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Example - 1000 RUs/hr and within hr 1000 RUs are eaten up, what happen to 1001 request?

BreakHead
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  • Please see my answer [here](https://stackoverflow.com/questions/63003768/how-is-cosmosdb-ru-throughput-enforced/63006251#63006251), to a question that's essentially a duplicate of yours, which explains exactly how Cosmos DB handles requests beyond your allocated Request Units / second. – David Makogon Aug 15 '20 at 18:04

2 Answers2

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That is a really in interesting question. Some information suggests if you hit the provisioned RU/s rate limit for any operation or query, a Cosmos DB service won't execute the operation and the API will throw a DocumentClientException exception with the HttpStatusCode property set to 429. This HTTP status code means that the request made to Azure Cosmos DB has exceeded the provisioned throughput and it couldn't be executed.

429 Too many requests
The collection has exceeded the provisioned throughput limit. Retry the request after the server specified retry after duration

Cosmos DB throttles your requests intelligently When you’re exceeding your RU quota, Cosmos DB doesn’t reject your additional requests by just screaming ERROR! Not only does it explicitly flag these throttled requests with the HTTP status code 429, but the response also provides a very useful header: x-ms-retry-after-ms. As its name implies, this header tells you how much time you should wait before re-trying. Although this hint has its own limits (it may not be very reliable if multiple clients overload your RU quota at the same time), it’s still a very useful information to have in order to define the cool-off period one should wait and avoid a retry policy that would be too aggressive.

Ref

https://learn.microsoft.com/en-us/rest/api/cosmos-db/http-status-codes-for-cosmosdb#:~:text=429%20Too%20many%20requests,more%20information%2C%20see%20request%20units.

https://medium.com/@thomasweiss_io/how-i-learned-to-stop-worrying-and-love-cosmos-dbs-request-units-92c68c62c938

scott_lotus
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Cosmos DB request units are per second not per hour. So with 1000 RU/s you can run 1000 queries per second that each take 1 RU or you can run 100 queries per second that each take 10 RUs.

You might be confused because the billing is per hour based on the maximum amount of RUs you defined in that hour. So if you set 1000 RUs on a collection and then change to 400 RUs, you still get billed for 1000 RUs for the current hour.

If you exceed that number, you get a 429 error back from Cosmos DB like the other answer states. If you use the Cosmos DB SDK, you don't need to worry about this usually as they will automatically retry the query after some time if they get a 429. This retry policy is configurable, so you can decide how many times retry should be attempted.

Getting some 429s is usually expected, otherwise you might be over-provisioning throughput in Cosmos DB.

juunas
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