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.

Types of throttles in BCS:

1. One is a row per fetch against a database, which limits the number of rows BCS will retrieve from the database.
2. The second is the typical timeout for your operations against the backend.
3.   The third type is global, meaning that regardless of the type of connection throttle, you can throttle the number of connections to the backend system, so you don’t overload the backend with a lot of connections.

           So, here are the out of the box throttling limits for external lists.  Note that these limits are applied per instance of the Business Data Catalog service application; if you have multiple instances then each one can have different values:

Connection Type	Type of Throttle	Limit
Database	Rows per fetch	2000
Database	Timeout	3 Minutes
Web Services/WCF	Size of fetch	30MB
Web Services/WCF	Timeout	3 minutes
Global	Connections	100

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