Data Quality Testing Patterns
The insights generated by a data platform are only as good as the quality of the underlying data. I briefly mentioned data quality in my Notes on Data Engineering blog post and elaborated on the DataCop solution in the Partnering for data quality Medium article I co-authored with my colleagues.
In this post I want to talk about some of the requirements and common
patterns of data quality test solutions. I maintain that in the future
data quality testing will be commoditized and offered
as a service
by cloud providers. As of today, it is still something we have to stitch
together or onboard a 3rd party solution. Below is a blueprint for such
Any data quality test run is eventually translated into a query executing on the underlying data fabric. In Azure, this can be, for example, Azure SQL, Azure Data Explorer, Data Lake Storage etc. Data quality frameworks need to support multiple data fabrics for several reasons. First, in a big enough enterprise, data will live in multiple systems. Second, different data fabrics are optimized for different workloads, so even if we are very strict about the number of technology choices, sometimes it simply makes sense to leverage different data fabrics for different workloads.
We probably don't want to manage multiple data quality solutions specializing in different data fabrics, since data quality solutions become part of our infrastructure. Regardless of target data fabrics, any data quality solution needs to implement some form of test scheduling, alerting etc.
The best pattern is to support multiple data fabrics through a plug-in model, so support for additional data fabrics can be easily added by implementing a new plug-in, while the core of the system is shared for all data fabrics plugged into the system.
Types of Tests
The 6 Dimensions of Data Quality article talks about completeness, consistency, conformity, accuracy, integrity, and timeliness. I think of data quality testing like unit testing for code, so I will present a slightly different take on this. Some of the dimensions are harder to test for than others, for example checking consistency across multiple data systems is non-trivial (more equivalent to an integration test). Some of the dimensions have nuances -- for example detecting anomalies in day over day data volume change to identify potential issues vs. just looking at a point in time.
The simplest type of data test is availability, meaning is data available for a certain date. If, for example, we ingest yesterday's telemetry data every night, we expect to have yesterday's telemetry data available in our system tomorrow.
This type of test can be implemented as a query against the underlying data fabric that ensures some data is there. That can mean a query that returns at least one row passes the test. This can be an early smoke test we can run before performing more involved testing.
This type of test ensures data is correct, based on some criteria. For example, ensuring columns that shouldn't be empty are not empty, values are within expected ranges, and so on.
This type of test can be implemented as a query against the underlying data fabric that ensures no out-of-bounds values are returned. Going back to the unit test equivalency, we likely want multiple correctness tests per dataset, one or more per column we want to test.
Completeness tests expand on availability tests and validate not only that some data is available, but that all the data is available.
In some cases, this can be an exact query: for example, if we are expecting some data for all 50 states of US, we can check that the count of distinct states is 50.
More advanced checks look at historical data and ensure the volume of the data we are checking is within a certain threshold -- for example, our telemetry data volume should be within +/-5% of the data volume we observed yesterday.
We won't go to deep into this, but there are some complexities associated with the previous type of historical data checks. For example, website traffic volume, depending on the website, might be very different day over day between weekends and workdays, or during holidays etc.
For these situations we can use more complex AI-based anomaly detection to track volume over time until the system can identify anomalous data.
Queries, Code or Configuration
The implementation of tests can be done as queries, code, configuration, or a mix.
Implementing all tests as queries means writing each test as a stored procedure (or equivalent), so it is fully implemented on the data fabric it executes against. The test framework just invokes the test and reads the result. The main challenge with this is that there is not a lot of reuse. Since the framework ends up just calling a stored procedure, it is up to the test author to write the test, which is ultimately an arbitrary query.
Implementing all tests as code means wrapping each data quality test into a test method in some programming language. The main advantage of this is we can use an off-the-shelf test framework with all its benefits. Unlike data quality testing, the field of software testing is mature. On the other hand, the main drawback is that authoring tests as code really raises a barrier to entry, making it harder for non-engineers to create tests.
Tests as configuration means defining tests using text configuration files in a format like JSON, YAML, or XML. The framework interprets these and translates them into the final queries to execute against the data fabric. The main advantage of this approach is a fairly data fabric-independent way to specify tests, around which we can build schema validation etc. The disadvantage is increased complexity in the framework, as it must translate configuration to queries.
What works best, from my experience, is a mix of queries and configuration: give enough flexibility in the config schema to author custom queries and let the framework handle some of the common concerns like scheduling.
Another important concern is when we want to run a given data quality test.
In some scenarios we want to run tests as part of a data movement workflow. We want to either check that the data is in good shape at the source, before we copy it, or check that the data is in good shape after it got copied to the destination. Let's call this type of execution on demand. This type of test execution is integrated in the ETL pipeline.
In other scenarios we want to run the tests on a schedule, as we expect the data to be available. For example, if data should have arrived in our SQL database by 7 AM every morning, we want to run our data quality tests at 7 AM to make sure everything is in good shape. In this case, we don't really care how the data gets here, so we are running independent of the ETL pipeline that brings the data in. Let's call this scheduled execution.
A good data quality test framework should support both on demand and scheduled test execution, as there is room (and need) for both types of tests.
Monitoring and Alerting
A data quality framework must be integrated with an alerting system such that whenever a data quality test fails, data engineers are proactively alerted so they can start investigating/mitigating the data issue.
A dashboard showing the overall health of the data platform is another important component. This should include data quality for all datasets under test, so stakeholders suspecting a data issue can quickly check to see when did the latest tests run for a given dataset and what were the results.
A slightly more advanced capability is lineage tracking -- if we identify an issue with a dataset, do we know what is the impact? What analytics or machine learning models or downstream processing is impacted? Tracking metadata like lineage is a deep topic itself but integrating this with a data quality framework enables very powerful observability.
In this post we covered some patterns for data quality testing:
- Supporting multiple data fabrics via a plug-in model.
- Types of tests, from simple availability to anomaly detection.
- How to best specify tests as a mix of queries and configuration.
- Test execution, both on a schedule and on-demand.
- Monitoring and alerting to bubble up data quality issues.
A data quality test framework is a critical part of a data platform, giving confidence in the results produced by all workloads running on the platform.