Choosing a Shard Key
The first step in sharding a large collection is to decide on a shard key that will be used to determine which documents should be stored in which shard. The shard key is an indexed field or an indexed compound field that must be included in every document in the collection. MongoDB uses the value of the shard key to split the collection between the shards in the cluster.
Selecting a good shard key can be critical to achieving the performance you need from MongoDB. A bad key can seriously impact the performance of the system, whereas a good key can improve performance and ensure future scalability. If a good key does not exist in your documents, you might want to consider adding a field specifically to be a sharding key. ...
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