What is ETL Testing and How it Works

In this post we define ETL and discuss what does it mean exactly. We will then define ETL testing, the process, how it works, and why it’s important. Moreover, we will discuss the relevance of ETL today, common users and use cases, and pros and cons. Last, we will provide some best practices to be followed for optimal ETL testing.

 

What is ETL?

ETL is a process in data warehousing that is responsible for pulling data out of the source systems and placing it into a data warehouse. This process includes the performance of three tasks: Extracting the data, transforming the data, and then loading the data. Data is extracted from an OLTP database, transformed to match the data warehouse schema and loaded into the data warehouse database.  Many data warehouses also incorporate data from non-OLTP systems such as text files, legacy systems and spreadsheets.

For more info on ETL, check my other blog post regarding this

ETL Testing

ETL testing is done to ensure that the data that has been loaded from a source to the destination after business transformation is accurate. It also involves the verification of data at various middle stages that are being used between source and destination.

 

ETL testing is performed in five stages. First one is identifying the data sources and requirements for the system in use.Second is performing an efficient and timely data acquisition. Followed by implementing business logics and dimensional Modelling. Then, building and populating general and targeted data. Finally, generating and analyzing reports that reflect the performance.

 

ETL testing is about discovering problems in the data stream and correcting them. In the past, this was done in a waterfall approach by identifying problems in the data or the ETL process, building a system to resolve those problems, testing that everything works, and rolling into production. But it is expected that the future of this field will be more of an agile process in which data issues are fixed on the fly, and largely automatically, with no interruption to data ingestion.

 

The general methodology of ETL testing is usually to use SQL scripting, which can time-consuming, error-prone and seldom provide complete test coverage. To accelerate, improve coverage, reduce costs in production and development environments, ETL can be automated completely using various tools.

 

Why is it relevant?

Data Warehouse testing assures that information is not just loaded correctly, but that it is appropriately aggregated, catalogued and verified so that it is useful and accurate for analysis and decision-making. It is worth noting that the ETL process is particularly vulnerable in this regard.

 

An example of its necessity with regards to performance and scalability related issues, a hypothetical successful client who opens new branches in other cities and has not tested the scalability of their data warehouse. As queries to the BI system increase, the system bogs down and the response time slows down. This will cause customers to contact a competitor who is able to handle the increased business.

 

While it seems obvious it is also worth noting that testing should go hand-in-hand with any application changes or new releases. This is regression testing, in fact, automated regression testing is typically performed weekly or even daily, depending upon users’ or the system’s requirements.

 

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Common Challenges

ETL Testing is different from application testing because of the fact that it operates on a data centric testing approach. Many challenges are faced sometimes when dealing with ETL, some of which are: ETL Testing involves comparing large volumes of data, that is usually in the millions of records, the data that needs to be tested is in heterogeneous data sources (eg. databases, flat files), data is often transformed which might require complex SQL queries for comparing the data, ETL testing is very much dependent on the availability of test data with different test scenarios.

 

Best Practices

Many users are capable of increasing the efficiency of their tools notably by simply getting to know how to use it well. First advice always is to research, ask, question and wonder around, there are many resources with different features for different purposes.

 

Achieving proficiency in using ETL Testing can be achieved generally by working on multiple points, the user, for example, should make sure that the data is transformed correctly without any data loss and that the truncation projected data is loaded into the data warehouse. The user should also ensure that the ETL application appropriately rejects and replaces with default values and reports any dysfunctions or errors. You need to also ensure that the data loaded in the data warehouse is within the prescribed and expected time frames to confirm scalability and performance. All the methods should have appropriate unit tests,that should use appropriate coverage techniques to measure their effectiveness. Finally, the user should create unit tests that target outliers, exceptions and unusual cases that might be a sign of a a specific error or malfunction.

 

Closing Thoughts

Building a data warehouse is a major undertaking that’s expected to yield substantial business benefits in order to justify the cost and effort. To ensure that your ETL data warehouse project lives up to expectations, ETL testing must be front and center, particularly in the all-important early stages of the project being worked on.

 

We have seen the definition of ETL and ETL Testing. After that, we saw different reasons that make relevant for lots of users. Later on, we elaborated on some best practices and a checklist to follow when working on an ETL tool, finishing with those closing thoughts.

 

Reference

ETL Testing the Future is Here

https://www.alooma.com/blog/etl-testing-the-future-is-here

ETL Testing or Data Warehouse testing tutorial

https://www.guru99.com/utlimate-guide-etl-datawarehouse-testing.html

Basics of ETL Testing

http://www.datagaps.com/concepts/etl-testing

Why is ETL Testing so Important?

https://www.coherentsolutions.com/blog/why-is-etl-testing-so-important/