The Internal Mechanics of Request Unit (RU) Consumption during Index Transformation

Index transformations in Cosmos DB are performed in the background using 'spare' RUs, but they can still impact overall system performance if provisioned throughput is too low.

When you update an indexing policy (e.g., adding a composite index), Cosmos DB performs an online transformation. Unlike traditional databases that might lock tables, Cosmos DB does this asynchronously. It uses internal RUs not utilized by client workloads. However, the transformation itself competes for local storage I/O and CPU. If the container is under heavy load (high 429 rate), the index transformation will slow down significantly as it yields to user requests.