What is a Legacy System in Software Engineering: Strategic Evolution Options

What is a Legacy System in Software Engineering: Strategic Evolution Options - GuidingCode

When dealing with software engineering and development, you may come across the term “legacy system”. So, what exactly is it?

This guide goes in-depth to explain the characteristics that classify a legacy system from a software engineering standpoint, along with strategic ways to evolve, update, or deal with such systems.

With that said, let’s get right into it!



What are Legacy Systems?

Legacy systems are older generation operational systems in an organisation that depend on languages or technologies that are obsolete for newer system developments. Additionally, these legacy systems may also rely on older processes and protocols, as well as older hardware, like mainframe computers. Hence, legacy systems involve the entire environment, including the hardware, software, libraries and additional supportive software that’s essential to its business functionality.

Nevertheless, despite being old or often obsolete, these systems are essential to the organisation’s business model and environment. Plus, replacing legacy systems, is often a risky and costly endeavour, promoting businesses to pursue these older legacy systems.


Strategic Ways or Options to Evolve Legacy Systems

Having said that, the strategic evolution options or methods for these legacy systems are how these organisations aim to handle these legacy systems for the future with evolving technologies and changing business requirements. Essentially, these organisations have to determine whether to reuse or replace these systems entirely.

For this, there are 4 primary strategic options for evolving such legacy systems, and the strategy chosen depends on the system quality and business value:


1. Scrap or remove the legacy systems entirely, abandon their maintenance, and replace them with a newer system

This outcome is chosen when the system is no longer effectively functioning or in any way contributing to the business operations. Thus, the software’s purpose is said to be outdated. This typically happens when the business processes have been changed, ever since the software was first deployed or installed. Therefore, the business processes are no longer dependent on such obsolete legacy systems.

In that case, it’s best to abandon the maintenance of the legacy system and proceed to replace the entire system with a newer system that fulfils the newer business processes and requirements. Usually, legacy systems that are of low quality and serve low business value, are recommended to be scrapped or abandoned.

What is a Legacy System in Software Engineering: Strategic Evolution Options - GuidingCode


2. Make no changes to the legacy system and proceed with regular maintenance

We opt for this outcome when the legacy system is required and still found to be relatively stable and reliable. Thus, even the system users will make a lesser number of change requests or bug and issue reports, since the system is mostly functional and still fulfils most of the business requirements.

Such legacy systems that are high or acceptable quality and serves high business value, can continue with their operations under normal system maintenance procedures or schedule. In such cases, the system may not even need to be improved in terms of maintainability either.


3. Repurpose or reengineer the legacy system to enhance its maintainability

This option is favoured when the system quality has reduced as a result of changes in the business environment and due to proposed changes to the system. Also, it’s considered when the system’s maintenance cost is seen to be excessive, even exceeding the cost of reengineering the system.

Hence, this procedure involves constructing new interface elements or components, allowing the initial system to work with other current systems. Overall, the aim of this procedure is to improve the system in terms of reducing the workload and cost of maintaining the system.

What is a Legacy System in Software Engineering: Strategic Evolution Options - GuidingCode


4. Replace all or part of the legacy system with a more recently developed system

This step is advised when the newer hardware and tools available and off-the-shelf ready-made systems and technologies allow the newer systems to be developed at a more affordable cost. Thus, it’s financially unwise to continue maintaining an older legacy system that uses older technologies and hardware that costs more to maintain. Instead, it’s better to use these newer technologies to develop parts or entirely new systems.

Therefore, this evolutionary replacement strategy or procedure is typically employed, when major system components are able to be replaced by ready-made systems and additional components whenever possible.


When Should a Legacy System be Replaced Partially or Completely?

From there, a key dilemma that business owners or developers have is when and whether to replace parts or an entire system or instead continue with the regular maintenance of the system. Therefore, after evaluating the system’s quality and business value, it’s best to replace a system or parts of it than continue maintaining its software if:


1. The legacy system is of a low quality and yields low business value

What this means is that the system is expensive to operate and maintain. Plus, its output is inadequate as well and rigged with tonnes of bugs. Thus, leading to multiple user reports regarding issues with the system. Overall, such a system may not fulfil the original requirements effectively.

Plus, with the maintenance costs in mind, it might even cost less to replace the system with newer off-the-shelf systems that are available in the market. Even though it isn’t expensive to maintain, if the system doesn’t contribute to the business requirements, it’s best to abandon it, since maintenance can become too costly down the line.


2. The system’s hardware is already undergoing replacement

Additionally, one would opt for this replacement approach in scenarios where the hardware platform for the system is already being replaced. Thus, the organisation may intend to standardize the system operations to remain consistent and compatible with the current systems, where some large sub-systems are being replaced or when the technical quality of the legacy system is inadequate and possess no new tools available to reengineer it.

What is a Legacy System in Software Engineering: Strategic Evolution Options - GuidingCode


3. The current system’s software is obsolete

Along with that, looking from the software aspect, if all the current software (methods, tools, technologies) is obsolete or irrelevant, in that case, the entire system should be replaced. If the new hardware or costs rise but the software remains reusable, only part of the system will need changing as these software can still accommodate newer changes.

Therefore, we can repurpose or reuse as much of the software as possible since it can still fulfil business requirements. However, if the system requires little change and there are only a few user requests and reports, then the system is still relevant. So, it’s best to be left unchanged and it continues with the regular operations and maintenance.



To recap, in software engineering, a legacy system is essentially an older software system that depends on older-generation technologies that are becoming obsolete. However, it can be challenging to determine how one treats these types of systems, whether it’s wiser to replace it partially, completely, or to be kept as is and proceed with the regular maintenance operations.

This should be considered based on the system’s quality and business value as well as whether it makes sense financially. It’s also worth noting, that if no replacement is performed, and replacement is required down the line, it may be more troublesome and costly as the degree of the problem might grow, a larger portion of the system may require replacing, etc.

Overall, this guide explores in detail what software exactly classifies as a legacy system as well as the considerations and strategic options to handle such systems.

We hope that you found this guide helpful in shedding more light on this topic of legacy system in software engineering. Before we end, do comment down below, if you have any questions regarding this topic or if you have any additional information or experience to share regarding legacy systems. We’ll love to hear it!

Feel free to share this post with your fellow software engineering enthusiasts or somebody you know that needs help understanding the term “legacy system”.

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