The Basics of ETL Testing

Imagine spending lots of time and resources to move your data to a new system, only to realize that the data is incomplete or inaccurate. Not only would this result in costly mistakes, but it could also lead to costly business decisions based on incorrect data. That's a problem many face in their Extract, Transform, and Load (ETL) process.  

So if you're on a mission to integrate and migrate your data to a new system, ETL testing is a process you cannot afford to miss. It acts as a safety net for your data that ensures completeness, accuracy, and reliability.  

ETL testing can be challenging simply because of the volume of data involved. Plus, the data is almost always heterogeneous, which adds another layer of complexity. But it doesn't have to be difficult. With the right knowledge and tools, even beginners can get started with ETL testing in no time. 

In this blog, we'll cover everything you need to know about ETL testing, including ETL testing concepts, processes, challenges, and best practices. By the end of this blog, you'll be ready to do your own ETL testing like a pro.  


What is ETL testing?

ETL testing is the process of verifying that the data from source systems has been extracted completely, transferred properly, and loaded into the new system in the correct format. It's a crucial component of data warehouse testing. 

By identifying and preventing data quality issues such as duplicate data or data loss, ETL testing saves you precious time and resources in the long run. It also ensures that your integration and migration processes are smooth and efficient, and that there are no bottlenecks that might impact the performance of either the source or destination systems.  


When do you need ETL testing?

ETL testing can be helpful:

  • When setting up a data warehouse for the first time, after data is loaded.
  • After adding a new data source to your existing data warehouse.
  • After a data integration project.
  • After a data migration project.
  • When moving data for any reason.
  • If there are suspected issues with data quality in any of the source systems or the target system.
  • If there are suspected issues with the performance of ETL processes.


An ETL tester's roles and responsibilities

As an ETL tester, you play a critical role in ensuring the accuracy and completeness of data during the ETL process and ultimately helping the organization make informed business decisions based on reliable data. Here are some of the most important responsibilities of an ETL tester:

  • Test planning and preparation: Develop the overall testing strategy, test plan, and test cases for the ETL process.
  • Data analysis: Analyze source data to ensure it is complete, accurate, and consistent, and ensure that the data is transformed properly during the ETL process.
  • Test execution:  Execute test cases to validate the ETL process, ensuring that the data is loaded correctly and that the business rules and requirements are met.
  • Defect management: Identify and report defects or issues in the ETL process, as well as work with the development team to resolve them.
  • Communication: Communicate testing results, defects, and other relevant information to the development team and other stakeholders.
  • Continuous improvement: Work to continually improve the testing process, incorporating best practices and new technologies to ensure the accuracy and efficiency of the ETL process.


4 types of ETL testing

There are four main types of ETL testing for different stages and functions.

  • Unit testing tests individual components of the ETL process, such as source extraction, data transformation, and target loading. It's critical for identifying defects in individual components and ensuring that they function as expected.
  • Integration testing tests the interaction between various components of the ETL process. Integration testing is critical for identifying defects in the interaction between components and ensuring that the ETL process functions as a whole.
  • Performance testing tests the ETL process under different load conditions. Performance testing is critical for identifying performance bottlenecks and ensuring that the ETL process can handle large volumes of data.
  • Regression testing tests the ETL process after any changes or upgrades. Regression testing is critical for ensuring that the changes or upgrades do not impact the existing functionality.


8 steps of ETL testing process: How to test ETL effectively?

Creating effective ETL tests requires a thorough understanding of the ETL process, business requirements, and data sources. Here are 8 steps of an effective ETL testing process:

  1. Identify your requirements. Before you start your ETL testing, you need to first understand your business requirements, which involve defining data sources, the target system, and the level of transformation required between them. Look at the existing data model and update it if necessary.
  2. Assess your data sources. Perform a count of records of the primary source table so that you can later confirm that all of the data was moved.
  3. Create test cases. Test the data on the target system for completeness, quality, and performance. The test should cover all possible scenarios and edge cases.
  4. Extract that data. Begin your ETL testing by extracting data from the source systems. Make sure all of the data has been extracted properly and completely.
  5. Transform that data. Perform the necessary data transformations to ensure that the data is in the appropriate format for the target system and make sure that it complies with business rules.
  6. Load that data. Load the transformed data into the target destination. Check the record count to confirm that all of the data has been moved. Confirm data integrity by checking to see if any records were rejected by the target system and not loaded.
  7. Document your findings. Document any bugs or issues that you’ve identified during the test process. If necessary, adjust your processes to address the issues and repeat steps 4 through 6 again.
  8. Conclude testing and proceed with ETL. The testing process is complete! You can now proceed with ETL, confident in the knowledge that your data is accurate and consistent.

