Observe rate-limiting events


In this unit, we'll discuss methods of how to identify rate-limiting events.

While using the Azure Cosmos DB for NoSQL, Azure Cosmos DB might return a 429 status code error. This error code indicates that a Request rate too large exception has occurred. This exception means that Azure Cosmos DB requests are being rate limited.

When provisioned throughput is used, the request units per second (RU/s) is set for the workload. Operations (read, writes, queries) against the service consume request units (RUs). If in any given second, the operations consume more RUs than the provisioned RU/s, Azure Cosmos DB will return a 429 exception.

There are three main reasons why we get a 429 exception:

  • Request rate is large.
  • The request did not complete due to a high rate of metadata requests.
  • The request did not complete due to a transient service error.

We'll look at each of these reasons and how to identify them.

Request rate is large

Out of the three reasons for this exception, this reason is the most common one. Azure Cosmos DB returns this exception when the RUs by operations on data exceed the provisioned RU/s.

To investigate the possible cause for a 429 exception:

  1. Open up your Azure Cosmos DB account page in the Azure portal.
  2. Under Insights->Request menu, look at the Total Request by Status Code charts, and search for occurrences of the 429 exception.
  3. If needed, filter the charts by Time Range and Database to your desired time span and database.

For most applications, it's normal to have less than 5% of the requests with 429 exceptions.

Diagram that shows the charts by Status code and Throttled Request (429 exceptions).

If the percentage of 429 exceptions is higher than 5%, it's possible that the exceptions are caused by a hot partition.

To verify if the database access is coming across a hot partition, in the Azure portal under the Azure Cosmos DB account, under Insights->Throughput, review the Normalized RU Consumption (%) By PartitionKeyRangeID charts.

Diagram that shows the charts by throughput of a hot partition.

Rate limiting on metadata requests

A 429 exception can occur when doing a high volume of the following metadata operations:

  • Create, read, update, or delete a container or database
  • List databases or containers in a Cosmos account
  • Query the current provisioned throughput

To investigate if 429 exceptions are caused by Metadata requests, in the Azure portal under the Azure Cosmos DB account, under Insights->System, review the Metadata Requests That Exceeded Capacity (429s) charts.

Diagram that shows the charts for metadata access.

Rate limiting due to transient service error

Around the time you're noticing 429 exceptions happening, you might also notice 503 errors being reported in the Insights->Request Total Request by Status Code charts. These exceptions could indicate that the 429 exceptions are happening because of transient service errors.