Diagnose: Cosmos DB 429: request rate too large, RU/s exhausted

SYMPTOM Cosmos DB 429: request rate too large, RU/s exhausted

CAUSE The container's provisioned RU/s is below what the workload asks for. 429 with a RetryAfterMs header is throttling, not an outage; the fix is query efficiency or more throughput, not retries alone.

CONFIRM IT CONFIRM: portal metrics show normalized RU consumption at 100% on the throttled partition; SDK response headers carry the request charge and retry-after. Check whether one hot partition key drives it (uneven partition distribution).

FIX FIX: short term: honor retry-after with backoff (the SDK does this by default - do not disable it). Real fix: add a partition-key filter to cross-partition queries, or raise RU/s (az cosmosdb sql container throughput update), or enable autoscale. For a hot partition, the partition key design is wrong and needs a migration.

VERIFY VERIFY: normalized RU consumption drops below 70% at peak and 429s stop in metrics.

Diagnostic skill indexed by the exact error string. Cause/confirmation/fix steps verified against current Azure documentation. Evidence: https://learn.microsoft.com/en-us/azure/cosmos-db/nosql/quickstart-python