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Find out how to modernize business systems using microservices APIs


What are microservices APIs?

Posted by Angela Davis on February 17, 2023
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As more and more organizations embrace digital transformation, there’s a need to change their systems and processes from the monolithic architecture they’ve used for years. A huge part of that transformation includes moving to cloud-based alternatives, which can save money and offer better security.     

The legacy systems of many businesses, particularly larger ones, are now mostly outdated server-side. They can struggle to deal with the demands of a fast-moving digital business environment and a new technology stack with accompanying know-how. With a need to rapidly improve and refactor those systems, it’s time for businesses still using legacy systems to look to the future. 

Two keywords for many businesses are flexibility and customizability. They can achieve these goals by implementing microservices architecture networks with API-powered connections. This increases efficiency and offers scalability when needed. So, what should organizations know about microservices application programming interfaces (APIs)? 

APIs for modern digital infrastructure

The main issue with outdated legacy systems is that they need to be designed to deal with the challenges of the digital transformation era. However, completely replacing those systems is both costly and unnecessary. An alternative  solution is to modernize those systems in place, allowing you to integrate critical legacy data and processes with a modern API and microservices-based architecture). 

Using OpenLegacy Hub helps you move away from unwieldy monolithic application legacy systems and create an abstraction layer of microservices that deal with your most critical processes. For example, the payments system for a financial institution could consist of a mesh of microservices that handle different elements of that process. This can move you towards a more modern architecture, as you see with companies such as Netflix or Amazon AWS. 

This method enables an organization to introduce modern technologies and operate them harmoniously and continuously with its existing legacy infrastructure, making the best out of both. 

When you have a legacy system, breaking tangled applications into smaller microservices helps improve functionality. It also makes it easier to maintain oversight of that microservices-based application and develop it further as needed, including testing the app as you make changes. OpenLegacy Hub can play a major role if you are modernizing your system.  

The REST API framework

Flowchart showing the REST API framework

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When you modernize your architecture, APIs play a major role in your integration framework, handling any requests from external sources. For example, suppose you are a bank, and a customer initiates a request. In that case a REST API controller will send that request to the relevant resource handler, which in turn interacts with the appropriate resources to execute the process. 

Once that external request has been executed successfully, any resulting resource is processed by something known as a resource serializer which creates a resource response. These serializers can work in both XML and JSON. As your REST API is part of your integration framework, it can access automated authorization, system properties, and authentication. 

External and internal APIs

Many organizations do not understand the differences between internal and external APIs, yet this understanding is crucial to efficient API management.

Internal APIs

An internal API allows your developers access to all application functionality and backend data when it comes to web development. Internal APIs help companies become more efficient in various areas by working with any in-house DevOps teams. Areas, where internal APIs may be used, include customer service and HR.

When your DevOps team creates a new application, it can be distributed publicly but with the actual interface only seen by those working with the API publisher. Internal APIs help your in-house team reduce the time to create new applications and provide a common pool of software resources that different developers can draw on. 

External APIs 

As the name suggests, external APIs are accessible by a larger group than just your in-house developers. While it is published by your in-house team, it can also be used by external developers who want to register into the interface. With so many free-agent external developers available, publishing an external API means you have access to a wider range of innovative ideas. 

The success of any external API interface lies mainly in its ability to attract quality developers and it can help them develop applications that offer real benefits to your customers. You also need good engagement with external developers to drive success. If you take the same approach to internal and external APIs, you are more likely to build a good API ecosystem and become an API-first organization. 

Microservices software architecture and how it differs from APIs

Diagram showing a basic microservice held together by internal APIs

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While you may see these terms used a lot, it’s important to understand how APIs differ from microservices software architecture. Although they are separate entities, APIs and microservices are often paired, as different services within any microservice use APIs to ‘talk’ to each other. You could consider microservices as enablers of API when it comes to DevOps. 


APIs enable communication with other applications. Every piece of software has an API governed by a set of rules that define the acceptable requests that can be made to that API and a set of responses that can be made to those requests. 


Microservices architecture is the approach you take to building an application. It breaks down the application into a series of separate but connected programs that make it easier for your teams to develop software and customize it or maintain it in the future.  

The interaction that can happen between microservices and APIs

Diagram showing an API microservices gateway example 

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There’s an almost symbiotic relationship between APIs and the microservices you use to modernize your legacy system. They work together to help you build a more efficient microservices-based architecture for your applications. Think of microservices as the bricks that build your application, while APIs are the cement that holds them together and integrates them into your app. 

Having your DevOps team combine microservices with APIs lets them create a more modular architectural style designed to fulfill different needs. By doing so, they improve efficiency, allow for scalability and upgrades, save money, and create a more secure, more resilient system to meet any regulatory or compliance needs. 

