What Is Mainframe Migration?
Mainframe migration is the process of transferring applications, data, and workloads from legacy mainframe systems to modern computing environments. This migration often involves moving to cloud platforms or other server architectures to improve performance, reduce costs, and improve scalability.
It requires a strategic approach to ensure that the migration does not disrupt critical business operations. Key considerations include assessing the existing mainframe applications and determining the best strategy to move them to new platforms. The purpose of mainframe migration is to leverage modern technologies that offer greater efficiency, flexibility, and adaptability.
Legacy systems often impose limitations, as they may lack support for modern software development practices or integration capabilities with newer technologies. By migrating mainframe applications, organizations can benefit from reduced operational costs, improved performance, and increased agility.
This is part of a series of articles about mainframe modernization.
Why Modernize Mainframes Today
Modernizing mainframes is increasingly urgent as organizations face pressure to increase agility, reduce costs, and adopt digital-first strategies. Many mainframe systems were built decades ago and are now incompatible with modern development practices such as DevOps, continuous integration, and cloud-native architectures. These limitations hinder innovation and delay the rollout of new features or services.
Operational costs are another key driver. Maintaining legacy hardware and securing rare expertise in mainframe technologies can be prohibitively expensive. Modern environments—especially cloud platforms—offer scalable and cost-effective alternatives that reduce infrastructure overhead and simplify system maintenance.
Security and compliance are also critical. Legacy systems often lack up-to-date security features required to meet current regulatory standards. Modern platforms enable better monitoring, faster patching, and easier compliance with data protection laws.
Lastly, customer expectations have evolved. Users demand fast, reliable, and personalized digital services. Meeting these expectations requires IT systems that can integrate quickly with APIs, support real-time data access, and scale on demand.
Strategies for Mainframe Application Migration
1. Replatform (Lift-and-Shift)
Replatforming involves migrating applications to a different runtime environment with minimal code changes. This typically means moving COBOL or CICS-based applications to platforms that emulate mainframe behavior, such as cloud-based mainframe runtimes or on-prem x86 servers.
While the business logic remains largely unchanged, expect some adjustments—file handling, job schedulers, or data store conversions (e.g., from VSAM to relational databases) often need attention. Tools can help emulate the mainframe environment, but testing is essential to confirm correct behavior post-migration.
Pros: Quickest path off the mainframe with reduced disruption to application logic.
Cons: Doesn’t modernize the codebase. Retains old architectures and dependencies, which may still require specialized knowledge and ongoing maintenance.
2. Refactor (Re-architect or Rewrite)
Refactoring modernizes the application’s internals—either by rewriting legacy code in modern languages (like Java or C#) or by rearchitecting it into microservices or cloud-native patterns. The goal is to retain functionality while improving scalability, maintainability, and integration capabilities.
This approach often includes redesigning how the application works, breaking up monoliths, and upgrading associated data stores and interfaces.
Pros: Delivers a modern, extensible system aligned with current development practices.
Cons: High effort, long timelines, and significant risk if business logic is misunderstood or missed. Requires deep domain knowledge and extensive testing.
3. Replace (Repurchase or COTS Replacement)
Replacement means discarding the legacy application and adopting a commercial off-the-shelf (COTS) system or a modern prebuilt solution. For example, replacing a homegrown banking core with a vendor-provided platform.
While this can rapidly modernize systems, it often requires changing business processes to match the new system’s capabilities, along with customizations and data transformation.
Pros: Can speed up modernization with built-in best practices and vendor support.
Cons: Rarely aligns one-to-one with legacy features, making gap analysis and user retraining critical. Migration complexity and change management are high.
4. Retire (Eliminate)
Some mainframe applications are no longer relevant and can be safely decommissioned. The retire strategy involves identifying such unused or redundant systems and eliminating them rather than migrating.
This requires careful analysis to ensure no downstream dependencies or compliance obligations are overlooked.
Pros: Saves time, money, and complexity by not migrating what’s unnecessary.
Cons: Verification is crucial because missing a hidden dependency can cause operational disruptions. Requires stakeholder agreement and often archival of legacy data.
5. Wrap with APIs (Encapsulate)
Wrapping exposes legacy mainframe functionality through modern APIs without altering the core code. RESTful services or data streaming tools enable newer systems to interact with the mainframe as a backend service.
This approach is especially useful for enabling digital channels and creating an interim modernization layer while planning a deeper migration.
Pros: Quick wins through integration without modifying legacy code. Low disruption.
Cons: Underlying limitations (cost, performance, scalability) remain. API wrapping is best used as a transitional solution, not a permanent fix.
Related content: Read our guide to mainframe modernization tools
Tips from the expert
Mainframe Migration Challenges
Here are some of the main factors that can complicate mainframe migration.
Complexity of Legacy Systems
Mainframe applications are often decades old, written in languages like COBOL or PL/I, and tightly coupled with proprietary databases and batch processing workflows. Their documentation is frequently outdated or incomplete, and they may have evolved through years of incremental changes.
This complexity makes it hard to fully understand what the system does, especially when business logic is deeply embedded across different modules. Also, many of the people who originally built or maintained these systems may have retired, making institutional knowledge scarce.
