How to Know If You Truly Understand a Program
First Sip
Today’s coffee is a cherry vanilla blend from Coffee Beanery.
At first sip, it’s sweet. Clear. Simple.
But if you slow down, you notice layers — the balance, the aftertaste,
the structure underneath the flavor.
Programs are the same way.
You can skim the surface and think you understand them. Or you can sit
with them long enough to see what’s actually holding them together.
Most developers stop at the first sip.
Familiarity Feels Like Understanding
There’s a stage in every developer’s growth where reading code becomes
comfortable.
You can:
- Follow the indicators
- Understand the file I/O
- Trace the subprocedures
- Step through it in debug
You think: “This makes sense.”
But comfort is not comprehension.
Familiarity means:
You recognize the syntax.
Understanding means:
You grasp the intent, constraints, history, and consequences.
On IBM i, that distinction matters more than almost anywhere else.
Because many of our programs:
- Have been modified for 10, 20,
sometimes 30 years
- Encode business rules that are no
longer documented
- Serve multiple consumers without
anyone realizing it
You’re rarely looking at a simple script.
You’re looking at a layer in a system that evolved.
Depth Test #1: Do You Understand the
Business Tension?
Every meaningful program exists because of a business tension.
Speed vs accuracy
Flexibility vs control
Automation vs oversight
Revenue vs risk
If you don’t understand the tension the program resolves, you don’t
understand the program.
For example:
An order validation routine might look straightforward. But underneath it
may exist because:
- The company once lost significant
revenue from duplicate orders
- Auditors required stricter
enforcement
- Inventory reconciliation used to
fail nightly
Without knowing that history, a “small optimization” can quietly
reintroduce an old problem.
Understanding code without understanding business pressure is like
understanding grammar without understanding meaning.
Depth Test #2: Can You Spot the
Load-Bearing Walls?
In older systems, not all programs are equal.
Some are decorative.
Some are structural.
The problem is they often look the same.
- Is called more often than
expected
- Mutates shared state
- Is referenced in multiple job
streams
- Has side effects that are assumed
elsewhere
You know you truly understand a program when you can say:
“This is safe to refactor.”
“This must be isolated first.”
“This needs a safety net before touching it.”
That judgment doesn’t come from syntax. It comes from system awareness.
Depth Test #3: Do You Know What It Is
Not Responsible For?
This is subtle.
Many production incidents happen because someone assumes a program
guarantees something it does not.
For example:
- Assuming a validation routine
enforces uniqueness when it only warns
- Assuming a batch job reconciles
data when it only reports discrepancies
- Assuming a file update ensures
downstream recalculation
Understanding includes knowing the boundaries.
You must be able to say:
“This program ensures X. It does not ensure Y.”
Clarity about limits is a sign of maturity.
Depth Test #4: Can You Describe Its
Data Contracts?
IBM i systems are deeply data-centric.
Programs may change.
Files endure.
Every program participates in implicit data contracts:
- Field meanings
- Expected state transitions
- Timing assumptions
- Required sequencing
If you modify a program without understanding its data contract, you’re
not changing logic — you’re renegotiating trust.
Real understanding means you can articulate:
- What data state must exist before
execution
- What state must exist after
- What invariants cannot be broken
That’s engineering.
Depth Test #5: Can You Explain the
“Why Now?”
Programs often evolve in layers.
Look at almost any mature RPG program and you’ll see:
- Commented-out code
- Versioned logic
- Conditional branches tied to
dates or clients
Each layer exists because something happened.
If you understand the program deeply, you can often infer:
- What event caused this logic
- What failure drove this change
- What compromise was made
You start seeing history in structure.
That’s when you move from reader to interpreter.
The IBM i Reality: Time Compounds
Complexity
Modern platforms often rewrite aggressively.
IBM i systems accumulate.
That accumulation creates:
- Defensive logic
- Performance tradeoffs
- Assumption stacking
A program written simply in 1998 may now be part of:
- A web API
- A reporting pipeline
- An EDI exchange
- A scheduled automation chain
True understanding means you see the layers of time.
You recognize that the code in front of you is not just solving today’s
problem — it is carrying yesterday’s.
There’s a progression every developer goes through:
- Confused
- Comfortable
- Confident
- Responsible
Confused: “I don’t get this.”
Comfortable: “I can follow this.”
Confident: “I can change this.”
Responsible: “I understand the impact of changing this.”
The last stage is rare.
That’s where real understanding lives.
How to Build Deeper Understanding
Intentionally
This does not happen by accident.
Here are practical behaviors that accelerate depth:
Trace Beyond the Immediate Change
Never stop at the first program.
Trace upstream and downstream.
Study Successful Runs
Look at job logs when everything works.
Success reveals structure.
Review Incidents
If a program caused an outage five years ago, study it.
Pain leaves fingerprints.
Draw the Flow
Literally sketch:
Input → Transformation → Output → Consumers
If you can’t diagram it, you don’t fully see it.
Verbalize Assumptions
Before modifying logic, state your assumptions.
Then verify them.
Why This Separates Developers
Two developers can make the same change.
One:
- Tests the scenario
- Deploys it
- Moves on
The other:
- Evaluates side effects
- Checks job timing
- Reviews related programs
- Confirms invariants
- Documents reasoning
Both write correct syntax.
Only one truly understands the program.
That difference compounds over a career.
Final Sip
The longer you sit with that cherry vanilla coffee, the more you notice
the structure behind the sweetness.
Programs reward the same patience.
Understanding is not about speed.
It’s about depth.
If you can:
- Explain its purpose
- Predict its failure
- Map its dependencies
- Define its boundaries
- Change it responsibly
Then you understand it.
If not, you’re still tasting the surface.
And on IBM i, surface-level knowledge eventually gets tested.
The question is whether you’ll be ready when it is.
I need more coffee.
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