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NoSQL
The HTTP status code 409 represents the request had a conflict while processing.
Partition Key Collision
Azure Cosmos DB uses hash-based partitioning scheme to achieve horizontal scaling of data. All Azure Cosmos DB containers created before May 3, 2019 use a hash function that computes hash based on the first 101 bytes of the partition key. If there are multiple partition keys that have the same first 101 bytes, then those logical partitions are considered as the same logical partition by the service, which creates a partition key collision. This can lead to issues like partition key collisions, partition size quota being incorrect, unique indexes being incorrectly applied across the partition keys, and uneven distribution of storage. Users might get the error "Resource with id already exists with a conflicting hashed partition key, Please retry with a different partition key" with 409 response code on document creation. Large partition keys are introduced to solve this issue. Azure Cosmos DB now supports large partition keys with values up to 2 KB.
Large partition keys are supported by enabling an enhanced version of the hash function, which can generate a unique hash from large partition keys up to 2 KB.
Solution
Use large partition key. Enabling large partition keys can only be done at time of container creation. If you have an existing container that doesn't use large partition keys, you will have to create a new container and migrate your data to it.