Why does the API Gateway accept requests above the set value in the usage plan?

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The issue

In the API Gateway usage plan, I have set the rate and burst, but it still accepts requests exceeding the set values.
Why does the API Gateway accept requests above the set value in the usage plan?

The solution

The settings of the API Gateway usage plan are on a best effort basis, so it may accept requests exceeding the set values depending on the situation.
Therefore, a design considering the best effort is necessary.

Usage plan throttling and quotas are not hard limits, and are applied on a best-effort basis. In some cases, clients can exceed the quotas that you set. Don’t rely on usage plan quotas or throttling to control costs or block access to an API. Consider using AWS Budgets to monitor costs and AWS WAF to manage API requests.

AWS official documentation.

References

• Creating and using usage plans with API keys - Amazon API Gateway