Inside the Engine: How to Analyze and Map COBOL Systems for Modernization

by Jordan Silk, Sales Director

Modernizing a COBOL system is not as simple as translating code line-by-line into a modern language. These applications often comprise millions of lines, interlinked programs, batch jobs, VSAM files, JCL scripts, and custom logic built over decades.

At Astadia, we don’t treat these systems as monoliths to “lift and shift.” We deconstruct them — functionally, structurally, and semantically — using a deeply engineered algorithmic pipeline that transforms legacy code into modern architecture, module by module.

Here’s how it works.

Step 1: Source Code Ingestion and Parsing

Every engagement begins with a comprehensive extraction of source artifacts:

• COBOL programs (both batch and online)
• Job Control Language (JCL)
• Copybooks
• BMS Maps(for CICS screens)
• VSAM / Db2 schema definitions
• External scripts, triggers, or file definitions

Using a custom-built parser and Abstract Syntax Tree (AST)engine, we ingest and tokenize all of this code into a machine-readable intermediate format. This provides a structured baseline from which relationships and dependencies can be extracted.

We’re not just scanning code — we’re building a semantic map of the entire application environment.

Step 2: Application Inventory and Dependency Graphing

Legacy COBOL applications rarely exist as standalone programs. They rely on hundreds of external components and shared business logic. Our engine builds a full dependency graph, which includes:

• Program call hierarchies
•  Copybook reusability
• File and DB interactions
• Batch job flows and timing
• Shared utilities and routines

This dependency graph is visualized, enabling both Astadia engineers and clients to see the system as a living, interconnected entity —not just code. Think of it as reconstructing a blueprint of a skyscraper where no original plans exist.

 This step is vital. It ensures that any changes, replacements, or refactoring downstream preserve the logic, dependencies, and sequencing of the legacy system.

Step 3: Business Rule Extraction and Behavioral Modeling

Code is not the end — it’s a means to execute business logic. To truly modernize, we extract not just syntax, but semantics

Our toolset includes a GenAI modernization Agent that:

• Identifies conditional logic and decision trees
• Maps input/output flows between application components
• Identifies business logic across transactions
• Documents of program functionality   

This allows us to build a business behavior context model of the system — which becomes essential when transforming legacy logic into modern services, APIs, or cloud-native functions.

We effectively reverse-engineer what the system does, not just what it is. 

Step 4: Data Architecture Mapping

COBOL systems are tightly coupled with VSAM, DB2, or flat files. Data structure transformation is a key risk in modernization — get this wrong, and business logic breaks.

Our process includes:

• Full schema extraction from DB2 and VSAM
• Mapping to relational schemas
• Identifying normalization opportunities
• Providing the customized extract, transform and load (ETL) mechanism for bulk data migration

This structured mapping is automated — and validated. If a COBOL program writes to a VSAM file, and a batch job reads from it six hours later, we ensure that equivalent behavior is replicated in the refactored system.

We also enable simulated I/O environments for testing before cutover, minimizing risk.

Step 5: Automated Equivalency Testing Framework

As we build the transformed modern code — whether in Java,.NET, or Python — we maintain a 1:1 mapping of logic, inputs, and outputs.

We then run automated equivalency tests on the modernized codebase to ensure:

• Functional parity with the legacy system
• Business rule integrity
• Accurate handling of edge cases

This is more than QA — it’s about establishing provable equivalence between the old and new worlds. That’s what makes our approach enterprise-ready.

The Result: A Fully Mapped, Fully Testable Modern Codebase

At the end of this process, we don’t just hand off a pile of converted code.

We deliver:

• Refactored source code with modular architecture
• Service blueprints for APIs and microservices
• Cloud-ready deployment packages
• Full business rule documentation
• Audit trails and test coverage reports

This isn’t modernization via black box. It’s a full-stack engineering upgrade, complete with transparency, testability, and transformation continuity.

Why It Matters

You can’t modernize what you don’t understand — and most organizations don’t understand their own mainframes. Astadia’s approach changes that. We provide a mirror to the legacy world — then deliver a path out of it, engineered with confidence.

In the next piece, we’ll dive into how that transformed code becomes cloud-native, and what it takes to go from refactored to containerized, scalable, and integrated into modern CI/CD pipelines.

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Learn more

Why 2025 is the Year to Modernize Mainframe Applications

Strategies for Successful Mainframe Modernization

Migrating COBOL to Java with Automated Conversion

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