Expert Advice on Integrating APIs with Legacy Systems in 2025
Application programming interfaces (API) play a critical role in the digital ecosystem. This online middleman allows software systems to communicate with each other. But what is it exactly, and how does it work?
Case in point: When you use an app to check the weather today, the API pulls data from the weather service and shows you the information on your device. Or when you pay for an e-commerce product using your PayPal account, the API connects the click-and-order store with the PayPal service provider to ensure a successful transaction.
That’s why companies and organizations invest in API products/services for seamless communication and transaction. However, incorporating API into your legacy systems can be challenging due to compatibility issues and more. How do you address these problems?
Keep reading for some expert advice on integrating APIs with your legacy systems in 2025. Let’s dive right in!
Challenges of Integrating APIs with Legacy Systems
To begin, API has been growing and expanding. Now powered by robotic process automation (RPA) and artificial intelligence (AI), it has become more efficient and effective in sharing information across various digital platforms.
On the flip side, legacy systems remain as is—using old technology, whether software or hardware. Still, some companies have been using these systems since they have long worked for them.
The problem starts when you plan to integrate modern APIs with conventional legacy systems. For informed decisions, here are common challenges encountered with API-legacy system integration:
- Compatibility issues: As cited, APIs are modern with new standards, while legacy systems are traditional using old protocols. So, expect some API features and functions to be incompatible with your outdated systems. For instance, you might struggle to incorporate APIs with your landline phones for your contact center—leveraging voice over Internet Protocol (VoIP) is key!
- Lack of documentation: Some companies heavily rely on legacy systems as they’ve been working for them. However, these systems often lack proper documentation since their early deployment. As such, it can be challenging to understand how they work and ultimately connect with modern APIs. See how often organizations update their APIs on average below:
- Limited flexibility and scalability: Most legacy systems are old, outdated, and not designed to work with external systems. Hence, it will be hard for you to integrate APIs with these systems due to a lack of customization features, making it also difficult to scale your business in the long run.
- Privacy and security risks: Most legacy systems are designed not to support modern security protocols, such as OAuth or TLS. This makes them vulnerable to security breaches and privacy issues. Even API integration with advanced platforms still faces potential risks, like authentication problems (39%), privacy incidents (38%), and network vulnerabilities (37%):
- High integration costs: Integrating APIs with legacy systems can be costly due to complex installation and upgrade requirements. Often, you’d have to modify or upgrade your systems to ensure they work with APIs requiring specialized expertise. Meanwhile, modern systems have customization features, making API integration more seamless and less expensive.
Learn strategic tips from business experts on integrating API with legacy systems below.
Strategic Approaches to API Integration
There’s no denying the continued rise of the API management industry. In fact, its global market is projected to grow from $5.42 billion in 2024 to $34.17 billion by 2032 at a 25.9% compound annual growth rate (CAGR). Rapid digital transformation and early technological adoption drive this market growth.
Image source - fortunebusinessinsights.com
That’s why even big companies with established legacy systems invest in API development. They want to integrate API features and functions into their systems for seamless communication and collaboration. However, they confront underlying issues like those mentioned above when doing so.
Some business experts have shared how to integrate API with legacy systems—Here’s how:
1. Examine the systems and plan the integration thoroughly
As cited, modern APIs and legacy systems are worlds apart. APIs are constantly evolving while keeping up with industry trends. Meanwhile, legacy systems rely heavily on traditional databases with zero-to-minimal customization features. To meet halfway, you should examine your systems thoroughly and plan how to integrate APIs with them.
Reyansh Mestry, Head of Marketing at TopSource Worldwide, recommends system upgrades and modifications to make the integration plausible.
Their company has its fair share of undergoing full audit and integration designs when working with clients needing payroll services. “A thorough system audit is essential before integrating APIs with legacy systems. It helps identify limitations and plan the necessary upgrades or adjustments to ensure smooth and efficient integration.”
2. Bridge the gap with middleware or API gateways
API and legacy systems require a bridge to facilitate complex integration. First, you can capitalize on middleware—from SOAP to REST—to bridge the gap between old and modern systems. Second, you can establish centralized control by using API gateways (like Apigee API gateway) to handle traffic management, authentication, and scaling.
Take it from Jeffrey Zhou, CEO and Founder of Fig Loans. With the rise of mobile/online banking, he recognizes the value of API investments in the banking, finance service, and insurance (BFSI) industry. He has witnessed how banks have begun integrating APIs with their legacy systems.
Zhou suggests, “To bridge the gap between APIs and legacy systems, consider using middleware or API gateways. Middleware can connect older systems to modern technology, while gateways help manage traffic, security, and scaling effectively.”