Note: If you're using a cloud-native data integration platform that performs ELT instead of ETL, you'll need to switch steps 5 and 6 around – load the data to the cloud first, then transform that data in the cloud.  


ETL testing challenges

The ETL testing process is often not described as smooth sailing. It involves many challenges such as:

  • Complex data transformations: Transforming data from one format to another can be time-consuming and complicated, especially when dealing with large and complex datasets in different formats, which can feel like comparing apples to oranges at times 
  • Data quality issues: ETL testing requires clean and accurate data, but real-world data is often messy and inconsistent. Poor data quality can lead to inaccurate results and compromise the integrity of your data.
  • Limited testing resources: ETL testing can be a resource-intensive process, especially when dealing with large datasets or complex systems.
  • Changing data sources: Changing data sources can affect the accuracy and completeness of your data, making it difficult to maintain data quality over time.
  • Integration challenges: Integrating different systems and data sources can be challenging, especially when dealing with complex business processes.
  • Performance issues: Massive data volume can slow down your processing time or cause system crashes, which might affect the accuracy and completeness of your data.
  • Lack of expertise: ETL testing requires a specific skill set and expertise, which can be a challenge for organizations that lack experience in this area. But, worry not, by the end of this article, you should already be an ETL testing pro!


ETL testing best practices

These ETL testing best practices will help you get a better ETL testing experience.

  • Test early and often: Catching defects early in the testing process can save you time and resources in the long run.
  • Use realistic test data: Using realistic data ensures that your testing accurately reflects real-world scenarios.
  • Automate your testing: Automating your testing saves time and effort and ensures consistent and thorough testing.
  • Collaborate with stakeholders: Collaborating with stakeholders ensures that your testing aligns with business objectives and priorities.
  • Follow a structured approach: Following a structured approach ensures consistency and comprehensiveness in your testing.
  • Test for performance: Testing for performance ensures that your ETL processes can handle large volumes of data and are optimized for speed.
  • Test after any changes or upgrades: Testing after any changes or upgrades ensures that everything is still working as expected and that new changes haven't introduced any issues.


Choosing the right ETL testing approach

ETL testing is a crucial step in data integration, and it involves various stages and types of testing. By following the best practices and using the right tools and techniques, you can ensure successful ETL testing and make informed decisions based on accurate and consistent data. ETL testing requires patience, attention to detail, and a little bit of creativity. 

Before you get started with ETL testing, it's important to determine what methods or tools are right for your organization. Many organizations use SQL scripting to test their ETL processes. Doing your own scripting may save money in the short term, but could potentially take longer than using existing data integration solutions that support ETL testing. It’s important to work with all of the relevant stakeholders before making decisions about how to proceed. A kick-off meeting with key stakeholders can help you determine the best course of action.  

Get started with ETL testing in the cloud

Many organizations today encounter increasing volumes and complexity of data, which can only be handled in a cloud environment. However, testing cloud-based ETL processes requires different technologies and approaches than traditional ETL testing.  

Data integration platforms like the Matillion Data Productivity Cloud is purpose-built to provide the ETL tools and functionality that meet the scalability, flexibility, and cost-effectiveness demands of cloud data environments. 

Matillion extracts and loads the data into the cloud first and then takes advantage of the processing power of your cloud data platform to transform that data in the cloud.  This process, push-down ELT, instead of traditional ETL, leverages the flexibility and virtually unlimited scalability of the cloud. As a result, it is ideally suited for ETL testing of data that is migrated to the cloud.  

The only caveat is that, with a cloud-native push-down ELT solution like Matillion, you would switch the order of the loading and transformation steps outlined in the 8 steps above for ETL testing – load the data to the cloud first and transform the data in the cloud.  

Matillion Data Loader is a free SaaS-based data integration solution that seamlessly extracts data and then loads it into your cloud data warehouse. With a code-free, wizard-based pipeline builder to common data sources like Salesforce, Google Analytics, and more, Matillion Data Loader can help make your first data migration to the cloud and subsequent data loading quick and easy. It’s also free. Sign up today to try Matillion Data Loader. 

Matillion ETL is a cloud-native data integration and transformation solution, purpose-built to support leading cloud data warehouses, including Snowflake, Amazon Redshift, Google BigQuery, and Microsoft Azure Synapse, as well as Delta Lake on Databricks. From extracting and loading your data to performing powerful data transformations, Matillion’s cloud-native ETL solution offers cloud-native architecture and performance compared to legacy ETL tools. Matillion leverages the performance and scale of the cloud, making complex data transformations fast, secure, and cost-efficient. 

Request a demo to see a hands-on tutorial and learn how Matillion fits into your ETL requirements and process.

Andreu Pintado
Andreu Pintado