Mainframe Migration - Moving Legacy Systems to the Cloud with AWS Mainframe Modernization
This article explains mainframe cloud migration using AWS Mainframe Modernization. It introduces the two patterns, replatforming (Micro Focus) and refactoring (Blu Age), along with migration strategies.
Challenges of Mainframe Migration
Mainframes have supported core business operations at financial institutions, insurance companies, government agencies, and others for decades, but high licensing costs, a shortage of specialized talent, and technological obsolescence have become serious challenges. The mainframe-specific technology stack, such as codebases of millions of lines written in COBOL, CICS and IMS transaction processing, and batch processing with JCL, raises the hurdles for cloud migration. AWS Mainframe Modernization is a service specialized in migrating mainframe workloads to the cloud, and it offers two migration patterns: replatforming and refactoring.
Two Migration Patterns
The replatforming pattern uses the Micro Focus (now OpenText) runtime and runs COBOL/PL/I code on AWS almost as is. Compatible runtimes for CICS, IMS, and JCL are provided, and changes to application code are minimal. The migration period is short and the risk is low, but the benefits of cloud-native architecture (serverless, microservices) are limited. The refactoring pattern uses Blu Age tools and automatically converts COBOL/PL/I code to Java. The converted code runs as Spring Boot-based microservices and can be deployed on ECS/EKS. The migration period becomes longer, but you can modernize to a cloud-native architecture, and long-term maintainability and extensibility improve. Because both patterns are provided as managed runtime environments, AWS manages runtime installation / patching / scaling.
Testing and Phased Migration
The Application Testing feature automatically compares outputs for the same inputs between the pre-migration mainframe and the post-migration AWS environment, and verifies functional equivalence. It compares batch processing output files, transaction processing responses, and database states, and generates detailed reports if there are differences. As a phased migration strategy, the common approach is to first migrate development / test environments to the cloud, then batch processing, and finally online transaction processing. Risk is minimized by setting up a period of parallel operation of the mainframe and the cloud and switching traffic over in stages. Through integration with DMS (Database Migration Service), it also supports data migration from mainframe databases (DB2, VSAM) to RDS and DynamoDB.
Mainframe Modernization Pricing
The refactoring pattern (Blu Age) of Mainframe Modernization is priced based on the number of lines of code to be converted, and it is individually quoted with the AWS account team. The replatforming pattern (Micro Focus) is billed hourly for the managed runtime, at about $0.355 per hour for an m5.large equivalent (as of August 2026). With either pattern, post-migration AWS infrastructure costs are greatly reduced compared with the annual operating costs of the mainframe. The TCO of the current environment is analyzed with Migration Evaluator and compared with post-migration cost estimates to make the investment decision.
Choosing Between Refactoring and Replatforming
There are broadly two approaches to mainframe migration. One is refactoring, which automatically analyzes existing code and converts it to a new language and structure that runs in the cloud. You can gain long-term maintainability and ease of development, but verifying the conversion results takes effort. The other is replatforming, which reproduces the mainframe execution environment in the cloud and runs existing assets almost as is. You can migrate in a short period with few changes, but technical modernization is limited. It is important to determine which one is suitable based on the size of the assets, the future of maintenance, and the period acceptable for the migration.
Risk Management and Parallel Operation
Mainframes are often critical systems that have supported a company's core business operations for many years, and migration demands caution. It is necessary to thoroughly verify that the target system returns the same results as the existing one. Parallel operation, in which the source and the target run side by side for a certain period and their processing results are matched to confirm there are no differences, is an effective means of risk management. You switch over in stages while maintaining data consistency, and make it possible to roll back if there is a problem. Rather than switching everything over at once, proceeding carefully with repeated verification is the key to a successful core system migration.
Summary - Guidelines for Making Use of Mainframe Modernization
AWS Mainframe Modernization is a service specialized in migrating mainframe workloads to the cloud. It offers two patterns, replatforming (rapid migration with minimal changes) and refactoring (conversion to Java to go cloud-native), and you can choose a migration strategy that matches your organization's requirements. Equivalence verification with Application Testing and a phased migration approach minimize migration risk. It is suited to organizations that are considering mainframe cost reduction and technical modernization.
References (Official AWS Resources)
The primary sources for this page are the official AWS website and documentation. Check the official pages below for the latest specifications and pricing.
- AWS Mainframe Modernization official page
- AWS Database Migration Service official page
- AWS Migration Hub official page
- AWS Documentation (official)
If this page and the official documentation disagree, treat the official documentation as authoritative.