Thursday, June 26, 2014

Decoding Data Mapping Sheets - ETL Process

An ETL (Extract Transform Load) process is all about moving a variety of data from Source System to the destination Data Warehouse System by taking data through a number of extraction, transformation, data cleansing & data validation processes.

Just imagine how easy it will get for someone as an ETL developer if he gets a chance to visualize all the transformations & business rules up front in an easy to interpret format. This is where mapping sheets come into picture.  

A carefully designed mapping sheet up-front can save a lot of pain as handling mapping information increasingly gets difficult as application grows with time. Only downside of using them is it takes good effort to create them & then keeping them up-to-date. But, trust me, rewards of using them easily outnumber the pain of not maintaining one as the system grows overtime
Each data migration project uses mapping sheets in one form or the other but the one which I have used too often & has worked exceptionally well for me is what I am detailing in this here.

To start with a sample Mapping Sheet would look something like this for any given entity as represented in Figure 1. The sheet can be extended further by using multiple sheets in the Excel workbook to represent other entities of the system.

Fig 1 – Sample Mapping Sheet structure

Generally, the extreme left of the sheet represents data from the Source System or the staging area. The middle layer represents data validation, data verification, data cleansing & business rule validation with a number of data-centric transformation rules in place. There can be more than 1 transformation layer in the centre depending upon how complex the ETL process is. The extreme right mostly represent Data Warehouse system.

In the Figure 2 below, I have taken example of Employee table in staging area to represent how actual table structure from database gets represented in a mapping sheet.

Fig 2 – How a database table structure is represented in a Mapping Sheet Document

Carefully looking at the snapshot in figure 2, it provides almost all the information related to a table in a database. The information includes everything like - Schema name (stg), Name of the Table [Employee] along with name of the columns, data types & whether they allows NULL or NOT NULLS values. For eg - EmployeeID is INT & NOT NULL whereas EmployeeName is going to be varchar(255) NULL column.

On a similar line, mapping sheets can be extended to further represent structure of the given tables across different layers. Figure 3 below further helps you visualize how a field gets mapped, validated & transformed through different layers of the ETL process while being migrated from Source System to ODS to DW.

Fig 3 – Transformation rules at the table/field level

Another efficient use of mapping sheet would be to document Business rules in layman’s term against respective field right next to them. Please refer to Figure 4 below. The figure represents only a selective few & a very basic level transformation rules. Please note, the transformation rules represented here are only from informative purpose & real time transformation could vary considerably depending upon project requirements. The mapping sheets only works as placeholders to store transformation information.

Some of the sample business rules could be like
1.  Use database defaults to set to Current Date & Time for CreatedOn, ModifiedOn fields.
2.  Use SQL Case statement to set a field to an integer value in warehouse depending upon text information coming in from source feed.
3.  Placeholder to store I, U, D flags in ODS layer to perform respective actions in warehouse for a given entity – just to name a few
4.  Other transformations would be something like these -

Figure 4 – Sample Transformation Rules :

Some of the key points that must be considered while designing a mapping information document:    
1        Design staging area with minimal constraints/indexes and with no or minimum data integrity checks in place. Consideration for Extraction process must be to load data from Source System as quickly as possible with minimal data leakage. Ideally, data type in this layer should be minimally restrictive to allow full data to pass through to staging area without any data loss. Like allowing NULLs, data size sufficiently big enough to hold data from feeding source system. 

2        Operational Data Store Layer - In this layer most of the business rules are defined and data types are generally tightly coupled with data types in warehouse layer. Most of the data transformations, error handling & data filtering are done in this layer. An ideal ODS layer should be able to maintain audit trail information to keep track of I, U, D operations to Data Warehouse. In this example, “Action" field in ods.Employee table can be used to maintain current state of a record throughout the life cycle of ETL process.                         

3        Data warehouse Layer - This layer contains only current version of data. The records normally gets Inserted, Updated & Deleted in Data Warehouse depending upon incoming “Action” field from ODS source. Normally, only selective few transformation are performed in this layer, the ones which are specific to data available in data warehouse s.                          

With this, I will now sign off on this topic. I have tried keeping this information in its basic & simplest form so I hope this information will come handy in your respective projects. Please do share your inputs, feedback & comments & I will be more than happy to amend/improve this posting further.


Rishi Saxena
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