List Throttling in SharePoint 2010

1. Request throttling can be used to set the maximum retrieval limit level of the list items from the database.
2. Best technique to handle large data like when the size of folders and items increases.
3. One of the best techniques to optimize the performance.
4. The List view threshold limit is 5000 for users and 20,000 for auditors and administrators.
5. Possible to set the “Request throttling” limit is at the web applications level only [Go to Administration –> Application Management –> Manage Web Applications –> select site –> General settings menu –> select Resource Throttling].
6. The way to set the threshold limit from the Development side is by setting the “MaxItemsPerThrottledOperation” property value to the SPWebApplication.
7. But we can’t set the “Request throttling” limit value at the specific list and library level. The only possible way is either by enabling or disabling the “EnableThrottling” property of SPList. By default it is in disable state.
8. List View Lookup Threshold limit is 8 by default.

When/Where/Why Request Throttling or how it’s handle the performance:

One of the main sources of performance problem in SharePoint is either users or developers writing too many items to a list and then requesting a view or a query that tries to return a large number of items.

External list throttling in SharePoint 2010

 External list Throttling:

Same as internal list throttling, SharePoint also supports External list throttling. External lists are lists that are connected to other systems using BCS and External content type.

If you don't have any idea on list throttling concept, refer this post "List Throttling".

Why throttling for external lists also: sometimes when connecting to backend systems, the backend can take a long time to respond, or you might try to retrieve too many items, overloading the backend. To solve this problem, BCS implements a throttle per connection type whether that is a database or web service that you are connecting to.