An API is simply the way one piece of software asks another for something. Your online store asks your inventory system how many units are left. Your accounting software asks the bank for today’s transactions. A warehouse scanner tells the main system that a pallet has just shipped.
These connections usually start out small and convenient. But once several systems rely on them, they become part of how the business runs. Small unclear points, like what a particular error means or whether it’s safe to send a request twice, end up causing problems everywhere. Reliability starts with making those things clear.
Describe the job in plain words first
Before writing any code, I describe what the connection is actually for. What is one system asking the other to do? What needs to be true before it can happen? How do we know it’s finished? What are the normal “no” answers, like “out of stock”, and what counts as a genuine fault?
This keeps the connection focused on the business task rather than on how the database happens to be laid out today. It also makes sure everyone means the same thing by the same word. If “complete” means “shipped” in one place and “paid” in another, someone will eventually make a costly mistake.
Make error messages helpful
When something is rejected, the message should say what went wrong and what to do about it. “The delivery date can’t be in the past” is useful. “Something went wrong” is not.
Different problems deserve different answers: missing information, a record that doesn’t exist, two people changing the same thing at once, not having permission, or a real fault on the system’s side. Error messages also shouldn’t give away internal details like passwords or how the database is built. Those belong in private logs, where I can trace exactly what happened.
Plan for the request that gets sent twice
Here’s a common problem. One system sends an order, the other side saves it, but the reply gets lost on the way back. The sender doesn’t know it worked, so it tries again. Now there are two orders.
The fix is to give each request its own reference number, so the receiving system can say “I’ve already got this one” and simply send back the original result. For anything that creates orders, payments, or shipments, I treat this as essential.
Some jobs also take a while, like building a large report or processing a big batch of records. In those cases the system should reply straight away with “got it, here’s a reference to check on”, rather than leaving the sender to guess whether a long silence means success or failure.
Change it carefully
Once other systems depend on a connection, changing it is a bit like changing the rules of a game halfway through. Removing a piece of information is obviously risky. Quietly changing what a piece of information means can be just as bad, and much harder to spot.
When changes are needed, I add the new version alongside the old one where possible, give everyone notice and a timeline before anything is retired, and check who is still using the old version before switching it off.
Look after it once it’s live
A connection that works on a developer’s laptop isn’t finished. In real use it also needs to:
- check who is asking, and what they’re allowed to see
- limit how much any one caller can send, so one faulty system can’t overwhelm it
- keep information encrypted while it travels
- be monitored, so slowdowns and errors are spotted early
- come with clear examples, so the people connecting to it get it right the first time
The test of a good connection is simple: the systems using it can depend on it, and the people looking after it can understand what it’s doing.
In short
A dependable connection makes the right thing easy to do and makes problems easy to understand. Clear agreements, safe handling of repeats, careful changes, and good monitoring aren’t extras added at the end. They’re part of building it properly.