Routing doesn't increase the amount of shards and will route you documents to available shards. That means that you can have documents with different user_id in a single shard, so in addition to routing you should also add filtering for the same user_id if you wan to get documents for a particular user only.
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I'm curious about using routing.
Let's assuming I use my user_id for routing. If I understand it correctly, then all documents for that user id
will be in it's own shard.
I'm not sure how this co-exists with the need to specific the # of shards when creating an index.
Does routing automatically increase the # of shards when it sees a new routing id or does it map
routes into the existing set of shards.