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Parallel Transformer stages do not accept reference links so Parallel lookups are implemented using a dedicated Lookup Stage. S2PX will therefore convert Server Transformer stages used for lookup operations into Parallel Lookup stages.
The Server Transformer’s Key Expressions are not available in the Parallel Lookup Stage - the values being used for the join need to be pre-supplied, either directly from a data source or calculated using an up-stream transformer. To handle this S2PX will identify the use of Transformer Key Expressions and generate Parallel jobs containing a pre-lookup Transformer stage which pre-calculates keys for comparison in the Lookup stage.
Note that you cannot insert a Transformer stage between a Lookup Stage and the stage being used as that lookup’s reference.
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Because S2PX replicates Server Job designs using containers you’d you would expect a S2PX-generated Job to look like this:
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Unfortunately, DataStage does not support the use of sparse lookup operations inside Job Containers where the lookup sources is located outside of the Job Containers so Container. S2PX addresses this by bringing the reference source inside the Container, resulting in a Job design looking that looks like this:
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As mentioned above, the Parallel Lookup stage does not accept user-specified expressions for key lookups. S2PX therefore injects a pre-lookup Prepare Transformer stage which pre-calculates any required Key Expressions and supplies them as one or more ephemeral column(s). These columns are only used to support the Lookup functionality and aren't propagated downstream once they have been used. S2PX also generates a post-lookup Logic Transformer stage which calculates other Transformer Stage Variables and output link derivations as required. These Transformers also perform pre- and post-processing required to support the S2PX translation of Hashed Files - See Hashed File Lookups for more details.
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An important issue this introduces is that the specially - formatted data retrieved from the Hashed File-derived database tables will need to be pre-converted before they can be used in a Parallel Lookup (which does not permit Key Expressions). The requirement to introduce leading ! characters (used as a prefix for non-null fields) and @ characters (indicating Nulls) in order to ensure lookup keys match one another is another responsibility of the Prepare Transformer, introduced above. These temporary changes are then removed in the Logic Transformer, restoring the input data to its original format. See Using Hashed-File database tables in lookups for more information.
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