What is an API? Plain words and a real example

API stands for application programming interface. It is the way one program asks another program for something, or tells it to do something, with no person clicking in between.

Here is one we use every working day. When the order desk of our parts business books a FedEx label, nobody opens fedex.com. The software sends FedEx the address, the weight and the service, and FedEx sends back the label and a tracking number. That exchange is an API call.

Our order deskInvent, at the packing bench

address, weight, servicelabel, tracking number

FedEx’s APIthe published list of requests FedEx accepts

book itbooked, pickup arranged

FedExrates, labels, pickups

One booking, start to finish: the request goes out with the parcel’s details and comes back with the label, ready to print on the thermal printer.

The four parts of every API

Every API you will hear about, from eBay’s to your bank’s, has the same pieces.

The request: What one program asks for, in a fixed shape the other side published: “a FedEx Priority label for this address, 1.9 kg, one box”.
The response: What comes back: the label as a file and a tracking number, or an error that says which field was wrong.
The key: A long secret that lets a program act for your account, so it never needs your password. Delete the key and the connection stops at once.
The documentation: The published list of what you may ask, which fields each request needs and how many requests you may send.

What an API integration is

An integration is two or more programs joined through their APIs, so that one event in one of them sets off the right steps in the others. Our listing app, ICE, is a good example. A person photographs a used part, ICE reads the nameplate and drafts the listing, and the person sets the price and presses List.

eBaythe listing buyers see

title, photos, category, pricelisting live, its number

ICEthe listing app on the phone

the listing and its boxesstock count updated

Inventthe order desk and stock

Pressing List makes two calls: one to eBay, one to our own order desk. Weeks later, when the part sells, the packer’s phone already shows the shelf and box.

Invent shipping screen with an address check and a customs declaration for a photoelectric sensor (example data) — Before the FedEx request goes out, Invent checks the address field by field and drafts the customs line.

ICE review screen with a nameplate photo, the fields read from it and a List on eBay button (example data) — ICE fills the listing from the nameplate photo. Pressing “List on eBay” is the request; the listing number is the response.

APIs our own software talks to

Every one of these runs in our order desk, our listing app or the bookshop we built. The left column is the service, the right one what our software asks it to do.

The bookshop also showed us why the details matter. A partial update on the marketplace wipes the fields it leaves out, so the shop always sends the complete record.

eBayNew orders fetched every five minutes, listings put live, tracking numbers sent back to the buyer’s order.
FedExLive rates, the label, the courier pickup and the customs invoice filed electronically, in one booking.
A Hungarian shipping brokerGLS, DPD, UPS and other carriers quoted next to FedEx through one connection.
számlázz.huInvoices issued with the right VAT line for each buyer, then sent to the tax authority by the invoicing service.
The EU VAT registerA buyer’s VAT number checked before an invoice uses it.
AbeBooksA book listing updated or withdrawn when the same book changes or sells on the bookseller’s own shop.
A card payment providerThe buyer pays on the provider’s page, and a signed message tells the shop the order is paid.
Google Merchant CenterA product feed of every book with a photograph, shipping cost per country included.

When there is no API

Not every system has one, and a missing API is a reason to plan, not a dead end. At the solar-panel recycler in Hungary we built plant software for, the national waste register stays the place where the papers are issued. The software records the load, links the scans and prepares exactly what the office would type, so nothing is typed twice.

Sometimes the answer arrives by email instead. When FedEx writes that a parcel is held at customs, our order desk reads that email, puts the parcel back on its list and chases it again every two days until it clears.

Questions about APIs

What does API stand for?

Application programming interface. “Interface” is the important word: it is the agreed way for one program to talk to another, the way a form at a counter is the agreed way for a person to ask for something.

What is an API in simple terms?

A set of requests one program will accept from another, with a fixed answer for each. Our order desk sends FedEx an address, a weight and a service, and FedEx answers with a label and a tracking number. No person opens a website in between.

What is an API integration?

Two programs connected through their APIs so data moves between them by itself. When a part is listed on eBay from our listing app, the app also tells our order desk which box the part sits in, so the packer’s phone shows it when the part sells. More on API integration.

What is a REST API?

The most common kind of web API. Each thing you can ask about has a web address (an order, an invoice, a listing), and you read, create, change or delete it with ordinary web requests. The data usually travels as JSON, a plain text format both sides can read.

Do I need a developer to use an API?

For two popular apps and a simple step, a ready-made connector like Zapier or Make does it without code. When the step needs your own rules, or the other side is not in the connector’s list, someone writes the integration. See Zapier or a custom integration.

Is it safe to give a program an API key?

A key can do whatever the account allows, so treat it like a password: keep it on the server, never in a web page, and delete it when the connection ends. We add a second guard: in our order desk only the production server is allowed to send anything to a buyer, after a test copy once sent live messages.

Get two of your programs talking

Thirty minutes with us. Tell us which tools your team copies between today, and we tell you whether their APIs can do it, what we would build and what it costs.

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