What is refining a shard key and how to do it?

Difficulty: 7/10

shard key selection, refine shard key, data migration

Refining a collection's shard key allows for a more fine-grained data distribution and can address situations where the existing key has led to jumbo chunks due to insufficient cardinality.

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To refine a collection's shard key, use the refineCollectionShardKey command. The refineCollectionShardKey adds a suffix field or fields to the existing key to create the new shard key.

For example, you may have an existing orders collection in a test database with the shard key { customer_id: 1 }. You can use the refineCollectionShardKey command to change the shard key to the new shard key { customer_id: 1, order_id: 1 }:

Scenario Questions

0-2 years experience

1We have a collection sharded on {country:1}. After a few weeks most queries also filter on city. How would you refine the shard key to include city, and what MongoDB steps would you take?
2If you try to add a new field to an existing shard key without recreating the collection, what error do you expect and why?
3You need to switch from a hashed shard key to a compound key to support range queries. Which commands would you run and what immediate considerations are there?

Follow-up Questions

What are the risks if the new shard key isn’t a superset of the old one?
How would you verify that chunk distribution improved after the refinement?
Can you roll back a shard key refinement if something goes wrong, and how?