, however I'm not sure what will hibernate do when multiple sells are being made at the same time, and I haven't been able to successfully replicate the problem which I think will happen, that is, the current_stock won't be saved as current_stock -1 but as the value i setted to the object's instance attribute. <strong>e.g.</strong>, I have 100, two transactions start at the same time to sell a product and both do in code </p> <pre><code>instance.setCurrentStock(instance.getCurrentStock()-1); </code></pre> <p>setting both the stock to 99, then the transactions end and the current stock is saved to 99 when it should have been 98, am I right?</p> <p><strong>The question:</strong> Should/must I make an HQL statement to update the stock directly to -1 instead of updating the object's value from code?</p> <p><strong>UPDATE:</strong></p> <p>I was able to replicate the error by load testing the application with many users making a purchase at the same time, but what is actually happening is not that Hibernate is saving the stock wrong, what is happening is a <strong>deadlock</strong> at database level. We are currently working on it and the answers given are being very helpful.</p> <p>The problem I'm now facing is that handling the locks might be tricky, because the current_stock table and specifically some rows of it might have many reads and many writes at the same time. I'll post the progress and hopefully the final solution</p>
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