How does API work? WHere's the manual?

So far, we’ve talked about many things.

One system shares information.
Another system consumes it.
Both agree on how that exchange works.

That agreement is the foundation of all system communication.

But here’s the real question.

If two systems have never met before, how does one system know:

1. that the other system exists?
2. how to talk to it?
3. which channel to use?
4. what kind of information is allowed?
5. what happens if something goes wrong?

Systems don’t guess. They don’t experiment blindly. They rely on instructions to communicate and that instruction is Documentation.

How does one system know how to communicate?

Let’s go back to a simple example.

The TV remote again

Imagine buying a new TV, but there is no manual. You pick up the remote and start pressing random buttons. One button increases volume. Another suddenly changes language. Another switches input

Another turns the TV off completely

You could eventually figure it out, but only through trial and error.

That’s exactly what happens when systems try to integrate without documentation.

They keep guessing:

1. Does this value work?
2. What happens if I send this?
3. Why did it fail?
4. What does this response even mean?

It’s slow, frustrating, and risky.

Documentation is the remote’s manual.

It clearly says:

1. This button does this
2. This input is expected
3. This behavior is guaranteed

Channels matter, not just messages

Earlier, we talked about different ways systems communicate:

1. Asking for data
2. Being notified by events
3. Streaming updates
4. Sending files
5. Waiting for callbacks

But how does a system know which channel to use?

Think of communication in daily life.

1. You don’t send a wedding invitation via a fire alarm.
2. You don’t call someone on the phone to send them a photo album.
3. You don’t shout breaking news through a letter.

The channel matters.

Documentation tells systems:

1. Where to send requests
2. Where to listen for events
3. When to wait and when to react
4. What kind of communication is expected
5. Without that clarity, systems talk past each other.

The fishing hook example, but with instructions

Earlier, we used the fishing hook example for webhooks.

Now imagine someone gives you a fishing rod but tells you nothing else.

1. Where do you throw the hook?
2. What kind of fish are you catching?
3. How do you know when something is caught?
4. What do you do after the hook moves?

You could try randomly, but success would depend on luck.

Documentation is what tells you:

1. Where to place the hook
2. What kind of fish to expect
3. What movement means “something happened”
4. What to do next

Systems need the same clarity. This is where API documentation comes in

API documentation exists to answer very practical questions like:

1. Where do I send information?
2. What information is allowed?
3. What information is required?
4. What will I get back?
5. What events can I listen to?
6. What does success or failure look like?

To help teams answer these questions consistently, developers use tools like:

1. Swagger
2. Postman

And similar documentation platforms

These tools don’t just list APIs. They describe the rules of communication.

They act like:

1. A remote control manual
2. A translator’s guide
3. A communication contract between systems

What happens without documentation?

Without documentation, integration becomes guesswork.

Teams are forced into trial-and-error:

1. Sending random values to see what works
2. Reading error messages like puzzles
3. Learning behavior only after something breaks

This wastes time and creates fragile systems.

Two systems might appear to work today, but the moment something changes, everything falls apart.

Documentation prevents this by:

1. Defining standards
2. Setting expectations
3. Making communication predictable

The quiet truth

Well-designed APIs can still fail if their documentation is poor. And average APIs can succeed if their documentation is clear.

Because systems don’t just need a way to talk. They need a shared understanding of how to talk.

In the next part, let's talk about why many API documentation tools focus more on technical completeness than human understanding, and how that gap shows up the moment non-engineers, partners, or investors look at them.

That’s where things usually break.