Understanding of Transformer Looping & Caching  

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 Understanding of Transformer Looping & Caching:-

 Power of transformer looping is introduced from IIS Data stage 8.5 and above versions. Transformer looping is made developers life easy in developing complex scenarios which was almost difficult in earlier versions.

 loop variables:-

You can use loop variables when a loop condition is defined for the Transformer stage. When a loop is defined, the Transformer stage can output multiple rows for every row input to the stage. Loop variables are evaluated every time that the loop is iterated, and so can change their value for every output row. Such variables are accessible only from the Transformer stage in which they are declared. You cannot use a loop variable in a stage variable derivation.

Loop variables can be used as follows:

·                     They can be assigned values by expressions.

·                     They can be used in expressions which define an output column derivation.

·                     Expressions evaluating a variable can include other loop variables or stage variables             or the variable being evaluated itself. 

@ITERATION: - System variable used for transformer looping

 What is Caching in the Transformer?

 The Transformer Cache is an in-memory storage mechanism that is available from within the Transformer stage and is used to help solve complex data integration scenarios. The cache is a first-in/first-out (i.e. FIFO) construct and is accessible to the developer via two new functions:

•  SaveInputRecord: stores an input row to back of the cache

•  GetInputRecord: retrieves a saved row from the front of the cache

I will try to explain the transformer caching and looping using below scenario:-

Below is my input data:-

Customer	Store	Products	Value
600785011	103	10	100
600785011	104	20	200
600785011	109	30	300

Output:-

For each customer get a list of the stores he shopped, with this list output every combination of stores dropping customer from the output.

Home Store	Products	Value	Away Store	Products	Value
103	10	100	103	10	100
103	10	100	104	20	200
103	10	100	109	30	300
104	20	200	103	10	100
104	20	200	104	20	200
104	20	200	109	30	300
109	30	300	103	10	100
109	30	300	104	20	200
109	30	300	109	30	300

Below is Job Design:-

  SeqFile -------> TRFM1-------> TRFM2--------->TempDataset

TSFM1:-

 Define below stage variables in transformer:-

 NumSavedRows= SaveInputRecord

IsBreak= LastRowInGroup(to_tsfm1.Customer)

svCurrStore= If IsBreak Then to_tsfm1.Store:"|": svPrevStore Else to_tsfm1.Store

svPrevStore= If IsBreak Then to_tsfm1.Products else  to_tsfm1.Products :"|": svPrevProduct svCurrProduct= If IsBreak Then to_tsfm1.Products :"|": svPrevProduct Else to_tsfm1.Products

svPrevProduct= If IsBreak Then to_tsfm1.Products else  to_tsfm1.Products :"|": svPrevProduct

svCurrValue= If IsBreak Then to_tsfm1.Value :"|": svPrevValue Else to_tsfm1.Values

svPrevValue= If IsBreak Then to_tsfm1.Value else  to_tsfm1.Value :"|": svPrevValue

NumRows= If IsBreak Then NumSavedRows Else 0          

 Define below loop conditions & Derivation:-

                       @ITERATION <= NumRows

OutputRecord= GetSavedInputRecord()

Output Derivations:-

Store=Inputlink.Store

Product=InputLink.Product

Value=InputLink.Value

ConcatStore=svCurrStore

ConcatProduct=svCurrProduct

ConcatValue=svCurrValue

Customer=InputCustomer

The logic of this transformer is as follows:

1 input data is read and stored in the cache via the "SaveInputRecord" function.
2.key breaks are tested via the "LastRowInGroup.
3.svCurrStore, svCurrProduct, svCurrValue will concatenate with pipl all stores,Products & values in the same group.
4.if there is a key break, the " NumRows " variable is set to the number of records on the cache and we'll skip to step 6
5.if there is no key break(NumRows=0) the Loop Condition will be false.  This will cause the next input record to be read and processed .
6.finally, the transformer will loop while "@ITERATION <= NumRows " and read the cache records.

TSFM2:-

Define below Stage Variable:-

                         RowNum=Count(InputLink1.ConcatStore,”|”)

 Define below loop conditions :-

                          @ITERATION <= RowNum

 Output Derivation:-

Store=InputLink1.Store

Product=InputLink.Product

Value=InputLink.Value

AwayStore= Field(to_tsfm2.ConcatStore,"|",@ITERATION)

Product= Field(to_tsfm2.ConcatProducts,"|",@ITERATION)

Value= Field(to_tsfm2.ConcatValue,"|",@ITERATION)

Note:- I have just tried to explain better understanding of Transformer looping using this scenario in my blog