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Note that sharding is not fully supported (or at least not fully tested) as of the March 2012 release. Apart from one weekly maintenance script (that is awaiting a new MongoDB) feature, we believe it should work. As of 3M documents indexed, sharding is not necessary in case. Assuming sharding is enabled, the top-level design page explains how the system scales.
As an alternative to load balancers, DNS round robin load balancing (using Amazon's Route 53) has also been tested and works well.
The remaining sections describe the different steps necessary to get up and running. Note that steps 3-5 can be performed interchangeably, and it is not necessary to finish one step before starting the next. Also API nodes can be added to the load balancer before they are complete (they will appear as out of service until the system is working).
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