Why You Should Care About the Robustness Principle

Photo by Nick Design @nickshuperdesign on Unsplash

Ever felt like the world is out to get you? That users will try anything to break your well-crafted application? Welcome to the party, pal.

That’s because when you don’t listen to Jon Postel, your app and your career explodes.

What With The What?

Jon Postel, one of the original Internet architects, dropped a nugget of wisdom we should all be engraving into our keyboards:

“Be liberal in what you accept, and conservative in what you send.”

In plain English: be chill when receiving stuff, and paranoid when sending stuff.

What Postel Meant (and Why You Should Listen)

Postel’s Law came from the wild west days of TCP/IP. The idea was that if everyone receiving data played it cool when things weren’t perfect, and everyone sending data made sure their stuff was airtight, we’d all get along a lot better.

You want specifics? Imagine ordering a “Big Mac, no cheese, extra salad.” You’d better be clear about what you want. But if your colleague says, “get me McDonald’s” and doesn’t specify, you don’t need to waterboard them for clarification. You guess intelligently (it’s chicken McNuggets, of course).

In Code:

Sending

Be specific, clear, and follow the protocol to a T.

Receiving

Assume the other person might have had a stroke mid-request and build your code to handle it gracefully.

The Takeaways for Developers

• Assume user input will be trash (you’ll usually be right).

• Build APIs that don’t collapse when some joker sends a smiley face instead of a number.

• Validate the living daylights out of your output before it leaves your service.

• Where possible, gently guide users (and upstream systems) back toward sanity.

I’m working in a place where one senior dev still doesn’t understand async calls, so this all feels like a stretch for them, but hey, let’s not expect miracles.

Why Flexibility is Your Secret Superpower

Every service today talks to other services. Some built yesterday. Some built by interns. Some built by people who thought COBOL was a cool trend. If your system freaks out every time someone sneezes at the input, you’re not resilient — you’re fragile.

Handle a little weirdness? Congrats: you’re now the backbone of the system.

Can You Be Too Nice?

Yes. Accepting everything without judgment is how you end up letting malicious payloads, SQL injection attacks, and data corruption waltz in wearing party hats.

Balance matters. Liberality is great on input — but even then, with boundaries. Validation and clear error handling are not optional.

How to Actually Implement Postel’s Law Today

API Design

Be strict on output. Validate it like a Victorian nanny inspecting your homework.

Error Handling

Fail gracefully, informatively, and without crashing.

Security

Don’t let “liberal acceptance” mean “open season on our backend.”

Testing

Simulate the craziest edge cases you can imagine — and then test for three crazier ones.

My Experience

Applying Postel’s Law requires teamwork and shared goals. At my current shop, we have neither. Contracts aren’t respected, compatibility isn’t considered, and cross-team communication is like a bad game of telephone. 

No Postel’s Law here, and I’m not able to push it unfortunately. Even if I could you can’t polish a turd. I don’t think this is actually going to help somewhere where things are this far gone.

Conclusion

Postel’s Law isn’t a license for chaos. It’s permission to survive in the chaotic reality of software engineering without becoming part of the problem.

You need robustness. You need flexibility. And if your team thinks “it’s good enough” means “send garbage and pray,” you may need a new team.

Create systems that survive. Do this and maybe, just maybe, you’ll survive this crazy industry too.