Question: How does MongoDB handle unique indexes in sharded collections?

Answer

MongoDB uses sharding to distribute data across multiple servers, improving read/write performance and providing horizontal scalability. When it comes to implementing unique indexes in a sharded environment, there are specific considerations and constraints to ensure data integrity and uniqueness across shards.

Unique Indexes on Sharded Collections

In MongoDB, a unique index ensures that the collection does not have more than one document with the same value for the indexed field(s). However, achieving uniqueness in a sharded collection is more complex due to the distributed nature of the data.

Shard Key and Unique Indexes

• Unique Shard Key: By default, if you create an index on the shard key itself (or a compound shard key), MongoDB will enforce uniqueness for that shard key across all shards. This is straightforward because the shard key determines the distribution of documents among shards.

db.collection.createIndex( { \"shardKeyField\": 1 }, { unique: true } )

• Non-Shard Key Unique Indexes: Creating a unique index on a field (or fields) that is not part of the shard key requires careful consideration. For such indexes, uniqueness is enforced globally, across all shards. However, to maintain global uniqueness, all insert, update, or delete operations involving the uniquely indexed field must include the shard key. This requirement ensures that MongoDB can route the operation to the correct shard(s) and verify uniqueness efficiently.

db.collection.createIndex( { \"nonShardKeyField\": 1 }, { unique: true } )

Note: If a unique index does not include the shard key, MongoDB will still enforce uniqueness across all documents in the collection. This can lead to performance issues during write operations, as MongoDB may need to check multiple shards to ensure uniqueness.

Considerations

1. Performance: While unique indexes are beneficial for data integrity, they can impact write performance, especially in sharded collections. Ensure that your application's design accounts for this.

2. Shard Key Selection: Choosing an appropriate shard key is crucial. It affects not only query performance but also how you can implement unique indexes. In some cases, including additional fields in a compound shard key might be necessary to support unique indexes on non-shard key fields efficiently.

3. Data Modeling: Sometimes, adjusting your data model can avoid the need for unique indexes on non-shard key fields. Consider embedding related information directly within documents or using application-level logic to enforce uniqueness when possible.

In summary, while MongoDB supports unique indexes in sharded collections, it requires careful planning around shard key selection and understanding the limitations and performance implications of enforcing uniqueness at scale.