WordPress and sharded clusters

Query: WordPress and Sharded Clusters

To handle duplicate posts when syncing data between WordPress and MongoDB, you can use a combination of strategies:

1. Unique Indexes on Shard Key

- Ranged Shard Key: MongoDB can enforce uniqueness on a ranged shard key index. This means that if the shard key is a range, MongoDB will ensure that no two documents have the same shard key value within that range[2][3].

2. Unique Indexes on Fields Other Than Shard Key

- Not Supported: MongoDB does not support unique indexes on fields other than the shard key in sharded collections. This is because insert and indexing operations are local to each shard, and MongoDB does not support creating new unique indexes in sharded collections[3].

3. Unique Indexes on `_id` Field

- Default `_id` Index: MongoDB creates a default `_id` index on each collection, which is always unique. This index is used to enforce uniqueness on the `_id` field, but only within a single shard. If the `_id` field is not the shard key or the prefix of the shard key, the `_id` index only enforces uniqueness per shard and not across shards[2][4].

4. Workarounds for Unique Constraints

- Secondary Collection: To enforce uniqueness on fields other than the shard key, you can create a secondary collection that contains the unique field and a reference to the document in the main collection. This approach ensures that the field is unique across the entire cluster[3].
- Guaranteed Unique Identifiers: Using universally unique identifiers (UUIDs) like `ObjectId` can ensure that a field has unique values. This approach is particularly useful for the `_id` field, which must be unique[3].

5. Limitations and Considerations

- Compound Shard Key: If you use a compound shard key, you can only enforce uniqueness on the entire key combination and not on individual components of the shard key[2][3].
- Sharding and Unique Indexes: If you need to shard a collection with unique indexes, you must shard on the unique field. However, if you have multiple unique indexes on a single field, you cannot shard the collection[3].

Conclusion

MongoDB sharding is a powerful technique for horizontally scaling and distributing data across multiple servers or “shards.” Understanding the components of a sharded cluster and how sharding works is crucial for managing and scaling database infrastructure efficiently. By following best practices and optimizing queries, you can ensure optimal performance and data distribution in your sharded cluster.