What You Should Document and Why¶

Good documentation isn’t about writing endless pages of text. It’s about capturing the right information so developers can understand, maintain, and extend a system without reverse-engineering everything from the code.

If you’ve ever struggled to debug a system, onboard a new developer, or figure out why something was designed a certain way, chances are critical documentation was missing.

So, what should you document? And more importantly, why?

1. High-Level System Design (So No One Has to Guess How It Works)¶

Every system has an overall architecture—even if it’s not formally defined. Without documentation, developers have to trace through code, dependencies, and configurations to figure it out.

What to Document¶

System architecture – High-level diagram showing major components, services, or modules.
Data flow – How information moves through the system, including storage and processing.
Third-party dependencies – External APIs, libraries, and integrations.

Why It Matters¶

✅ Helps new developers understand the system quickly.
✅ Prevents breaking changes by making dependencies clear.
✅ Reduces debugging time when troubleshooting system-wide issues.

2. Key Design Decisions (So No One Has to Re-Litigate Old Choices)¶

Code tells you how something works, but not why it was built that way. Without documenting key decisions, developers waste time re-evaluating past trade-offs—or worse, they undo critical design choices without realising why they existed.

What to Document¶

Architecture Decision Records (ADRs) – Short documents explaining major design choices.
Trade-offs and alternatives considered – Why one approach was chosen over another.
Known limitations – What the system can’t do, or what to watch out for.

Why It Matters¶

✅ Avoids repeating past debates and mistakes.
✅ Helps developers understand why things work the way they do.
✅ Prevents unnecessary rewrites of working code.

3. State Machines & Workflow Logic (So No One Has to Reverse-Engineer Every Possible Case)¶

State transitions and complex workflows aren’t always obvious from code, especially when they depend on external events, concurrency, or business rules.

What to Document¶

State diagrams – Visual representation of valid transitions.
Event triggers – What causes a state change?
Edge cases & failure modes – What happens in unexpected situations?

Why It Matters¶

✅ Reduces the risk of introducing invalid state transitions.
✅ Helps developers debug edge cases without guessing.
✅ Saves time when adding new features that interact with existing logic.

4. Concurrency & Synchronisation (So No One Introduces Subtle Race Conditions)¶

Multithreaded applications, real-time systems, and distributed microservices all involve concurrent execution. If you don’t document how tasks interact, you’re asking for deadlocks, race conditions, and subtle timing bugs.

What to Document¶

Threading model – How tasks are scheduled and synchronised.
Shared resources – What’s accessed by multiple threads/processes, and how it’s protected.
Critical sections & locks – Where synchronisation mechanisms (mutexes, semaphores, atomic operations) are used.

Why It Matters¶

✅ Prevents thread safety issues and unexpected behaviour.
✅ Reduces debugging time by making synchronisation points clear.
✅ Helps developers understand how performance and responsiveness are affected.

5. APIs & Contracts (So No One Has to Read Through Code to Use Them)¶

If an application exposes an API—whether internally or externally—it needs clear documentation. Otherwise, consumers will either misuse it or have to experiment to figure out how it works.

What to Document¶

Endpoints & request formats – What methods exist and what data they accept.
Response structures & error handling – What comes back, and what to expect when things go wrong.
Rate limits, authentication & security – Who can access the API, and how.

Why It Matters¶

✅ Enables faster development by making API usage clear.
✅ Reduces misuse and unexpected failures.
✅ Helps maintain consistency across multiple services.

6. Configuration, Deployment, & Infrastructure (So No One Has to Figure It Out From Scratch)¶

Software doesn’t just run—it needs to be configured, deployed, and maintained. If those steps aren’t documented, setting up a new environment becomes a nightmare.

What to Document¶

Configuration files & environment variables – What settings control behaviour?
Deployment process – How is the system built, packaged, and deployed?
Infrastructure dependencies – What databases, queues, or cloud services does it rely on?

Why It Matters¶

✅ Reduces downtime caused by misconfigured environments.
✅ Makes onboarding faster for new developers.
✅ Prevents deployment errors that break production.

7. Error Handling & Debugging (So No One Has to Spend Hours Digging Through Logs)¶

When something goes wrong, developers need to quickly understand why. Without documentation on failure modes, error codes, and logs, debugging becomes a frustrating exercise in guesswork.

What to Document¶

Common failure cases & expected behaviour – How should the system react to failures?
Logging strategy – What gets logged, and where?
Monitoring & alerts – How to detect when something is wrong?

Why It Matters¶

✅ Saves hours of debugging by making failure handling clear.
✅ Helps identify and fix issues faster in production.
✅ Improves system reliability and resilience.

The Bottom Line: Good Documentation Saves Time & Prevents Mistakes¶

Documentation isn’t about writing everything down—it’s about capturing what actually matters so that developers can work efficiently without unnecessary roadblocks.

🔹 If it helps someone understand the system faster, document it.
🔹 If it prevents a future mistake, document it.
🔹 If it saves hours of debugging, document it.

The best documentation answers why things are the way they are—so developers don’t have to reverse-engineer every decision from the code.

Final Thought¶

Bad documentation leads to wasted time, frustration, and costly mistakes. If you’ve ever spent hours figuring out something that could have been explained in five minutes, you already know why documentation matters.

So document the right things, and do it well. Future-you (and your team) will thank you.