Architecting Legacy Applications and Paths for Modernization

Learn more about IT Architecture and legacy applications on the path to modernization.

There is no doubt that all of us working in a software engineer or an architect role have engaged with legacy applications at any point in time. In this article, we shall aim to learn about the legacy system and ways to refactor it and also explore various migration paths to the cloud.

We shall also examine other aspects of a legacy application that can be modernised, including software development methodology, and the build and deployment procedures.

Legacy Applications

Many of us enjoy working in brand new Greenfield software systems because they involve a modern technology stack designed from the ground up. Such systems have little constraints to the prior work and do not need to be integrated with the existing systems. However, whatever new systems we have today will become a legacy tomorrow.

We often find ourselves working on existing software systems and being able to do so is a valuable skill. A legacy application is an existing software system which is still in use but also difficult to maintain. A handful of challenges arise from using a legacy application.

Issues and Challenges

Working with a legacy application usually brings with various problems that need to be overcome. The most important of them is difficult to maintain and extend. It may include older, outdated technologies and in addition very often development best practices not being followed. Apart from this few other concerns such as security, inefficiency, incompatibility and compliance/GDPR-related things.

A legacy application tends to be older, and over time, several modifications are added by developers. As these modifications continue to be made on a codebase, the code can become untidy and messy, resulting in a spaghetti code.

A legacy system will also accumulate some technical debt. Any design decisions that were made for an application, in which an easier, quicker solution was selected over one that was a cleaner solution but would have taken longer to implement, incur technical debt. Technical debt entails future rework for system improvement.

Refactoring Approaches for Legacy Applications

When we engage with a legacy application, we want to refactor it in order to augment the maintainability factor. It could be an addition of new features, bug fixes, design improvement, increase quality or overall optimisation. In order to perform these tasks, the legacy application must be in the desired state so that enhancements or changes can be easily made and without much risk.

Also, the right attitude is required before touching upon the legacy codebase refactoring. New developers/architects should have respect for the earlier development team because there are reasons why things were done in a certain way and newbies may not always be aware of all the decisions that took place and the rationale behind them.

As software developers/architects, we should aim to modernise and improve the legacy application that you have been overseeing. We should avoid making unnecessary modifications, especially if we do not fully understand the impact of the changes. The following tasks are involved during the legacy refactoring phase.

1.     Making legacy code testable

2.     Removing redundant code

3.     Use tools to refactor the code

4.     Making small, incremental changes

5.     Transforming monoliths to Microservices.

Making Legacy Code Testable

A lot of legacy software applications lack automated unit tests and only some of them have adequate code coverage. Also, there were few legacy applications which were developed and unit testing was not even considered as a scope then. Owing to this, it was difficult to add tests later. Proper unit tests in addition to a legacy system should be of utmost priority. It is because the unit tests are those pillars which will ensure new changes did not introduce new defects and that the functionality still works. 

Regular execution of unit tests post-change as a part of the build pipeline will make debugging a cakewalk. 

Removing Redundant Code

Legacy applications are quite older and there is a high probability of having duplicate code segments. The reason being it was maintained by various people and for this increased instance of code that is either repetitive or obsolete.

Reducing the total lines of code will for sure reduce the complexity and easier to understand by other fellow engineers.

Some useful tools can identify some types of code that are unnecessary. Refactoring unobtainable, dead, reflected- out and duplicate code will enhance the maintainability of the system. 

Using Tools to Refactor

It is advisable to exploit the advantage of development tools that comes with licensed IDE workbenches. These utilities can detect areas of your codebase that can be refactored. 

Making Small, Incremental Changes

 During refactoring work, some of the changes may be pretty large. In this case, small incremental changes sometimes are the right approach to improve a legacy codebase. To avoid negative consequences, writing and execution unit test is strongly recommended. 

Whenever there are normal tasks for developers like bug fixes, or small enhancements, it is an opportunity to improve the area of the code base that is being changed. Over time, more parts of the codebase will get improved.

Transition to Microservices

Legacy applications have a monolithic architecture where all modules are packed together as a single unit. 

Converting a monolithic application into a decentralised Microservices-based architecture solves plenty of problems that exist in legacy monoliths. Microservices are independent, self-contained miniature services that are deployed individually. This section will be described in more detail later in this document.

Legacy Modernisation Strategies

There is a variety of strategy options that can be adopted for legacy modernisation.

Encapsulate. Leverage and extend the application features by encapsulating its data and functions, making them available as services via an API.

Rehost. Redeploy the application component to another structure( physical, virtual or cloud) without modifying its code, features or functions. 

Replatform. Resettle to a new runtime platform, making minimum changes to the code, but not the code structure, features or functions.

Refactor. Restructure and optimize the existing code (although not its external behaviour) to remove technical debt and improve non-functional attributes.

Rearchitect. Materially alter the code to shift it to a new application architecture and exploit new and better capabilities. Rebuild. Redesign or rewrite the application component from scratch while preserving its scope and specifications.

Replace. Eliminate the former application component altogether and replace it, considering new requirements and needs at the same time.

Convert the Legacy To Cloud-based Microservices Architecture

Out of the above approaches, the best is to use a combination of refactoring and re-architecting which is arguably the most effective technique in the long run. Also, it brings a few uncertainties and skills and expertise. This strategy can be implemented with the help of microservices architecture hosted over the cloud.

Let us see some of the technical aspects of microservices migration strategies.

1.     Packaging: Instead of bundling up the related modules together, split the modules into independent packages. This involves minor changes to the code or more static content

2.     Containers: Apply the “container per service” deployment pattern to package into its individual server space or preferably its own container.

3.     DevOps: Once all the components are broken down, you can manage each package through an automated delivery pipeline. The idea is to build, deploy and manage independently.

Strategy 1 Strangler Pattern

Today, the Strangler Pattern is a popular design pattern to incrementally transform a monolithic application into microservices by replacing a particular functionality with a new service. Once the new functionality is ready, the old component is “strangled,” that is, decommissioned, and the new service is put into use. Any new development is done as part of the new service and not part of the monolith.

Strategy 2 Domain-Driven Design

Domain-driven design is an effective pattern to build software that has complex and ever-changing business requirements. Most organizations are in this situation, as they have complex business processes that are evolving to become even more complex.

Domain-Driven Design (DDD) is a software development approach introduced by Eric Evans in 2003. It requires an understanding of the domain for which the application will be written. The necessary domain knowledge to create the application resides with the people who understand it: the domain experts.

The general migration has three steps:

  • Stop adding functionality to the monolithic application
  • Split the front from the backend
  • Decompose and decouple the monolith into a series of microservices

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Relevant Blogs:

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3 Easy Steps for a (Dev)Containerized Microservice With Jolie and Docker 

Is Python Effective for Microservices Architecture? 

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