API integration services connect the software a business already uses, such as its website, online store, CRM, accounting, stock and booking tools, so that data moves between them automatically instead of being copied by hand. A good partner scopes which systems must talk, what data flows in which direction and what happens when something fails, then builds, tests and looks after those connections.
Last updated: 4 November 2026.
This guide explains what API integration is, what an integration partner actually does, how to scope a project, how to choose a partner in Europe and the questions to ask before you sign.
What is API integration?
An API (application programming interface) is a published set of rules that lets one piece of software request data or actions from another. API integration is the work of using those interfaces to make two or more systems exchange information reliably: a new order in the online store creates an invoice in accounting, or a form on the website creates a contact in the CRM.
Most modern APIs work over HTTP, the same protocol browsers use, and exchange data as JSON. Mozilla’s overview of HTTP is a clear primer for anyone who wants the technical background.
Integrations come in three common shapes:
- Point to point: one system calls another directly. Simple, and fine for two or three connections.
- Middleware or integration platform: a central service passes data between many systems and keeps a log of every exchange.
- Event-driven: systems announce changes (webhooks, message queues) and others react, which suits high volumes and real-time updates.
In short: an integration is a small software product. It needs a clear scope, error handling, monitoring and an owner after launch. Partners who treat it as “just connecting two tools” leave you with fragile links that fail quietly.
What do API integration services include?
A partner takes an integration from idea to a running, monitored connection. At Odduel, a creative and technology agency in Barcelona and São Paulo, integrations sit within our application development service, because they follow the same discipline as any other software we build.
In practice the work covers:
- Discovery. Listing the systems, reading their API documentation, checking limits, authentication methods and data formats.
- Data mapping. Deciding which field in one system matches which field in the other, and what to do when they do not match.
- Architecture. Choosing point to point, middleware or events, and where the integration code runs.
- Build. Writing the connectors, transformations, retries and logs.
- Testing. Running real scenarios against test accounts, including failures, duplicates and edge cases.
- Launch and monitoring. Alerts when exchanges fail, dashboards for volumes, and a runbook for the team.
- Maintenance. Updating connectors when a provider changes its API or retires a version.
Which integrations do European businesses ask for most?
The same handful of connections appears in most projects, whatever the sector.
| Integration | Typical data flow | Main risk to plan for |
|---|---|---|
| Website or store to CRM | Leads and customers into the sales pipeline | Duplicates and missing consent records |
| Online store to accounting | Orders, refunds and VAT into the books | VAT rules per country and rounding |
| Store to stock or ERP | Stock levels both ways, orders out | Overselling when updates are delayed |
| Booking tool to calendar and payments | Appointments, deposits, reminders | Time zones and cancellations |
| Payment provider to store and accounting | Payments, refunds, payouts | Webhooks missed or processed twice |
| Marketing tools to CRM | Campaign responses, consent status | Consent not travelling with the contact |
How do you scope an API integration project?
Good scoping turns a vague wish (“make the systems talk”) into a document a partner can price and test against. Work through these steps with your partner.
- Name the trigger and the outcome. “When an order is paid in the store, an invoice is created in accounting within five minutes.”
- List every system and its owner. Include the plan or edition you use: APIs often differ between tiers.
- Map the fields. For each flow, a table of source field, destination field, transformation, required or optional.
- Decide direction and source of truth. For each piece of data, which system wins when two disagree?
- Define volumes and timing. Orders per day at peak, acceptable delay, nightly batches or real time.
- Describe failure. What should happen when the other system is down, a record is rejected or a duplicate arrives?
- Agree who is told. Alerts by email or chat, to whom, and how quickly someone must act.
- Set acceptance tests. The scenarios that must pass before launch, written in plain language.
A scope like this is usually a few pages long. It protects both sides: the partner can quote a fixed amount for known work, and you can check what you receive.
What does an API integration cost to build and run?
The cost depends on how many systems and flows are involved, how good their APIs are, and how much data transformation is needed. A fixed quote follows a short brief and a review of the APIs. Until then, these are the lines that move the price:
- Number of flows, not number of systems: three systems with six flows is more work than four systems with two.
- API quality. Well-documented APIs with test environments are faster to work with than old or undocumented ones.
