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You can modernize a mainframe without replacing it. Treat modernization as a set of choices for individual applications: expose selected functions through APIs, connect the platform to cloud services, improve delivery and operations, add capabilities where useful, and move a workload only when its business case supports the change.
What mainframe modernization can mean
Modernization is not one migration project with a single destination. IBM describes a range of possible changes, including API modernization, hybrid-cloud integration, DevOps integration, AI integration, and infrastructure optimization. Some extend or improve existing applications while they remain on IBM Z; infrastructure optimization can also mean selectively rehosting or replatforming an application.
The practical unit of choice is the application or workload, not the mainframe estate as a whole. A core transaction system, a reporting process, and a low-criticality utility can have different requirements and therefore different modernization paths. Keeping one workload on the mainframe does not commit every workload to staying, and moving one does not require replacing the rest.
Choose a path for each workload
These approaches can be combined. For example, an organization might keep a system of record on the mainframe, expose some functions through APIs, and send a suitable analytics workload to cloud infrastructure.
#1 Best Overall
| Approach | What changes | When it may fit |
|---|---|---|
| API modernization | Selected business functions or data become available through managed interfaces; the underlying application can remain in place. | Other systems need controlled access to established mainframe capabilities. |
| Hybrid-cloud integration | Mainframe and cloud environments exchange data, events, or service requests; selected workloads may run in cloud. | An organization needs to connect platforms or place elastic, analytic, or otherwise suitable work outside the mainframe. |
| Delivery and operations modernization | Source control, builds, testing, deployments, and operational workflows are improved around mainframe and connected systems. | Teams need more consistent delivery practices without assuming that the application itself must be rewritten. |
| Selective optimization or relocation | A particular application is rehosted, replatformed, refactored, or migrated. | Evidence supports the move and its dependencies, service obligations, and operating model can be addressed. |
Expose capabilities through APIs
An API can give another application a governed way to request a business function or access selected data without making the mainframe application disappear. That can help connect established capabilities to newer applications and services. Decide which operations to expose, who may call them, how they are authenticated and monitored, and what service levels callers can expect. API access is not a substitute for understanding the underlying data and transaction behavior.
Connect mainframe and cloud deliberately
IBM and AWS describe hybrid patterns that include data synchronization, real-time event exchange, APIs, hybrid storage, and infrastructure management. The design question is not simply whether to connect the environments, but which data and events need to cross the boundary, how quickly, and under what security controls. Data volumes, latency, recovery requirements, and regulatory obligations all affect the choice.
Rank #2
Cloud integration can extend access or put selected workloads on cloud platforms while the mainframe continues to serve other applications. IBM Redbooks also frames application modernization in a hybrid-cloud context. These are architecture options, not evidence that every workload will benefit from cloud placement.
Improve delivery without assuming a rewrite
Modernization can target the way teams change and operate software: source control, build and test automation, deployment practices, and operational workflows. Mainframe and cloud-native toolchains may differ, so plan how teams will handle shared changes, testing across system boundaries, releases, and incident response. AWS Prescriptive Guidance recommends planning migration incrementally in waves; applying that staged discipline to modernization helps teams validate outcomes before expanding a change.
Relocate only when the case supports it
Rehosting, replatforming, refactoring, and migration are distinct choices with different levels of change. A relocation decision should account for dependencies, data movement, service obligations, and the target operating model—not just the destination infrastructure. Some workloads may be suitable to move while others remain where they are. No universal rule establishes that all applications should stay on, or move off, a mainframe.
Build the decision around evidence
Start with an application-level inventory, then compare options against the requirements that matter to the business. The same mainframe environment can contain workloads with very different criticality, data flows, performance needs, and constraints.
- Map the workload. Record business ownership and criticality, dependencies, interfaces, data flows and volumes, transaction and batch behavior, operating requirements, and support arrangements.
- Set outcomes and constraints. Specify availability and recovery needs, latency and throughput, security boundaries, regulatory obligations, integration and data-access goals, developer workflow, skills, costs, time to value, and sustainability objectives.
- Compare plausible paths. Assess API exposure, hybrid integration, delivery changes, selective optimization, and relocation against the same requirements. Include implementation and ongoing operational consequences.
- Stage the work. Choose a bounded first change, identify how its outcome will be validated, and use what the team learns to shape later work. Avoid treating an estate-wide conversion as the default.
For every candidate change, make the trade-offs explicit: expected business outcome, operational risk, resilience, performance, security and compliance, integration effort, skills, cost, time to value, and sustainability. A claimed benefit in one area does not establish a benefit in another. In particular, survey-reported ROI is context, not a substitute for a workload-specific business case.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What Kyndryl’s 2025 survey says—and does not say
Kyndryl’s vendor-sponsored 2025 State of Mainframe Modernization survey report describes responses from 500 senior IT and business leaders. Its figures characterize those respondents; they are not forecasts or guaranteed results for another organization.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware match- Kyndryl reports that 80% of respondents changed their mainframe modernization strategy in the prior year. Among respondents changing approach, 43% put more focus on modernization directly on the mainframe, 34% on cloud integration, and 16% on moving more applications off the mainframe.
- One of the 500 respondents planned to move entirely off the mainframe. This is a survey response, not evidence that a full exit is right or wrong for other organizations.
- Kyndryl reports survey ROI figures of 288% for modernization on the mainframe, 297% for cloud integration, and 362% for moving applications off the mainframe. These reported values do not guarantee returns for a particular project and should not be treated as a directly comparable local forecast.
- The report gives an average cost of mainframe modernization of $7.2 million in its 2025 survey, compared with $9.1 million in its 2024 survey. Differences in the surveyed population and reporting methodology limit what that year-to-year comparison can establish about an individual organization’s likely cost.
- Kyndryl reports that 94% said regulation strongly influences modernization, while 32% said security was a reason for keeping an application on the mainframe. These responses underline that placement choices can be shaped by compliance and security concerns, but do not determine the right controls or location for a specific workload.
- Kyndryl also reports that 88% were deploying or planning GenAI on the mainframe. That describes reported activity among survey respondents; it does not establish that GenAI is necessary or suitable for every mainframe environment.
The range of strategies in the survey is consistent with a portfolio approach: organizations reported attention to on-platform work, cloud integration, and selective movement rather than a single universal destination. The survey does not establish which path will produce the best result for a particular application.
Quick Recap
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Common mistakes to avoid
- Making the estate the decision unit. One application’s performance or compliance needs do not automatically apply to every workload.
- Confusing access with replacement. An API or hybrid connection can extend use of existing capabilities without removing the underlying system.
- Moving a workload before mapping its dependencies. Data flows, connected systems, batch schedules, and service obligations can make an apparently isolated application less straightforward to relocate.
- Assuming modernization means cloud-native tooling everywhere. Delivery practices can improve, but teams need a plan for differences between mainframe stacks and cloud tools.
- Treating survey returns as a business case. A reported average, ROI, or respondent preference cannot replace local cost, risk, and outcome analysis.
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