How is a new application integrated with existing systems?

Oscar Bout · 01 Oct 2026

Connecting a new application to existing systems is done through system integration: a technical process in which two or more software environments exchange data with each other via agreed-upon connection points. The most commonly used method is an API connection, but depending on your situation, multiple approaches are possible. In this article, we answer the most frequently asked questions about system integration, from technical preparation to post-delivery management.

What integration methods exist for connecting systems?

There are four widely used integration methods: API connections, file exchange (such as CSV or XML), database connections, and middleware platforms. Which method fits best depends on the systems you want to connect, how up-to-date the data needs to be, and how much control you want to maintain over the process.

Each of these methods has its own area of application:

API connections are suitable for real-time data exchange between modern applications. They are flexible, well-documented, and widely supported.
File exchange works well when systems do not need to communicate in real time. Think of a nightly export of order data to an accounting package.
Database connections provide direct access to the underlying data, but carry risks in the areas of data security and consistency.
Middleware platforms such as an ESB (Enterprise Service Bus) or iPaaS solution act as an intermediary that connects multiple systems and manages message traffic.

In complex environments with multiple systems, a middleware approach is often the most manageable choice. For simpler connections between two modern applications, a direct API connection is sufficient.

What is an API and how does it work in system integration?

An API (Application Programming Interface) is a standardized interface that allows two applications to communicate with each other. In system integration, one application sends a request via the API, and the other application sends back a response with the requested data or a confirmation of an action.

You can compare an API to a waiter in a restaurant: you (the application) give your order (the request) to the waiter (the API), who forwards it to the kitchen (the other system) and brings back the result. The kitchen doesn’t need to know what you look like, and you don’t need to know how the kitchen works.

In practice, we distinguish two commonly used API types:

REST APIs are lightweight, work via HTTP, and are the most common choice for modern web applications and mobile apps.
SOAP APIs are older and more strictly defined, but are still widely used in sectors such as finance and government where strict protocols apply.

A good API has clear documentation, error messages you can debug, and an authentication mechanism such as OAuth or an API key to prevent unauthorized access.

How does the technical preparation for a system connection proceed?

The technical preparation for a system connection begins with mapping out the systems involved, the desired data flows, and the technical capabilities of each system. Without this inventory, you risk discovering halfway through the project that a system does not support an API or that data formats do not match.

A structured preparation typically proceeds in these steps:

Inventory: Which systems are being connected? What data needs to be exchanged and in which direction?
Technical analysis: Does each system offer an API? What are the limitations, such as rate limits or outdated protocols?
Data modeling: Do the data structures match? If not, you need to define a mapping that translates the fields of one system to the other.
Authentication and security: How are the systems secured? What permissions does the connection need?
Test environment: Set up a sandbox or test environment before going into production, so you can identify errors without touching live data.

This preparation takes time, but saves you many problems later in the process. The better the analysis, the smaller the chance of unexpected costs or delays.

What challenges are most common when integrating legacy systems?

When integrating legacy systems, the most common challenges are the absence of a modern API, outdated data formats, poor documentation, and limited testing options. Legacy systems are often built in an era when integration was not a priority, which makes the connection technically more complex.

Concrete problems you encounter with legacy integrations:

No API support: Older systems sometimes only communicate via direct database access or file exchange, which requires additional middleware.
Outdated protocols: Some systems use FTP, SOAP, or proprietary formats that require additional conversion layers.
Missing documentation: If no one knows exactly how the system works anymore, reverse-engineering the data structure takes a lot of time.
Limited testing options: Many legacy systems have no separate test environment, meaning you have to work directly on production data.
Performance issues: Older systems are not always designed for the load that real-time API requests bring with them.

The solution is often to build an intermediate layer that handles communication with the legacy system and converts the data into a format that the new application understands. We also refer to this as an adapter or integration layer.

When do you choose custom integration versus an off-the-shelf connector?

You choose an off-the-shelf connector when a reliable, well-maintained connection is available for the systems you want to connect and when the standard functionality meets your needs. You choose custom integration when the standard connection does not exist, is not flexible enough, or when you want to incorporate specific business logic into the integration.

A comparison to clarify the choice:

Off-the-shelf connector: Faster to implement, lower initial costs, but less flexible. You are dependent on the vendor for updates and maintenance. Suitable for standard connections such as an online store to an accounting package.
Custom integration: More development time and higher initial investment, but fully tailored to your processes. No dependency on external vendors. Suitable when you have unique business logic or work with systems for which no standard connector exists.

In practice, many companies opt for a hybrid approach: standard connectors where possible, custom work where necessary. This keeps costs manageable without sacrificing functionality.

Who is responsible for managing a system connection after delivery?

After delivery, responsibility for managing a system connection lies with the party that built the connection or with a designated administrator, depending on the agreements made in advance. Without a clear owner, connections quickly become outdated or break when one of the systems is updated.

Good management of a system connection includes:

Monitoring: Actively tracking whether the connection is working correctly and quickly flagging error messages.
Version management: Keeping track of which versions of the systems and APIs involved are in use, so you can anticipate changes in a timely manner.
Documentation: Recording how the connection works, so that future adjustments can be made quickly and correctly.
Incident management: A clear process for when the connection goes down or data is passed on incorrectly.

At delivery, always make agreements about who manages what. Record this in an SLA or maintenance contract, so there is no ambiguity if something goes wrong.

How we help with connecting your applications to existing systems

At 3Bird, we help companies design and build system connections that truly work, even when the situation is technically complex. Whether you want to connect a new application to a modern SaaS platform or to a legacy system that has been in use for ten years, we ensure an integration that aligns with your processes.

What we bring to the table:

A team of experienced developers with expertise in REST APIs, SOAP, middleware, and custom integrations
Dutch fractional CTOs who provide technical direction and guide you in your own language
Flexible deployment: scale up or down as needed, without long-term commitments
Support for a wide range of technologies, including AWS, Azure, NodeJS, .NET, and more
Rates from €25 per hour, keeping high-quality software development accessible

Want to know which approach best fits your situation? Get in touch via contact@3bird.nl or call us at +(31)75-7993038. We are happy to think along with you without obligation.