Microservices’ need for API gateways 

While microservices architecture offers organizations many advantages, it also creates challenges that only API gateways can address. Microservices architecture separates the different functions of an application into modules that often focus on implementing a specific business rule. 

Modularizing different functions makes it easier for DevOps teams to develop, test, deploy, and customize/maintain the different functions and the overall capability of the application itself. As you would expect, there is an accompanying increase in the complexity of accessing services. 

Access to a microservice module

Each API gateway can handle a number of API calls at the same time and route them to the relevant backend microservice module. A gateway can also break a single call into multiple requests to other microservices and combine the responses.

Horizontal scalability of services

The use of API gateways allows you to scale horizontally. More API gateways can be added to a cluster and achieve load balancing across the cluster. By load balancing, you can virtualize your APIs and have them execute the same policies, meaning you can easily scale horizontally when needed. 

Translation of protocols

By using protocol translations, API gateways can help microservices and client requests communicate with each other even when using different programming languages. This allows for features such as microservices Java automation (javascript). Your gateways translate any API protocols from the devices used by end users to the different languages used by your microservices. For example, if the request is in XML, but the microservice uses JSON, the gateway will send the answer back in XML. 

Traffic control for request-response management 

The last thing you want is for your network to be overloaded or, even worse, face a distributed denial of service (DDoS) attack. Microservice-based API gateways help control traffic into your network. They can blacklist sources of malicious requests as well as reduce the number of requests made to affected services to prevent them from becoming unresponsive. 

Functions to expect from API gateways 

Image showing how API works

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Authenticate, authorize, and audit requests 

With cybercrime on the rise, an API gateway is your first defense against malicious attacks. It can perform security functions on requests that include virus scanning, decryption/encryption, authorization, authentication, and many other security functions. 

Manage a service registry for better routing

Your service registry contains all relevant data structures for different network service instances. The registry knows where every service is located (including node name, host, port, and all pertinent metadata). In effect, it’s a messaging system that communicates data at the application level.

Prevent security attacks 

API gateways mitigate the risk of security attacks by having an input validation for all incoming requests. This can help intercept any malicious requests before they reach your backend services. While they are efficient security guards, you should also implement other security measures to supplement them. 

Monitor data

You already know the importance of data collection and analytics. API gateways help you see how requests are sent to your gateway, from where, and when. They can monitor the regularity of rejected requests and the reason why. They can also help you monitor any trends in the data that passes through them. 

Microservices and API gateways – get the best of both worlds with OpenLegacy

Diagram showing how integration microservices have typically been hard to create, largely due to complex, manual effort

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Modernizing your legacy system is daunting but can be cheaper and just as efficient as replacing it completely. While it may be intimidating, and you will face hurdles during the process, digitalization is  necessary when operating in the digital transformation era. 

The biggest hurdle you will face is integration and migrating of core business architecture and systems to a wholly digital environment. OpenLegacy’s API integration platform can help you leverage your existing IT legacy system and modernize it by creating and deploying the microservice-based APIs your systems need to bring them up to the required efficiency. 

OpenLegacy allows you to leverage your existing legacy, core, and on-prem systems to create and deploy lightning-fast microservice-based APIs.  You can build microservices that enable you to implement innovations and deliver advanced services to your clients in days, and hours without making any changes to your existing system.


FAQS about microservices and APIs

Do all microservices have an API?

Although you can access microservices via APIs, not every microservice has to have an API or be part of one. The main purpose of microservices is to modularize your applications into separate and independent parts. This provides your DevOps teams with more flexibility and customizability when creating applications. 

What are the 3 types of APIs?

The three most common forms of API architecture are: 

  • REST: the most common type, REST, is a collection of rules/guidelines for simple but scalable web services and APIs. 
  • SOAP: is a stricter protocol used with more secure APIs. 
  • RPC: this API protocol allows you to invoke processes that can be written with either XML or JSON. 

What are the 3 main components of a microservice?

Three of the main components of a microservice are: 

  • Containers: are the executable parts of your software where code is packaged alongside any relevant libraries and dependencies. 
  • Service mesh: refers to the layer containing configurable and dedicated infrastructure that manages all network communications within the application. 
  • API gateways: are crucial when it comes to communication in any distributed architecture. They act as the singular entry point for external requests and as an access point to your apps and services. 

How can you test microservices APIs? 

There are three primary tests for any microservices: 

  • Unit tests: the most used test, they look at individual services such as class, method, latency, or function in isolation. 
  • Integration tests: these tests work differently from those in other architecture types. The idea is to identify any defects in interfaces by making your microservices interact in a series of use cases.  
  • End-to-end tests: this form of microservices API testing ensures that your system meets the needs of users at any endpoint and achieves all business capabilities and objectives by testing the entire application so that user experience is optimal. 

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