- Transformation logic. Matching product codes, tax rules or customer records across systems takes the largest share of effort in many projects.
- Real-time needs. Event-driven designs cost more to build than nightly batches.
- Running costs. Hosting, an integration platform subscription where used, and monitoring.
- Maintenance. Providers change their APIs; budget each year for updates.
How do you keep integrations secure and GDPR-compliant?
Integrations move personal data between systems, so they fall squarely under the General Data Protection Regulation. Treat security and data protection as part of the scope, not an add-on.
- Send only the fields each system needs (data minimisation).
- Store API keys and tokens in a secrets manager, never in code or spreadsheets.
- Use the narrowest permissions each API offers.
- Log exchanges without logging personal data you do not need.
- Check where each provider stores and processes data, and that a data processing agreement is in place.
- Make sure consent status travels with a contact between marketing tools and the CRM.
- Plan how deletion requests reach every connected system.
How do you choose an API integration partner in Europe?
Choose on evidence of similar integrations, clarity of process and a plan for after launch. Location matters less than working hours overlap, language and familiarity with European rules such as the GDPR and VAT across countries.
| What to compare | Strong sign | Warning sign |
|---|---|---|
| Discovery | Reads the APIs before quoting; asks about failure cases | Quotes from a list of tool names alone |
| Architecture | Explains why point to point, middleware or events | One approach for every project |
| Testing | Written acceptance scenarios, test accounts | “We test it when it is live” |
| Monitoring | Alerts, logs, a runbook for your team | No answer to “how will we know it failed?” |
| Ownership | You own code, accounts and credentials | Integration runs on the partner’s private account |
| Maintenance | A clear support arrangement and response times | No plan once the project is paid |
Our guide on how to choose an app development company covers the wider selection process, from shortlists to contracts.
Which questions should you ask before signing?
Put these questions to every shortlisted partner and compare the answers in writing.
- Which of our systems’ APIs have you reviewed, and what limits did you find?
- What happens to a record when the other system is unavailable?
- How do you prevent duplicates when a webhook arrives twice?
- Where will the integration run, and who pays for hosting?
- How will we be alerted to failures, and who responds?
- Who owns the code, the accounts and the API credentials?
- What is included when a provider changes or retires its API?
- How do you document the integration so another developer could take it over?
Should you use an integration platform or custom code?
Use an off-the-shelf integration platform when the flows are standard, volumes are modest and your tools have ready-made connectors. Use custom code when the logic is specific to your business, volumes are high, data must be transformed heavily, or platform subscription costs would outgrow the build.
Many projects combine both: a platform for simple flows and custom services for the core ones. A good partner explains the trade-off for your case rather than selling one answer. If the integration is part of a larger product, our MVP development guide explains how to phase it.
Questions people ask
What is an example of API integration?
A common example is an online store connected to accounting software: when a customer pays, the store sends the order through the accounting system’s API, which creates an invoice with the right VAT and marks it paid. Nobody re-types the order, errors fall, and the books are up to date the same day.
How long does an API integration take?
A single, well-documented flow between two modern systems can be built and tested quickly, while a set of flows across several older systems takes considerably longer. Discovery of the APIs, data mapping and testing of failure cases take more time than writing the connectors, so a clear scope is the best way to shorten the project.
What is the difference between an API and a webhook?
With an API, your system asks another system for data or an action when it needs it. With a webhook, the other system tells yours when something happens, such as a payment succeeding. Most integrations use both: webhooks to react quickly to events and API calls to fetch details or confirm the state.
Who maintains an API integration after launch?
Someone must own it, either your internal team or the partner under a support agreement. Providers change and retire API versions, credentials expire and volumes grow. Maintenance covers monitoring alerts, fixing failed exchanges, updating connectors and keeping documentation current, so the integration keeps working long after the project ends.
Do API integrations need to comply with the GDPR?
Yes, whenever they move personal data such as names, emails or orders. Send only the fields each system needs, protect credentials, make sure each provider has a data processing agreement, keep consent status with the contact and plan how deletion requests reach every connected system. These points belong in the scope from day one.
List the systems you use today and the information you copy between them by hand; that list is the start of a good integration scope. For how such projects fit a wider product, read our application development complete guide. When you want a partner to scope and build the connections, see our application development service or tell us what needs to connect.
