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SharePoint Online uses throttling to prevent users from over-consuming resources. When a user runs CSOM or REST code that exceeds usage limits, SharePoint Online throttles any further request from the user for a period of time.
What is HTTP throttling?
Throttling allows you to set the maximum number of concurrent messages that are processed by a particular endpoint. Increased message load and large message payloads can cause memory usage spikes that can decrease performance. Throttling limits resource consumption so that consistent performance is maintained.
Why is my SharePoint Server throttling HTTP requests?
SharePoint Foundation includes a system for monitoring various Windows Server 2008 performance counters and for throttling (that is, blocking) HTTP requests, if any of those counters indicates that a server is too busy to handle all the requests that it is receiving.
Why do we need to use HTTP throttling?
HTTP Request Monitoring and Throttling helps prevent a server from running out of resources used to serve existing jobs and high priority user requests such as PUT/POST and is configured on a per-Web application basis.
When a user runs CSOM or REST code that exceeds usage limits, SharePoint Online throttles any further request from the user for a period of time. The Core.Throttling code sample in the SharePoint Patterns and Practices organization shows how to implement the exponential back off technique to handle throttling in SharePoint Online.
For requests that a user performs directly in the browser, SharePoint Online redirects you to the throttling information page, and the requests fail. For all other requests, including CSOM or REST calls, SharePoint Online returns HTTP status code 429 (“Too many requests”) or 503 (“Server Too Busy”) and the requests will fail.