Cloud Endpoints provides quotas, which let you control the rate at which applications can call your API. Setting a quota allows you to specify usage limits to protect your API from an excessive number of requests from calling applications. The excessive requests might have been caused by a simple typo or from an inefficiently designed system that makes needless calls to your API. Regardless of the cause, blocking traffic from a specific source once it reaches a certain level is necessary for the overall health of your API. By setting a quota, you ensure that one application cannot negatively impact other applications that use your API.
To learn more about quotas, see the "About Quotas" page in the documentation for your API implementation: