A Guide to Modernizing Legacy Applications

Every organization faces the issue of legacy applications sooner or later. Maybe you’re already struggling with a legacy application today. Modernizing these legacy applications can often significantly extend their lifetime at a reduced cost compared to rewriting from scratch. This article is intended to guide you through the modernization of your legacy application. 

But before you even start your modernization efforts, I recommend analyzing whether or not it’s actually a good idea to modernize the application.

Is the Modernization of a Legacy Application a Good Idea?

Modernizing legacy applications isn’t always necessary. Maybe the application is running fine, doesn’t need modifying, and isn’t mission-critical. Maybe there are off-the-shelf alternatives you can purchase at a lower
TCO. Or maybe it even makes more sense to stop selling or using the software entirely. 

Another case could be that the issues the legacy application is facing are so large that modernizing isn’t worthwhile. In this case, you may require a rewrite. 

But in cases where the application must remain supported, needs new features in the future, and is important enough to the organization, it’s a good idea to modernize a legacy application. Why, you ask?

What Are the Benefits of Application Modernization?

By modernizing your legacy application, you can reap one or more of these benefits: 

  • Improved security: older components often lack modern security patches, placing your application at risk.
  • New features: sometimes, it can be nearly impossible to implement certain required features in a legacy application.
  • Improved developer satisfaction: legacy code often causes frustration in a team, which can lead to high turnover if there’s no roadmap for improvement.
  • Increased productivity: modern applications allow developers to use modern tooling that increases their productivity; it also makes onboarding newcomers faster.
  • Reduced costs: legacy applications often operate on older hardware that is costly to run or requires more expensive experts to manage.

This all sounds very compelling, but there is still another question to answer. 

When Should We Modernize?

Generally, the time is now. It makes little sense to postpone this unless there are some crucial events that need to happen first. For example, your business is going through a re-org and teams are changing, or an important feature must be delivered to customers first. 

But other than that, the longer you wait, the more problems will pile up and the harder the modernization effort will become—especially if developers still have to add and modify code. 

So, once you know when to start, how do you approach this challenge? 


How Do You Modernize Legacy Applications?

Let’s look at several techniques you can use to modernize legacy applications. Which ones you choose will depend on your specific context: what techniques you’ve already implemented, which direction you want to go, and the number of resources you have available. 

Automated Tests

Many legacy applications lack a good test suite. This can be because of time pressure in the past or lack of experience on the team. Whatever the cause, automated tests are the first step to modernizing legacy applications. Automated tests give a team a safety net to refactor and make changes. 


A legacy application is often a complex system of intertwined components. Splitting up the application into microservices is a good way forward. Microservices are services that encapsulate all logic for a specific functionality or feature. 

To do so isn’t always easy, however. Here are two techniques for making use of microservices. 

Strangler Pattern

In the strangler pattern, you start with your legacy application. Next, you put a facade in front of it whose only role is to forward traffic to the application. Next, you extract functionality out of the legacy application and into a new service. You then modify the facade to point the relevant traffic to the new service. Repeat this, and your legacy application becomes smaller and more manageable until it’s completely replaced over time. Sometimes, the microservices is internal logic and has no advantage of a facade. In that case, you just want to extract a microservices. 

Extracting Microservices

In this technique, you move specific pieces of code out to a new service. Then, instead of making in-memory calls, the legacy application will call the new microservices. If you repeat this, you’ll end up with a set of microservices that communicate with each other, and the legacy application might end up being the facade from the pattern above.  


Containers are a good way to modernize legacy applications. If you can run your application in a container, then deployment, scaling, and testing become easier. For example, it’s easier to update the underlying OS and deploy it to the cloud. 

If it’s currently not possible to run your application in a container, you could deploy it to a virtual machine in the cloud.

Modernize Communication Between Services

Another interesting aspect of modernization is the opportunity to update the communication between applications. Older applications often use older protocols. And if you’re splitting off pieces of your application into microservices, you’ll have to decide how the different applications communicate with each other. 

Choose modern standards and protocols. Also analyze if your applications need synchronous or asynchronous communication. Asynchronous communication can increase performance and reliability. 


Modernizing a legacy application isn’t just a technical effort. Many companies also need organizational changes. Development teams in the past weren’t responsible for running and monitoring the application. Today, it’s beneficial to give them this responsibility. This will incentivize them to write reliable code and will also empower them to make the necessary changes to modernize the application. 

Tooling & Training

Because the DevOps culture requires more from the developers, they’ll require better tools. Legacy applications were often developed in a different time, when developers could focus on the code. In a DevOps world, developers have more tasks and responsibilities, requiring training and better tooling. Developers will need to broaden their horizons beyond just coding. 

Leverage the Cloud

Major cloud providers offer many tools and services that can greatly improve the productivity of your teams. That is why moving to the cloud is an important part of modernizing a legacy application. If it’s still running on local servers, use the previous techniques to move (parts of) the application to the cloud. 

The above techniques should be able to reinforce each other. Doing one will make the other easier, which in turn will make the first easier again. 

But it definitely isn’t an easy endeavor. Let’s look at some common pitfalls you might encounter.

Common Issues & Best Practices

Update Old Tools

Legacy applications were often written in times when there were a lot fewer tools available to the team. Some teams developed their own tooling to improve their productivity. This creates extra assets to maintain and update. When modernizing an application, it’s best to look at what’s available in the current market. Some teams might argue their code is special and the existing solutions don’t address their problems. This is rarely the case. 

Include Multiple Teams

Another problem can arise when only the developers are tasked with the modernization. Ideally, all stakeholders are informed of the modernization effort. Customers will want to know there could be hiccups. Management should be aware that they have to reorganize teams and provide training. 

Set Clear Intentions

You’ll have to have a decent plan in order to go forward. Trying to speed up too much will leave pieces of the modernization unfinished or untested, creating new legacy code. The team must know where they’re heading and how they want to achieve their goals. In your plan, allow ample time to complete this work. 

Prepare for Future Problems

Ensure that changes are backwards compatible. Make sure you can roll back a change if you deployed something that breaks the user experience. 

Avoid a Rewrite

One last pitfall is the idea of a rewrite. You might be tempted to try a rewrite instead of modernizing, but a rewrite is many times more challenging than modernizing a legacy application. Legacy code contains many small fixes for obscure bugs, and those bugs could resurface in a rewrite, requiring you to fix them again. A rewrite also takes a long time and could give you a product that isn’t what the end user wanted. Comparatively, you’re sure the legacy application is currently addressing the user’s needs. 

Modernizing Legacy Application Is Worthwhile

While it can seem daunting to modernize a legacy application, it’s often worth the effort. But it can rarely be done in a few days or weeks. Plan accordingly, engage all stakeholders, and move at the correct pace to ensure nothing breaks. By adopting modern standards, both in code and in the organization, you’ll be able to modernize your legacy application successfully. 


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