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While preparing for the Databricks Data Engineer Professional Certification and exploring Delta Lake operations, I came across a real-world scenario that completely shifted my understanding of what “deleting data” actually means.
This wasn’t just another certification topic to memorize. It was a €20 million lesson hidden in plain sight.
I explored use cases from domains like retail, CPG (Consumer Packaged Goods), and healthcare, industries where sensitive customer data isn’t just important, it’s legally protected.
Think about:
Retail: Customer purchase history, payment details, addresses
Healthcare: Patient records, medical history, insurance information
CPG: Consumer preferences, loyalty program data, contact information
In all these scenarios, one thing is common: customers have the right to ask for their data to be permanently deleted.
I came across a scenario called “The Time Travel Compliance Failure”, and it had nothing to do with sci-fi.
The core issue? When a customer requests deletion of their personal data, and the team uses Databricks with Delta tables, something critical can go wrong.
Let me explain what I learned.
Before diving into the technical details, I had to understand the legal side. Under GDPR (General Data Protection Regulation), organizations must:
✅ Delete personal data permanently and irreversibly
✅ Do it without undue delay (typically within 30 days)
✅ Ensure it cannot be recovered by any technical means
This is not optional. Failing to comply can result in fines up to:
4% of global annual revenue
Whichever is higher.
Suddenly, understanding how Delta Lake handles deletions became much more than just a certification question.
A customer exercises their GDPR right and requests: “Delete all my personal information from your systems.”
A compliance team member executes the deletion:
DELETE FROM users WHERE user_id = '12345';Result: ✅ “1 row deleted successfully”
The engineer verifies:
SELECT * FROM users WHERE user_id = '12345';Result: No records found. ✅
Everything looks perfect. The data is “deleted.”
A few days later, compliance auditing begins.
The auditor, understanding Delta Lake’s capabilities, runs this query:
SELECT * FROM users TIMESTAMP AS OF '2024-09-09';Why September 9th specifically? The auditor was checking data from 3 days ago, before the deletion request was processed on September 10th. They wanted to verify that deleted data wasn’t just hidden in the current version, but truly inaccessible from all versions.
The result?
The screen fills with data.
User ID 12345. Name. Email address. Phone number. Home address. Purchase history.
All the “deleted” personal information appeared. 😱
This is where my certification study became eye-opening. I learned that when you execute a DELETE command in Delta Lake, here’s what actually happens:
DELETE FROM users WHERE user_id = '12345';What the team thought happened:
Data permanently removed from storage ❌
What actually happened:
Data deleted from current version ✅
Underlying Parquet files remain physically intact in cloud storage
Delta Lake’s transaction log records the deletion
Previous versions still point to the original data files
This is by design. Delta Lake maintains version history to enable powerful features like:
Recovering from accidental deletions
Auditing data changes over time
Time travel queries for analysis
Reproducing past datasets
But here’s the problem: This feature that makes Delta Lake so powerful is the exact reason the deleted data was still accessible.
I discovered that Delta Lake has a command specifically for physically deleting old data files: VACUUM.
The VACUUM command:
Identifies Parquet files no longer referenced by recent versions
Physically deletes these files from cloud storage
Makes old versions inaccessible via time travel
Sounds like the perfect solution for GDPR compliance, right?
Here’s where I learned about the hidden trap.
VACUUM has a default retention period of 7 days.
This means:
Even after running VACUUM, deleted data files are retained for 7 days
For those 7 days, time travel queries can still access the “deleted” data
Your compliance team believes the data is gone, but it’s still fully recoverable
Let me show you what this looks like in practice:
Here’s the exact scenario from my certification study:
September 10th - Deletion Executed:
-- User requests data deletion (GDPR requirement)
DELETE FROM users WHERE user_id = '12345'; Data deleted from current version ✅
Current version check:
SELECT * FROM users WHERE user_id = '12345';Returns: 0 rows ✅
Everything seems fine.
But the default VACUUM retention is 7 days.
For the next 7 days, this happens:
-- Auditor checks historical data from September 9th
SELECT * FROM users TIMESTAMP AS OF '2024-09-09'
WHERE user_id = '12345';Result: ❌ Deleted user data appears!
GDPR violation! ❌
The data was supposed to be permanently and irreversibly deleted. Instead, it’s still sitting in cloud storage, fully accessible to anyone who knows how to use Delta Lake’s time travel feature.
After understanding the problem, I learned the correct approach:
DELETE FROM users WHERE user_id = '12345';VACUUM users RETAIN 0 HOURS;Now the verification:
-- Try to access historical data
SELECT * FROM users TIMESTAMP AS OF '2024-09-09'
WHERE user_id = '12345';Result: ✅ Returns nothing (files purged permanently)
This two-step process ensures:
✅ Data deleted from the current version
✅ Physical files removed from cloud storage immediately
✅ Time travel cannot recover the deleted data
✅ GDPR compliance achieved
There isn’t a single universal solution to this problem. Different organizations may adopt different approaches depending on their governance policies, compliance frameworks, and risk tolerance. But what matters most is awareness, the right process, and proactive governance.
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