Good Practise in Transformation Rule

It is discouraged to use the field routine to translate certain fields to its mappings or read certain attributes from the master data if the source data is huge. This is because the data is processed one single row each time the routine gets processed when it goes through the main loop instead of as the complete data package in an internal table in the start or end routine. The best practice in a transformation is to:

• Filter data in start routine and perform business mapping rules in the end routine. Usually in the end routine,you will work with the result package to capture error stack records and for re-transformation.
• Declaring global variables and capturing data in Global internal table at start routine. 
• Using constant to pass derived value from one field to the next