Backward Compatibility in QA: A Practical Approach to Testing Legacy Systems

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Balancing innovation with backward compatibility, especially in legacy systems, is like renovating a historical building — you want to add modern amenities without compromising its structure or alienating the people who still love the original design.

Here’s a detailed, conversational take on how to handle this balancing act, peppered with real-world challenges and ways to test effectively.

1. Understand Your Legacy System’s “Sacred Cows”

Every legacy system has those untouchable features users rely on. These are the “sacred cows” you must tread lightly around.

• The Issue: Teams often assume they know what’s critical without validating. This leads to breaking something crucial while focusing on “cool” new features.
• How to Test: Start with user analytics or surveys. Identify the most used functions and prioritize regression tests for these areas.

Example: If a legacy system processes invoices, test old workflows with various edge cases like bulk uploads, outdated formats, or special characters.

2. Avoid the Temptation of the “Big Rewrite”