What Is mainframe modernization? 

Mainframe modernization refers to the process of updating and upgrading legacy mainframe systems to enhance their efficiency and integrate them with modern technologies. This transformation enables companies to meet current market demands and prepare for future challenges by leveraging new tools and architectures.

This process involves transitioning from older, often restrictive technologies to more scalable and cost-effective solutions. Organizations modernize mainframes to improve performance, incorporate new functionalities, and reduce dependency on outdated systems and software.

This is part of a series of articles about application modernization.

Benefits of mainframe modernization 

Modernizing the mainframe offers the following advantages:

  • Reduced costs: By modernizing mainframes, companies reduce costs associated with maintaining old hardware and software. Newer technologies often have lower operational costs due to improved energy efficiency and resource management. Cost reductions also stem from minimizing downtime and streamlining processes.
  • Improved automation and productivity: Integrating automation tools helps simplify complex tasks and workflows. Automation speeds up processes and reduces errors, boosting overall productivity.
  • Deeper insights from data: Modern mainframes can process and analyze vast amounts of data more efficiently, providing deeper insights that inform strategic decisions. Enhanced analytical capabilities allow organizations to identify trends, patterns, and opportunities that were previously obscured in large data pools.

7 approaches to mainframe modernization (the 7 Rs) 

There are several ways to modernize a mainframe system. These approaches are adapted from the “7 Rs” model, initially introduced by Gartner and expanded by AWS, which provided a framework for migrating computing systems to new environments. 


Replatforming involves moving applications from a mainframe environment to a new platform, often cloud-based, without significantly altering the application’s core architecture. This allows organizations to leverage modern infrastructure benefits such as scalability and flexibility while minimizing changes to the application itself. 

By replatforming applications, organizations can integrate with modern services and technologies more easily. This method also simplifies the adoption of DevOps practices, enabling more efficient development cycles and faster time-to-market for updates or new features. 


Refactoring involves modifying the internal structure of mainframe applications without changing their external behavior. This approach optimizes existing code to improve performance, maintainability, and scalability while enabling integration with modern technologies. By refactoring, organizations can extend the life of their critical mainframe applications.

This process allows for the gradual introduction of new features and the modernization of systems in a controlled manner. It reduces risk by maintaining application stability and user experience during the transition to more contemporary architectures or platforms. Refactoring is particularly useful for complex systems where a full rewrite is impractical.


Rehosting transfers mainframe applications to a new environment without altering the application code or features. This enables organizations to maintain their existing applications on more flexible and scalable platforms such as cloud infrastructure. By moving away from proprietary mainframe hardware, companies can reduce their operational costs and improve disaster recovery capabilities.

This strategy preserves the investment in existing mainframe applications while providing the benefits of modern infrastructure. Rehosting is particularly suitable for organizations looking to realize cost savings and operational efficiencies without the complexities of modifying application code.  


Rearchitecting involves transforming the underlying architecture of mainframe applications to better align with modern cloud-native or microservices architectures. This optimizes applications for current and future scalability, performance, and functionality requirements. Organizations can break down monolithic mainframe applications into more manageable, modular components.

Rearchitecting improves agility and allows for the integration of contemporary development practices such as DevOps and continuous delivery. It allows organizations to respond to market changes and customer demands with faster updates and innovations. This method is suitable for fully leveraging cloud capabilities. 


Replacing refers to the complete substitution of mainframe applications with new solutions designed to meet current business needs more effectively. This approach is chosen when existing systems are too rigid or outdated to be adapted through other modernization strategies. By replacing old systems, organizations can implement solutions that are more scalable, flexible, and capable of integrating with modern technologies and architectures.

While replacing mainframe applications can be resource-intensive and requires careful planning, it offers the opportunity to significantly improve operational efficiency, reduce maintenance costs, and enhance overall system performance and reliability.


Retiring involves phasing out mainframe applications or systems that are no longer needed or cost-effective to maintain. This includes identifying legacy applications that have become redundant due to changes in business processes or the adoption of more efficient technologies. By retiring these systems, organizations can eliminate unnecessary maintenance expenses and reallocate resources.

Successful retiring requires thorough planning and analysis to ensure business continuity and data preservation. Critical data must be migrated to newer systems, and dependencies on the retired applications must be resolved. Retiring applications can simplify IT operations and reduce the complexity of the IT landscape. 


Retaining in the context of mainframe modernization refers to the strategic decision to keep certain mainframe applications or systems operational while modernizing other aspects of the IT infrastructure. This approach acknowledges the value and reliability of existing mainframes, particularly for processing high-volume transactions securely and efficiently. 

Organizations can leverage the processing capabilities of existing mainframe systems while integrating them with modern technologies through APIs or middleware solutions. Successfully retaining mainframes requires identifying which applications are critical to retain due to their stability, efficiency, or unique functionality that would be difficult or costly to replicate on newer platforms. 

