Use another system's API when it already carries the record you need, and build your own when your system is the one other systems must call. Either way, the real work is writing the contract: who calls it, what they send, what they get back, and what happens when it fails. A URL with no failure behavior is not an API.
An API is a contract someone else can build against, so your system can talk to another one without sharing a database login. This guide gives you a short check for deciding which side of that contract you are on. It is the order we follow at Triwave Consulting before any build starts, and it describes a way of deciding, not a result we are claiming for anyone.
Takeaways
- Name the payload, the fields that go in and the fields that come back, before you name a tool.
- Use one test: another developer can build against the contract without asking you to explain it again.
- The common wrong start is opening the database to a partner, or shipping a sample response with no rules for errors.
- Use this article to make the decision, and use the service page to ask for the work.
The Choice in Plain Terms
The question is really about who has to call whom. If another system already exposes an API that carries your record, such as customers, orders, or appointments, then your job is to use that contract well. That is integration, and building a second API beside it would duplicate something that already works. If your system holds information that other tools, partners, or apps need to reach, then your system needs a contract of its own, because otherwise the only way in is a shared password or a file drop.
You are not choosing a philosophy about architecture. You are choosing where the payload lives on an ordinary day. If it moves cleanly between systems already, stay. If it moves by someone sharing a database login, or by a file dropped in a folder and picked up by hand, the stay has a cost you can describe without a pile of opinions.
A common situation is an owner whose other system already has an API, wondering whether to build one anyway. The plain job is to use the other system's contract when it already carries your record, and to build one only when your system is the thing others must call. If you came for a different job, such as choosing a software vendor, this page will not help much, and that is on purpose.
When Using an Existing API Fits
Using an existing API fits when the test already passes: another developer can build against the contract without asking you to explain it again. That is the whole green light. The other system has documented what goes in, what comes back, and what happens on failure, and it carries the record you need. Building your own beside it only gives you a second contract to maintain.
Be careful about changes that look like progress. Opening your database to a partner feels fast, but it ties them to your internal structure, so every later change risks breaking their work. Shipping a sample response with no rules for errors feels finished, but the first time something fails, nobody will know what to do. Both are the wrong start, and both give you a new place to be confused.
The second path fits when the first keeps dropping the step. You will know because the real work is a shared password to a database, or a file dropped in a folder. People are polite about this: they say the arrangement works, then keep a side path alive, such as a person who notices when the file did not arrive. Believe the side path. It is your specification, written in behavior.
Use this list as a single pass, not as a poster:
- Write down the record that has to move, and which system owns it.
- Check whether the owning system already offers a documented way to read or write it.
- Notice whether people still use a shared login or a file drop alongside it.
- If a documented way exists and covers your record, use it.
- If your system is the owner and others need in, that is the case for building.
When Building Your Own Fits
Feature lists hide this decision. They award a point for every integration a product advertises and none for the one your business needs. A box labeled "has an API" often means a read-only view of part of the data, which will not carry the record you actually need to change. Ask for one real item to be sent and received, with a deliberate failure, and watch what happens.
Building fits when your system is the source others depend on: a partner needs your inventory, a mobile app needs your customer data, or two of your own tools need to share the same record. Then a contract of your own is the safe way in. It lets you decide what is exposed, who can call it, and what happens when something goes wrong, without handing out database access.
The same checks sit in a table so you can see the fork at a glance. Every cell is a judgment you can make from the work in front of you. None of them is a score, a price, or a borrowed result.
| Question | Use an existing API (first path) | Build your own (second path) |
|---|---|---|
| What you are protecting | The way the work happens today | The step that keeps falling out |
| When it fits | Another developer can build against the contract without asking you to explain it again | The current setup is a shared password to a database, or a file dropped in a folder |
| When it fails | You open the database to a partner, or ship a sample response with no rules for errors | You cannot name the payload: the fields that go in and the fields that come back |
| What you still do | Test one real record, including a failure | Write the contract out loud before you build |
What a Checklist Hides
Decide in one sitting if you can. Write down who calls it, what they send, what they get back, and what happens when it fails. Then try to answer those four with what you already have. If you can, stop shopping. If you cannot, the discussion is about that missing answer. Avoid running five trials and a custom estimate in the same week, because you will remember the best demo and forget the contract.
The failure rules are the part most often skipped. What happens if the call times out, if the record is malformed, or if the same request arrives twice? A contract that answers those questions is one another developer can trust. A contract that does not will work fine until the day it matters.
Triwave Consulting will not pretend both paths are a tie that a case study could break, and we do not publish numbers we cannot stand behind. What we can do is listen for the contract during Initial Contact and decline a plan that ignores it. Detailed Discussion is where the chosen path gets a payload, a caller, and a boundary. Implementation follows the path you picked and changes if the path was described wrong. The page for that work is API Development. Use it when you already know which way you are leaning and want that tested against the work.
How to Decide This Week
Hold the choice to one record you can watch move. If a documented way already carries it, stop shopping. If it moves by password or file drop, the other path has earned a conversation, and only for that record. A second feature is not a reason to reopen a choice you have already watched play out.
The offer itself sits on API Development, under Software Development. If your question is really about connecting two systems that both already have contracts, API Integrations is the neighboring page that fits.
This article stays with API development because the situation matches that page. The parent page is the map back out if the match was only a similar word. Similar words are how offices buy a neighboring service and then wonder why the record is still moving by hand.
Before you leave, say the stance once more: an API is a contract someone else can build against, so your system can talk to another one without sharing a database login. If you cannot say that about your own week, do not force the rest of the article to agree with you.
Then walk through one ordinary week. Find the record people are careful with, and notice what happens when a second system needs it: does someone share a login, drop a file, or retype it? That moment is the subject. If the test already holds, your week does not need a new shape. If it fails, write down the failure in your own words, and resist translating it into a feature request.
Questions
What If the Other System's API Only Covers Part of My Record?
Use it for the part it covers, and be explicit about the gap. A partial API often means a second path remains for the rest, such as a file or a manual step. Decide whether that remaining step is worth a small contract of your own, or whether it is rare enough to leave alone.
Do I Need an API If Only Two of My Own Tools Must Share Data?
Not always. If both tools already offer integrations, connecting them may be enough. A contract of your own makes sense when neither tool exposes what you need, or when more systems will need the same record later and you want one dependable way in.
Why Is Sharing Database Access a Risky Shortcut?
It ties the other party to your internal structure and gives them more access than they need. Every later change to your tables can break their work, and there are no rules for what happens on failure. A contract exposes only what you choose and states how errors are handled.
What Should an API Contract Say About Failure?
It should say what the caller receives when a request is malformed, when it times out, and when the same request arrives twice. It should also say who is told when something breaks. The goal is that a developer who has never spoken to you can build against it and handle the bad day.
Can You Start Small and Expand the Contract Later?
Yes. Begin with the one record that causes the most hand-work, and make that contract dependable. Add more later, treating changes carefully so existing callers are not surprised. A small contract that works is better than a large one nobody trusts.
Conclusion
You came with a question about API development, and the aim here was a check rather than a pitch. Write who calls it, what they send, what they get back, and what happens when it fails. That contract is the API. Avoid the wrong start of opening your database to a partner or shipping a sample response with no rules for errors. The service page can take the conversation from here, and when your description is real, you already know what to say first.
When you have picked a direction and want it tested against the work, Speak With Us.