Legacy applications often have obscure dependencies, outdated plugins, or custom code that’s difficult to convert. These “low-volume but critical” components can become the hardest parts to migrate, especially when automated tooling doesn’t support them.
Learn more in our detailed guide to legacy system modernization
Verifying Data
Data integrity is central to a successful migration. Mainframes use data encodings like EBCDIC and file formats such as VSAM or IMS, which differ significantly from modern standards like ASCII or relational databases. Migrating this data requires careful conversion and validation.
Verification involves more than row counts—it requires spot-checking values, comparing key aggregates (e.g., total balances), and sometimes running parallel reports on the old and new systems to ensure consistent outputs. Automated comparison tools and checksum utilities help catch discrepancies, but even these need human oversight.
Sequencing of Data Cutover
Data migration requires more than converting the format, including ensuring the right timing and order. Organizations need to decide whether to move everything at once (“big bang”) or phase it over time. Many organizations prefer a hybrid strategy: preloading historical data in bulk, then using change data capture (CDC) tools to apply recent updates incrementally.
At cutover time, a final sync is needed. That means stopping mainframe updates, capturing the last deltas, applying them to the new system, and flipping users to the new environment. This cutover process must be carefully scripted and rehearsed, as errors can corrupt critical data or cause downtime.
Choosing a Migration Approach (Incremental or Bulk)
Choosing between an incremental or bulk migration depends on the complexity and criticality of the applications involved. A bulk cutover offers a clean switch but carries high risk if issues arise during the limited cutover window. An incremental approach reduces risk but adds overhead—both systems must stay in sync until the migration is complete.
Often, organizations use an initial bulk migration followed by real-time syncing of changes. This phased method allows for better testing and fallback options, but it requires a well-orchestrated pipeline for replication, verification, and final cutover execution.
5 Best Practices for Mainframe Migration
Organizations should consider the following best practices when migrating mainframe applications.
1. Perform a Comprehensive System Assessment
Begin with a detailed review of all mainframe applications. This includes mapping out code dependencies, understanding external interfaces, and analyzing how business logic is implemented. Without this knowledge, organizations risk missing key functionality or breaking downstream processes. It’s important to determine whether each application is a candidate for rehosting, refactoring, or replacement, based on its technical makeup and business value.
The assessment should also include an application rationalization step—asking what each program does, who uses it, and whether it still serves a valid purpose. This can uncover redundant or obsolete components that don’t need to be migrated at all. Rationalization helps define what should move, what can be archived, and what can be shut down entirely, simplifying the scope and reducing costs.
2. Document the Current System
Before any migration begins, teams must fully document how each mainframe application operates. This includes inputs, outputs, triggers, batch schedules, timing, and data dependencies. All integration points—both upstream and downstream—must be clearly identified. These details form the blueprint for migration and are essential for avoiding disruption to business workflows during cutover.
It’s also important to use this documentation process to identify opportunities for simplification. Some components may be outdated, unused, or already replaced by other systems. Removing or deprioritizing them saves time and prevents unnecessary effort. Additionally, documentation supports testing and validation efforts by clarifying expected behaviors, which is vital for ensuring that the migrated system performs correctly.
3. Ensure Stakeholder Buy-In
Securing buy-in from business users, IT teams, and executives is critical to migration success. Resistance often stems from concerns about performance, loss of functionality, or fear of job displacement. To counter this, leaders should communicate the concrete benefits of modernization—like improved scalability, lower costs, and easier access to data—and show how these translate into value for both the business and its people.
A practical strategy is to begin migration with teams that are eager for change. These early adopters can help demonstrate success, which can ease concerns in more skeptical groups. Emphasize how modernization can improve users’ day-to-day work—such as better interfaces, remote access, or opportunities to learn new technologies. Building momentum among supportive stakeholders helps create a foundation for broader acceptance.
4. Consider How Best to Rehost the Mainframe
Rehosting allows organizations to move mainframe workloads to modern infrastructure—like the cloud—without a full rewrite. This strategy is ideal when there’s limited time or budget, or when infrastructure contracts are expiring. Solutions include mainframe-as-a-service, outsourcing, or emulation, where applications run on platforms that replicate mainframe environments.
This approach provides breathing room for long-term planning. By quickly offloading infrastructure responsibilities, teams can buy time to analyze and modernize their applications at a more manageable pace. Rehosting is especially useful as a stepping stone—it reduces reliance on mainframe hardware while enabling incremental refactoring or replacement in the future.
5. Consider What Can Be Automated
Automation can accelerate migration significantly, especially for tasks like converting COBOL to Java or transforming VSAM data to relational formats. Many tools are available to help modernize code and move data efficiently. However, automation doesn’t cover everything. Older applications may contain rare code constructs or vendor-specific features for which no automated tools exist.
These difficult areas—like specialized plugins or rarely used legacy databases—need to be identified early in the process through a complexity analysis. Knowing where automation will fall short lets teams plan manual interventions, set realistic timelines, and assign the right experts. Addressing these “tough bits” upfront is essential to avoid project delays.
Supporting Mainframe Migration with Swimm
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