Demo 32: Legacy Migration

Overview

The Legacy Migration engine translates codebases from 18 source languages to 10 target languages using AI-powered translation rules, service boundary detection, and 6 migration strategies. With 31 supported language pairs and configurable strategies like Strangler Fig and Parallel Run, VibeCody handles everything from mainframe COBOL to desktop VB6 conversions.

Prerequisites

• VibeCLI installed and on your PATH
• At least one AI provider configured
• Access to the source codebase you want to migrate
• For VibeUI: the desktop app running with the Batch Builder panel visible (Migration tab)

Supported Languages

18 Source Languages

10 Target Languages

Rust, Go, Python, TypeScript, Java, C#, Kotlin, Swift, Ruby, Elixir

31 Supported Translation Pairs

Not every source-target combination is supported. VibeCody validates the pair before starting a migration. Common pairs include COBOL-to-Java, VB6-to-C#, Fortran-to-Rust, Perl-to-Python, and ABAP-to-TypeScript.

6 Migration Strategies

Step-by-Step Walkthrough

1. Analyze the Source Codebase

Scan the legacy codebase to detect languages, module boundaries, and dependencies.

CLI:

vibecli --legacymigrate analyze --source ./legacy-cobol-app

Example output:

Source Analysis: legacy-cobol-app
Language: COBOL (94%), JCL (6%)
Files: 342
Lines: 287,000
Modules detected: 18
Service boundaries: 5
External dependencies: DB2, CICS, MQ
Recommended strategy: StranglerFig
Recommended target: Java

VibeUI:

In the Batch Builder panel, switch to the Migration tab and click Analyze Source. Point it to your legacy project directory.

2. Configure the Migration

Choose the target language and strategy. Override the recommendations if needed.

CLI:

vibecli --legacymigrate configure \
  --source ./legacy-cobol-app \
  --target java \
  --strategy strangler-fig \
  --output ./migrated-java-app

VibeUI:

In the Migration tab, select the target language from the dropdown, choose a strategy, and set the output directory.

3. Review Translation Rules

Before running the migration, inspect the translation rules that will be applied. Rules map language constructs, data types, and patterns from source to target.

CLI:

vibecli --legacymigrate rules --pair cobol-java

Example output:

Translation Rules: COBOL → Java
  PIC X(n)       → String (length n)
  PIC 9(n)       → int / long
  PIC 9(n)V9(m)  → BigDecimal
  PERFORM THRU   → method call sequence
  COPY member    → import + class inclusion
  EVALUATE/WHEN  → switch/case
  88-level       → enum / boolean
  REDEFINES      → union type / inheritance
  ...
  Total rules: 47

VibeUI:

Click View Rules to see the full mapping table for the selected language pair.

4. Detect Service Boundaries

The engine identifies logical service boundaries in the legacy code to guide modular migration.

CLI:

vibecli --legacymigrate boundaries --source ./legacy-cobol-app

Example output:

Service Boundaries Detected: 5
  1. CustomerManagement (42 programs, 63K lines)
  2. OrderProcessing (78 programs, 89K lines)
  3. InventoryControl (51 programs, 44K lines)
  4. BillingEngine (38 programs, 51K lines)
  5. Reporting (33 programs, 40K lines)

5. Start the Migration

Launch the migration. For Strangler Fig, this creates the first batch of migrated modules.

CLI:

vibecli --legacymigrate start \
  --source ./legacy-cobol-app \
  --target java \
  --strategy strangler-fig \
  --boundary CustomerManagement

VibeUI:

Click Start Migration after configuring all options. Progress appears in the Monitor tab.

6. Validate the Output

Run the built-in validation to compare behavior between the legacy and migrated code.

CLI:

vibecli --legacymigrate validate --output ./migrated-java-app

Example output:

Validation Results:
  Modules migrated: 42
  Compilation: PASS
  Unit tests generated: 156 (all passing)
  Behavioral comparison: 98.7% match
  Warnings: 2 (manual review suggested for CICS transaction mapping)

7. Iterate on Remaining Boundaries

For incremental strategies, repeat steps 5-6 for each service boundary until the full codebase is migrated.

CLI:

vibecli --legacymigrate start --boundary OrderProcessing
vibecli --legacymigrate start --boundary InventoryControl
# ... continue for each boundary

Demo Recording JSON

{
  "demo_id": "32-legacy-migration",
  "title": "Legacy Migration",
  "version": "1.0.0",
  "steps": [
    {
      "action": "cli_command",
      "command": "vibecli --legacymigrate analyze --source ./legacy-cobol-app",
      "description": "Analyze the legacy COBOL codebase"
    },
    {
      "action": "cli_command",
      "command": "vibecli --legacymigrate configure --source ./legacy-cobol-app --target java --strategy strangler-fig --output ./migrated-java-app",
      "description": "Configure migration target and strategy"
    },
    {
      "action": "cli_command",
      "command": "vibecli --legacymigrate rules --pair cobol-java",
      "description": "Review the 47 COBOL-to-Java translation rules"
    },
    {
      "action": "cli_command",
      "command": "vibecli --legacymigrate boundaries --source ./legacy-cobol-app",
      "description": "Detect service boundaries in the legacy code"
    },
    {
      "action": "cli_command",
      "command": "vibecli --legacymigrate start --source ./legacy-cobol-app --target java --strategy strangler-fig --boundary CustomerManagement",
      "description": "Migrate the first service boundary"
    },
    {
      "action": "cli_command",
      "command": "vibecli --legacymigrate validate --output ./migrated-java-app",
      "description": "Validate migrated code against legacy behavior"
    },
    {
      "action": "vibeui_interaction",
      "panel": "BatchBuilder",
      "tab": "Migration",
      "description": "Configure and launch migration from the GUI"
    },
    {
      "action": "vibeui_interaction",
      "panel": "BatchBuilder",
      "tab": "Monitor",
      "description": "Watch migration progress per service boundary"
    }
  ]
}

What’s Next

• Demo 31: Batch Builder – Use Batch Builder for greenfield code generation
• Demo 33: App Builder – Scaffold new applications from natural language
• Demo 35: Compliance & Audit – Ensure migrated code meets compliance requirements