# API integration: what it involves and what can go wrong

An integration looks like a small job until the two systems disagree about what a customer, an order or a stock level is. Here is how to scope it before you pay for it.

- Page: https://www.cognivise.co.uk/blog/api-integration-guide
- Topic: Building software
- Published: 2026-10-08
- Updated: 2026-10-08
- Author: Cognivise

## Key takeaways

- An API integration moves data between two systems automatically, and most of the work is agreeing what the data means, not sending it.
- Scope an integration by listing each piece of data, which direction it moves, and what should happen when it fails.
- Check the other system's API limits, documentation and change policy before you commit to a build.
- Budget for monitoring and maintenance, because the other side will change.

## What is an API integration?

An API (application programming interface) is the way one piece of software lets another read or change its data. An integration uses those APIs so two systems stay in step without anyone retyping information, for example an online store sending orders to an accounts package.

The sending is the easy part. The harder part is deciding what each system means by the same word.

## What do you need to decide before anyone writes code?

List every piece of data that has to move, then answer four questions for each:

- **Direction.** One way, or both? If both, which system wins when they disagree?
- **Timing.** Instantly, every few minutes, or once a night?
- **Volume.** Ten records a day or ten thousand? This changes the design.
- **Failure.** What should happen if the other system is down or rejects a record?

Teams that skip the last question find out the answer in production, usually when an order goes missing.

## What do suppliers' APIs actually look like?

They vary a great deal. Before committing, check:

- whether documentation exists and is current
- rate limits, meaning how many requests you are allowed in a given period
- whether there is a test environment, so you do not experiment on live data
- how the supplier announces changes, and how long old versions keep working
- whether the API covers what you need, or only some of it

If the API cannot do what you need, an integration may be impossible or may need a workaround such as file exports. Better to learn that in a short discovery step than halfway through a build.

## What causes most integration projects to go wrong?

- **Mismatched data.** One system stores a name as a single field, the other as two. Duplicates and missing records follow.
- **No error handling.** A failed request is silently dropped and nobody notices for weeks.
- **Hidden manual steps.** The process only worked because someone fixed things by hand.
- **Ignoring limits.** A bulk import hits the rate limit and half the records never arrive.
- **No owner.** Nobody is responsible when the supplier changes their API.

## Should you buy a connector or build one?

- **Ready-made connector** — Quick and cheap for a common pairing with standard needs. Limited when your process is unusual.
- **Integration platform** — Suits several simple connections managed in one place. Ongoing subscription, and complex logic gets awkward.
- **Custom integration** — Suits unusual rules, high volume or a system with no connector. More to build, and you carry the maintenance.

> Choose the simplest option that handles your failure cases. Cost per month matters less than what happens when a record goes wrong.

## What should you expect after launch?

Integrations need looking after. Plan for logging so you can see what was sent, alerts when something fails, and someone responsible for updates when either system changes. Our guide to [what support and maintenance cover](https://www.cognivise.co.uk/blog/what-application-support-covers) explains what to ask for.

Cost depends on the number of systems, the data involved and the quality of each API. The factors in [how much custom software costs](https://www.cognivise.co.uk/blog/how-much-does-custom-software-cost) apply here too.

If you are scoping a connection between systems, see our [API development services](https://www.cognivise.co.uk/services/api-development-services) or [contact us](https://www.cognivise.co.uk/contact).

## Related

- [API development services](https://www.cognivise.co.uk/services/api-development-services)
- [How much does custom software cost?](https://www.cognivise.co.uk/blog/how-much-does-custom-software-cost)
- [What application support and maintenance actually cover](https://www.cognivise.co.uk/blog/what-application-support-covers)