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    <p>In the context of scaling MongoDB:</p> <ul> <li><p><em><a href="http://docs.mongodb.org/manual/core/replication/">replication</a></em> creates additional copies of the data and allows for automatic failover to another node. Replication may help with horizontal scaling of reads if you are OK to read data that potentially isn't the latest.</p></li> <li><p><em><a href="http://www.mongodb.org/display/DOCS/Sharding+Introduction">sharding</a></em> allows for horizontal scaling of data writes by partitioning data across multiple servers using a <em>shard key</em>. It's important to <a href="http://www.mongodb.org/display/DOCS/Choosing+a+Shard+Key">choose a good shard key</a>. For example, a poor choice of shard key could lead to "hot spots" of data only being written on a single shard. </p></li> </ul> <p>A sharded environment does add <a href="http://www.mongodb.org/display/DOCS/Sharding+Introduction#ShardingIntroduction-ShardinginaNutshell">more complexity</a> because MongoDB now has to manage distributing data and requests between shards -- additional configuration and routing processes are added to manage those aspects.</p> <p>Replication and sharding are typically combined to created a <em><a href="http://www.mongodb.org/display/DOCS/Simple+Initial+Sharding+Architecture">sharded cluster</a></em> where each shard is supported by a replica set.</p> <p>From a client application point of view you also have some control in relation to the replication/sharding interaction, in particular:</p> <ul> <li><a href="http://docs.mongodb.org/manual/applications/replication/#read-preference">Read preferences</a></li> <li><a href="http://docs.mongodb.org/manual/applications/replication/#write-concern">Write concerns</a></li> </ul>
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    1. POIn Mongo what is the difference between sharding and replication?
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    1. CO"In mongodb each shard is a replica set" - this is not true. You can add individual servers as shards.
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      1. USSergio Tulentsev
    2. CO@SergioTulentsev: thanks, clarified "is" as "can be". Ideally your shard *should* be a replica set so there is some redundancy/failover. Similarly, a replica set *should* run on multiple servers (but technically, does not have to).
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    3. COI suggest removing "builds on replication" clause. It does not build on it. The two things are completely orthogonal. :)
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