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When to use the Connected and Unconnected Lookup Transformation?
If you want to return only one port from lookup, use a unconnected lookup. You want to return multiple ports go for a connected lookup. Unconnected lookup can be called multiple time within the same mapping. Use connected lookup in case you need a dynamic cache or you want more than one output ports.
Why unconnected lookup cannot be dynamic?
The primary purpose of Unconnected lookup is to be called for the lookup value multiple time based on the need and it is not a direct link to the mapping flow It is, just like a variable defined in an Expression transformation. We define the variable once and use it in multiple output ports (with the same Expression transformation).
Dynamic caching is to verify if the dataset having ‘n’ number of records have any latest updates to it which should be updated in the same run along with the insert in the first place. If the 10th record is a new customer record and the 100th record is also of the 10th Customer with an update to the email address then the dynamic cache will identify the records as the same the it will flag the 10th record as Insert and 100th record as Update to the 10th record.
Unconnected lookup cannot insert values into lookup cache file as it is not possible to find out from which part of the mapping flow and the transformation that is called from and how many number of times that is called. This functionality reduces the boundaries of Unconnected lookup to perform well only with static cache.
To be more clear. If there is a mapping with three expressions, EXP1, EXP2 and EXP3 and the Unconnected lookup is being called in all three of them for a business logic. Now, how does the Integration Service determine in which expression out of these three the Unconnected has derived a record for Insert or an Update and in what order.
Here are the differences between Connected and Unconnected lookup transformation.