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    <p>The way you want to do it is the right way. One of advantages to something like MongoDB is that you can embed data to reduce the queries you need to run on a page. Which is what you're doing by caching the followers count.</p> <p>Both of your options work, it's just about the tradeoffs you want to make.</p> <p>1) Less efficient, but the most accurate. Any sort of desync that somehow occurs will be fixed when they next follow someone. This is a decent way to start off and then change it later if the follower counts or the user database gets large.</p> <p>2) Most efficient, you need to make sure you always call $inc when changing the follower count or it'll become desynced until you do #1.</p> <p>Alternatively, you can do #2 and then run a reconciliation job in the background that will resync it on the offhand chance it does become desynced. You can do this easily with the <a href="http://docs.mongodb.org/manual/core/aggregation/" rel="nofollow">MongoDB aggregation framework</a>, you can speed up the reconciliation job by using a timestamp in the <code>User</code> collection and only recalculating the followers for those who were updated since the last time the job ran.</p> <p>To expand as the original question was updated:</p> <p>In general, if you can avoid doing a <code>count()</code>, you should.</p> <p>Nothing is wrong with starting off using <code>count()</code> and then swap to a more efficient solution (#2) later on. Caching the <code>count()</code> results somewhere like memcache can also be used to reduce performance issues until you stop using <code>count()</code> directly.</p>
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    1. POData modelling for 'followers count' in MongoDB app with RESTful API
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    1. USZachary Anker
    1. USZachary Anker
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    1. COHow would a desync occur with option one? Do you think MongoDB is appropriate if this was to scale to thousands of users?
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    2. COIf you were running replication, a query against a slave could be inaccurate due to replication lag. If you're never going to run replication, then it shouldn't really be possible to desync. As for scaling, it depends on what your server is, but thousands of users is fine. The only issue is that Mongo can be slow at counting, so it's something to watch. It's not an issue you would need to worry too much about in all likelihood.
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      1. USZachary Anker
    3. COit's possible to be out of sync if the same thread updates following array but then dies before incrementing a counter on the user add/removed (note that array reflects whom this user is following, and count is kept on how many users are following a specific user so it's not redundant information in the same document). You also could have the "maintenance" job use aggregation framework rather than map/reduce - it'll be much faster.
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      1. USAsya Kamsky
 

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