Challenges of mainframe modernization

There are several challenges involved in modernizing a mainframe system:

Understanding the legacy system architecture and code 

Legacy mainframe systems often have decades of accumulated code, which may not be well-documented or understood by current IT teams. The complexity of legacy architectures, combined with the use of outdated programming languages and proprietary technologies, complicates the process of modernizing these systems. 

Without a deep understanding of the existing architecture and codebase, it’s difficult to assess what needs to be preserved, modified, or discarded during the modernization process. Managing many legacy systems requires specialized knowledge that is increasingly scarce.  

Learn more in our detailed guide to legacy system modernization (coming soon)

The mainframe skills gap 

As experienced mainframe professionals retire, there’s a noticeable scarcity of new talent with the requisite skills in COBOL, JCL, and other mainframe-specific technologies. This gap makes it difficult to maintain existing systems and complicates modernization efforts that often require deep understanding of old and new technologies. 

Data migration 

Data migration involves the transfer of data from legacy mainframe systems to new platforms. This process is complex, as it requires ensuring compatibility between old data formats and modern database systems. Migrating data without losing integrity or causing disruptions to business operations requires careful planning and execution. 

Budget uncertainty

Estimating the costs associated with updating legacy systems can be complex due to unforeseen issues and the unique requirements of each organization’s IT infrastructure. This unpredictability can lead to budget overruns, making it difficult for decision-makers to commit to modernization initiatives without clear financial projections.

Best practices and strategies for mainframe modernization 

Here are some of the measures that organizations can take to ensure the successful modernization of their mainframe systems.

Leverage cloud platforms

Transitioning mainframe functionalities and applications to cloud environments, either partially or fully, enables organizations to benefit from the scalability, flexibility, and cost-efficiency of cloud services. Cloud platforms provide the infrastructure and tools necessary to support the varying demands of applications, improve disaster recovery capabilities, and enhance security. 

By moving to the cloud, companies can reduce the physical footprint of their data centers, lower energy consumption, and streamline management processes, allowing for a more dynamic allocation of resources.

Optimize the code 

Mainframe code optimization focuses on refining and improving existing application code to enhance performance and compatibility with new technologies. This process involves analyzing the current codebase to identify inefficiencies, redundancies, or outdated coding practices that can be streamlined or upgraded. 

Techniques such as optimizing algorithms, removing unused code, and applying more efficient data structures are common. The goal is to make the code leaner and faster, thereby improving response times and reducing processing power requirements.

Use microservices and containers

Microservices and containers are often used to decompose traditional monolithic mainframe applications into smaller, independent units that can be developed, deployed, and managed separately. A microservices architecture allows organizations to make selective updates, scale specific functions as needed, and improve resilience without impacting the entire system. 

Containers provide an isolated environment for each microservice, ensuring that dependencies are contained and conflicts are minimized. This approach enhances the agility of development teams, enabling quicker adjustments to changing market conditions and customer needs.

Manage and modernize APIs 

API management and modernization involve developing and maintaining a secure and scalable interface layer that allows different applications and services to communicate. By modernizing APIs, organizations ensure that their mainframe systems can interact seamlessly with modern applications and cloud services.

Effective API management also includes securing APIs from unauthorized access, monitoring usage to optimize performance, and implementing version control to manage changes without disrupting existing services.

Adopt an incremental approach

An incremental approach to mainframe modernization involves gradually replacing or upgrading parts of the mainframe system rather than attempting a full-scale overhaul at once. This strategy allows organizations to manage risks in a controlled manner, with each phase thoroughly tested and adjusted before proceeding to the next. 

Incremental modernization can include step-by-step application refactoring, module replacement, or functionality enhancement.

Train and upskill the workforce

It’s important to prepare the workforce to operate and innovate with new technologies. The updated mainframe may require new technical skills to manage and develop it. 

Offering comprehensive training programs and continuous learning opportunities enables employees to use the latest technologies. It also ensures that the organization has the internal expertise needed to make use of the modernized mainframe.

Documenting legacy code with Swimm

Legacy code represents a significant challenge as well as an opportunity for software organizations. Managing legacy code entails more than just dealing with untidy or outdated code; it involves transforming it into a reliable and efficient foundation that supports future development initiatives. Handling legacy code effectively can lead to considerable long-term savings and heightened productivity, marking it as a strategic priority for any R&D organization. 

Swimm helps enterprise engineers and their teams document legacy code in an easy, manageable, and effective way. Utilize AI to create docs about legacy code and then discover those docs directly in your IDE. Those docs then stay up to date with ongoing code changes, boosting your team’s productivity and enhancing the quality of your codebase. 

Learn more about modernizing legacy code with Swimm