What is an API integration?

What is an API integration?

An API integration is a piece of software that lets two systems exchange data. It fetches something from one system and puts it in the other. Nobody then has to retype it.

The integration uses the API of both systems for this. An API is a fixed entrance to a software package. Behind that entrance there are agreements: which data you may request, which you may send and in what form. You can think of it as a service counter. You hand in a request in the agreed way, and you get an answer back.

Many business packages have such an entrance. Think of accounting packages such as Exact Online, AFAS and SnelStart, but also webshops, CRM systems and planning software. Want the short meaning of the word? Then read the term API integration.

What is the difference between an API and an API integration?

An API is the entrance to one system. An API integration is the connection that uses two of those entrances. The API is already in the package; the integration has to be built or switched on by someone.

A comparison: the API is the door of a building. The integration is the courier who picks something up at one door and delivers it at the other. Without a door, the courier cannot get in. Without a courier, nothing happens.

How does an API integration work, step by step?

An integration follows fixed rules, in a fixed order. For example like this:

1. There is something new. At set times, the integration asks the first system what is new. Or that system reports it itself, with a webhook.
2. Fetch. The integration fetches the data, for example a new order with the customer attached.
3. Translate. Every system calls things by different names. One has a ‘customer’, the other a ‘relation’. VAT codes and item numbers differ too. The integration converts everything into the language of the second system.
4. Write. The integration puts the data into the second system.
5. Check. Did it go well? Then the integration records that in a log. Did it go wrong? Then a notification follows.

That translating is often the most work. Two systems can each work perfectly well and still think differently about what a customer or an order is. You set those rules down in advance.

Which data can you integrate?

You can integrate almost anything that a system makes available through its API. For SMEs it is, for example, about:

• customers and suppliers (names, addresses, contact persons);
• orders and quotes;
• invoices and payment status;
• hours worked and planning;
• stock and items;
• documents, such as a PDF of an invoice.

Think of combinations such as webshop and accounts, CRM and accounts, planning and invoicing, or a web form that puts a request into your CRM. Do you retype things too at the moment? Then read how to recognise double data entry.

What is the difference between a standard integration and a custom integration?

A standard integration is ready-made. A custom integration is built for your situation. In between sits the integration platform.

Standard integration

Many packages have their own app store or overview of integrations. Exact Online has an app store with apps and integrations from other companies. SnelStart has an overview of integrations, for example with webshops and payment services. You switch such an integration on and fill in a few settings. The maker maintains it. What it costs differs per integration: check that with the provider.

The downside: a standard integration works the way it works. Want an exception, an extra check or a different field? Then that is often not possible.

Integration platform

An integration platform is an online service in which you click integrations together yourself. You choose a starting point (‘new order in the webshop’) and an action (‘create an invoice in the accounts’). That works well for simple steps. With many rules and exceptions it quickly becomes hard to follow.

Custom integration

A custom integration follows your rules. It can carry out checks that suit your business, and it sends a notification to the right person if something deviates. The downside: someone has to build and maintain it. That costs more than switching on a standard integration.

My advice: first check whether there is a standard integration that does what you need. If there is, use it.

Does an integration work one way or both ways?

You choose that yourself. One way is the simplest: the order goes from the webshop to the accounts, and nothing comes back. Both ways means that a change in one system also reaches the other, and vice versa.

With both ways, you must answer one question carefully: which system has the final say? If someone changes an address in the CRM and in the accounts, which version applies? If you don’t agree on that, the systems overwrite each other. That is why both ways is more work, and therefore also more expensive.

What happens if an integration fails?

Then a piece of data does not arrive, or it arrives only partly. That can have different causes:

• one of the systems is temporarily unavailable;
• the access has expired, for example a key or password;
• the supplier has changed the API;
• the data does not fit, such as an order without a customer number;
• the integration sends too many requests in a row. Many APIs have a limit on the number of requests per minute or per day.

A good integration handles that. It tries again later if a system was briefly gone. It makes sure an order is not booked twice. And it sends a notification to a designated person if it doesn’t succeed, with the reason.

Why is a notification on an error so important?

An integration works in the background. Nobody looks at it. Without a notification, you only notice an error when a customer calls about an invoice that is wrong, or when the figures don’t reconcile at the end of the month. With a notification, you know the moment it happens. Then you fix one order, not a whole pile.

What do you need for an API integration?

You need access to both systems, and agreements about the data. In a list:

• An API in both systems. Check whether your package has one, and whether it is included in your subscription. Sometimes you have to request access first. SnelStart, for example, has a request form for this.
• Login details for the integration. Usually an API key or a login via OAuth. Give the integration only the permissions it needs.
• Agreements about personal data. Does customer or employee data pass through the integration? Then the GDPR applies. Does another party process that data for you, for example because the integration runs with that party? Then you conclude a data processing agreement with that party.
• Someone who knows the work. A colleague who knows how things go now, and which exceptions there are.
• A designated person for notifications. Who fixes it when something goes wrong?

Is an API integration secure?

An integration is as secure as the way you set it up. A few basic rules help. Give the integration access only to the data it needs. Keep keys and passwords in a safe place, not in an email or a shared document. Only pass on data that the other system needs. And record who manages the integration and who receives the notifications.

An integration also removes typing errors: it takes the data over as it is. That cuts both ways. If there is an error in the first system, it will soon be in the second one too.

When is retyping cheaper than integrating?

Retyping is fine if it is only a small amount of work. An integration costs money to build and to maintain. That has to be outweighed by the work it takes over. Retyping is often the better choice if:

• it concerns a few lines a week;
• your way of working still changes often;
• you will soon replace one of the systems;
• one of the systems has no API. Then an export that you import at set times is sometimes a good alternative.

Unsure? Then write down for a few weeks how often someone retypes, how long it takes and which errors come out of it. Then you will see for yourself whether an integration fits. Also read what an API integration costs.

How do I help you with an API integration?

I start with a business analysis. I look at which data is retyped where, which systems you have and whether a standard integration already exists that is good enough. I receive no commission from suppliers. After that you get a proposal with a fixed price, so you know where you stand in advance.

If a custom integration is needed, I build it myself. It includes a notification if something goes wrong. What is added per month for hosting and maintenance is also stated in the proposal. What determines the price of an integration is explained in What does an API integration cost? Read more about having an API integration built, or book a call.

Frequently asked questions

Sources: Exact Online app store (in Dutch), SnelStart: integrations (in Dutch), SnelStart: requesting an API integration (in Dutch), Exact Online REST API, AFAS: connecting external software to Profit (in Dutch) and Autoriteit Persoonsgegevens (the Dutch Data Protection Authority): controller and processor (in Dutch). Consulted on 7 October 2026.