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POAccessing IndexedDB from multiple javascript threads
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  1. POAccessing IndexedDB from multiple javascript threads
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    copied!<p><strong>Overview:</strong> I am trying to avoid a race condition with accessing an IndexedDB from both a webpage and a web-worker. </p> <p><strong>Setup:</strong> Webpage that is saving items to the local IndexedDB as the user works with the site. Whenever a user saves data to the local DB the record is marked as "Unsent".</p> <p>Web-worker background thread that is pulling data from the IndexedDB, sending it to the server and once the server receives it, marking the data in the IndexedDB as "Sent".</p> <p><strong>Problem:</strong> Since access to the IndexedDB is asynchronous, I can not be guaranteed that the user won't update a record at the same time the web-worker is sending it to the server. The timeline is shown below:</p> <ol> <li>Web-worker gets data from DB and sends it to the server</li> <li>While the transfer is happening, the user updates the data saving it to the DB.</li> <li>The web-worker gets the response from the server and then updates the DB to "Sent"</li> <li>There is now data in DB that hasn't been sent to the server but marked as "Sent"</li> </ol> <p><strong>Failed Solution:</strong> After getting the response from the server, I can recheck to row to see if anything has been changed. However I am still left with a small window where data can be written to the DB and it will never be sent to the server.</p> <p>Example: After server says data is saved, then:</p> <pre><code>IndexedDB.HasDataChanged( function(changed) { // Since this is async, this changed boolean could be lying. // The data might have been updated after I checked and before I was called. if (!changed){ IndexedDB.UpdateToSent() } }); </code></pre> <p><strong>Other notes:</strong> There is a sync api according to the W3 spec, but no one has implemented it yet so it can not be used (http://www.w3.org/TR/IndexedDB/#sync-database). The sync api was designed to be used by web-workers, to avoid this exact situation I would assume.</p> <p>Any thoughts on this would be greatly appreciated. Have been working on it for about a week and haven't been able to come up with anything that will work.</p>
 

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