Every developer knows that feeling:
You're staring at a bug for days. You don't have enough information. You don't even know where to begin.
You read the error again and again, hoping for clarity, but it tells you nothing.
You start to wonder: Maybe this just can't be solved?
But the truth is… even when it seems impossible, it's not.
In our system, one of the scheduled cron tasks suddenly started failing.
It runs daily, and every single execution was ending in failure.
The only error message I got was:Variable 'W0' is not defined.
Where is this variable used? Why is it undefined? Which question does it belong to? Which course?
No answers. Just one vague error.
It was a classic needle-in-a-haystack situation.
So I built myself the needle.
Step 1: There Wasn't Enough Information
The first thing I realized was this:
The system wasn't giving me enough information to debug.
The message 'W0' is not defined meant absolutely nothing on its own.
I tried running an SQL query to find all references to W0, and got thousands of results.
That approach was a dead end.
But I had one small thread to pull, the stacktrace from the logs.
It pointed me to the function from where the error comes.
That’s where I decided to take control.
Step 2: I Built the Tool I Needed
Instead of trying to debug blindly, I patched the code to give me the details I actually needed.
I modified the error output to include:
- The question ID (
questionid) - The missing variable name (
missing_var) - The full variable stack (
vstack) - And some extra internal state
As soon as I did that - boom!
I had a complete picture.
Now I finally had a way forward.
Step 3: SQL Detective Work
With the question ID in hand, I ran an SQL query to locate the exact quiz and course.
That gave me the exact location of the problematic question inside our system.
From there, everything became solvable.
I fixed the question, re-tested the cron task, and it passed successfully.
Final Thoughts
An unclear error isn't the end.
It’s just a sign that your system isn't telling you what you need to know.
As a developer, I'm not limited to what the logs give me.
I can go into the code and build the tools I need.
If there's not enough information - generate it.
If you don't have a tool - create one.
And when everyone else only looks at what's visible -
go explore what's still hidden.
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