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    <p>There was a presentation from AOL/About.me on "<a href="http://www.slideshare.net/mongodb/mongodb-as-message-queue" rel="nofollow noreferrer">MongoDB as message queue</a>" at the Silicon Valley MUG in April, 2012 that you may find helpful in planning your approach.</p> <blockquote> <ol> <li>can I use any sort order for the cursor?</li> <li>does a tailing cursor work with a filter query (to skip some documents I don't want to process)? If so, how does it handle insertion of new documents - will I be notified only if the new document matches the query criteria?</li> </ol> </blockquote> <p>A tailable cursor tails the <em>end</em> of a capped collection and only in natural order. If you want a sort order you need to use an indexed query instead.</p> <p>Please have a read of the <a href="http://docs.mongodb.org/manual/tutorial/create-tailable-cursor/" rel="nofollow noreferrer">Create Tailable Cursor</a> documentation page for more information.</p> <blockquote> <p>Are tailing cursors fully supported by C# driver?</p> </blockquote> <p>Yes, tailable cursor support was implemented in the 1.1 C# driver (see: <a href="http://www.pastie.org/1977343" rel="nofollow noreferrer">code example</a>).</p> <blockquote> <p>I'd like my queue to guarantee single delivery - I mean if two clients try to read messages from the same collection they shouldn't be able to 'consume' the same message. I think this could be achieved with findAndModify, but how to do that with tailing cursor?</p> </blockquote> <p>There are a few approaches:</p> <ul> <li>Have a single reader with the tailable cursor distribute tasks to worker threads (i.e. based on worker queue depth or round-robin). This will be more efficient than having multiple readers, but is potentially a single point of failure.</li> <li>(suggested by @Contango in comment below) Have multiple readers, but use optimistic concurrency to ensure that only one worker "owns" each task. This will result in more I/O, but may simplify your design if the workers are also tailing directly.</li> </ul> <p>For more information on atomic operations, see <a href="http://docs.mongodb.org/manual/tutorial/isolate-sequence-of-operations/" rel="nofollow noreferrer">Isolate Sequence of Operations</a> in the MongoDB manual.</p>
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    1. POMongoDB as a queue, capped collection + tailable cursor
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    1. COThanks, I think this explains why there are so few examples of using tailable cursor
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    2. COThe code sample appears to be down. I mirrored it here: https://gist.github.com/tekmaven/5842521
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      1. USRyan Hoffman
 

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