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2010-10-27T07:55:01.947
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2010-10-27T07:55:01.947
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The best strategy depends on the nature of your map-reduce job. If you're using a separate map-reduce call for each individual user, I would go with the first or second strategy. The advantage of the second strategy over the first strategy is that you always have a result ready. So when the user makes a request and the result is outdated, you can still present the old result to the user, while running a new map-reduce in the background to generate a fresh result for the next requests. This has the following advantages: <ul> <li>The user doesn't have to wait for the map-reduce to complete, which is important if the map-reduce may take a while to complete. The exception is of course the very first map-reduce call; at this point there is no old result available.</li> <li>You're automatically running map-reduce only for the active users, reducing the load on the database.</li> </ul> If you're using a single, application-wide map-reduce call for all users, the third strategy is the best approach. You can easily achieve this by specifying an <a href="http://www.mongodb.org/display/DOCS/MapReduce#MapReduce-NoteonPermanentCollections" rel="nofollow">output collection</a>. The advantages of this approach: <ul> <li>You can easily control the freshness of the result. If you need more up-to-date results, or need to reduce the load on the database, you only have to adjust the schedule.</li> <li>Your application code isn't responsible for managing the map-reduce calls, which simplifies your application.</li> </ul> If a user can only see his or her own ratings, I'd go with strategy one or two, or include a <code>lastActivity</code> timestamp in user profiles and run an application-wide scheduled map-reduce job on the active subset of the users (strategy 3). If a user can see any other user's ratings, I'd go with strategy 3 as well, as this greatly reduces the complexity of the application.
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1. POStrategies for keeping Map Reduce results around for subsequent queries
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1. USNiels van der Rest
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1. POStrategies for keeping Map Reduce results around for subsequent queries
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