3. Modernize without replacing through API wrappers
Integrating APIs with legacy systems can be challenging due to compatibility issues, compelling businesses to replace their systems altogether. However, there’s a trick of the trade—build API wrappers to modernize old systems without replacing them entirely. The idea is to add a modern layer on top of old tech without messing with its core.
Learn from Gary Hemming, Owner and Finance Director at ABC Finance. As a financial institution, they aim to keep up with industry trends but find technological upgrades challenging. However, they see how API wrappers are valuable for integration with legacy systems.
Hemming explains, “To modernize legacy systems without a full replacement, consider using API wrappers. They add a modern interface to outdated technology, allowing seamless integration while preserving the system’s core.”
4. Leverage digital tools and technologies for integration
The best way to combat outdated technology is to use modern technology. But as mentioned, you don’t need to replace your legacy systems—only upgrade them. To make API-legacy system integration possible, utilize AI-powered platforms like MuleSoft, Zapier, or Apache Camel to streamline integration.
Stanislav Khilobochenko, VP of Customer Services at MacKeeper, recommends harnessing the power of digital tools and technologies. He believes that they enable seamless integration between APIs and legacy systems. Employ automated testing platforms to test compatibility and performance issues and API monitoring tools to see areas for improvement.
Khilobochenko shares, “Using digital tools like AI-powered platforms can make API integration with legacy systems much smoother. These tools streamline the process, enable compatibility testing, and help monitor performance to identify areas for improvement.”
5. Standardize API practices and optimize continuously
Finally, performance optimization should be part of the overall integration equation. The end goal is to standardize API workflows and optimize them for significant improvements. For one, use modern standards, such as REST, GraphQL, or OpenAPI specifications. Then, guidelines should be set in place, such as naming, error handling, and data formats.
Further, Chris Aubeeluck, Head of Sales and Marketing at Osbornes Law, emphasizes the need for performance tracking and optimization. He suggests setting API metrics and using application performance monitoring to track performance. However, he believes optimizing your
API-legacy system integration should be based on modern standards.
Aubeeluck shares, “To get the most out of your API-legacy-system integration, standardize workflows and use modern specifications. Continuously track performance and optimize based on clear metrics to ensure everything stays efficient and effective.”
Best Practices for API-Legacy-System Integration
At this point, you learned strategic approaches to integrating API with legacy systems. Now, it’s time to implement some best practices for such an integration to ensure stability. Below are some:
- Conduct a full audit regularly. A thorough examination should start before the actual integration to avoid issues in the long term. However, make it a habit to check your systems regularly as part of your upkeep. That way, you can fix minor issues before they escalate to major ones.
- Comply with API standards. To begin,API integration with legacy systems defies modern standards. But strive to follow standard protocols as much as possible to keep up with the latest innovations. This way, your business won’t be left behind.
- Design for scalability and flexibility. Integrating APIs with legacy systems might be difficult. That’s where the challenge begins—resort to modifications and upgrades to make your systems flexible and scalable. Gartner predicts that by 2026, over 30% of the growing demand for APIs will come from AI and tools powered by large language models.
- Perform rigorous testing. Testing is a crucial part of the overall equation. First, conduct a pilot run before the integration and adjust until all systems work smoothly. Also, test your system integrations regularly, whether using functional, performance, or usability testing. Lastly, fix your systems as needed.
- Document the integration. Since legacy systems lack proper documentation, the integration with legacy systems now requires API documentation. Document everything, from the technical structure to the API and legacy system details to data mapping and security measures. Doing so will make your system maintenance, repair, and future integration much easier, simpler, and faster.
- Focus on security and privacy.Both should be your top priority during the API-legacy system integration, as you don’t want to put your business at risk. Consider the rise of cyberattacks, such as phishing, malware, and Denial of Service (DoS) attacks, and invest in security. The API security market could achieve a 17.5% CAGR—so capitalize on it!
Image source - marketsandmarkets.com
- Track and measure performance. Start by setting API metrics, whether core usage (API call volume, usage growth, and latency) or user-specific (feature usage frequency, endpoints access, and usage patterns. Then, conduct regular API monitoring and measure your performance against the set metrics. Finally, fix and optimize your systems to ensure they’re working properly.
Final Words
API is a critical component across different industries in today’s digital landscape. It enables systems to communicate with each other, allowing stakeholders to work together. However, integrating API with legacy systems is one major problem due to compatibility issues, limited documentation, flexibility concerns, security risks, and high costs.
That said, consider some business experts’ strategic approaches to API integration with legacy systems. Likewise, follow the best practices shared to ensure a seamless and successful integration. With all these practical tips and steps, you can maximize the API potential to establish a collaborative digital platform for your business!
Planning to incorporate API products into your legacy systems? Consider integrating API Products automatically with Moesif for a smooth transition. Request a demo today and get started with us for free!
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