Senior Software Engineer

Role Description

We are sharing a specialised consulting opportunity for experienced Software Engineers with deep expertise in legacy codebase migration, large-scale systems modernisation, and complex production-grade software environments. This opportunity is designed for engineers who have personally executed substantial migrations across mature, mission-critical systems. The work involves designing, implementing, reviewing, and evaluating complex modernisation tasks while preserving functionality, compatibility, data integrity, security, performance, and production reliability.

Key Responsibilities

• Legacy Codebase Migration & Modernisation
  • Design, implement, review, and evaluate complex codebase migration and modernisation tasks
  • Analyse mature production systems to determine safe and technically sound migration approaches
  • Review legacy application architecture, dependencies, APIs, databases, build systems, and infrastructure
  • Identify technical debt, compatibility constraints, migration risks, and hidden dependencies
  • Develop migration strategies appropriate for large, interconnected production environments
• Language & Runtime Modernisation
  • Evaluate migrations involving major language or runtime upgrades
  • Work with scenarios such as:
    • Java 8 to Java 17/21
    • Legacy .NET to modern .NET
    • Python 2 to Python 3
    • PHP 7 to PHP 8
  • Review language rewrites such as:
    • COBOL to Java/Python
    • JavaScript to TypeScript
  • Assess behavioural equivalence and compatibility across legacy and modern implementations
  • Identify regressions introduced through language, runtime, or compiler changes
• Framework & Application Migration
  • Review major upgrades or replacements involving frameworks such as:
    • Angular
    • Rails
    • Django
    • Spring/Spring Boot
    • ASP.NET
    • Next.js
    • Astro
  • Assess architectural implications of replacing or substantially upgrading mature application frameworks
  • Evaluate dependency compatibility and integration risks
  • Review implementation approaches for maintainability and production readiness
  • Identify subtle behavioural differences introduced during framework migrations
• Database & Data-Layer Modernisation
  • Evaluate migrations such as:
    • Oracle/MySQL to PostgreSQL
    • SQL to NoSQL
  • Review ORM and data-access transitions including:
    • Hibernate to jOOQ
    • Sequelize to Prisma
    • Raw SQL to ORM
  • Assess data integrity, schema compatibility, migration sequencing, and application dependencies
  • Identify risks involving data loss, behavioural changes, transaction semantics, and performance
  • Review validation approaches for complex production data migrations
• Infrastructure & Cloud Modernisation
  • Review transitions from VMs or bare metal to containerised and cloud-native architectures
  • Evaluate migrations involving:
    • Docker
    • Kubernetes
  • Assess deployment architecture, operational dependencies, scalability, and production resilience
  • Review staged rollout and rollback strategies
  • Evaluate whether infrastructure changes preserve reliability and operational visibility
• CI/CD & Build Tooling
  • Review modernisation of deployment pipelines such as:
    • Jenkins/Travis
    • GitHub Actions
    • GitLab CI
  • Evaluate build and package-tool migrations including:
    • Maven to Gradle
    • Webpack to Vite
    • CRA to Next.js
    • npm to pnpm
  • Assess build reproducibility, dependency management, deployment safety, and automation quality
  • Identify migration risks within mature CI/CD environments
  • Review changes for maintainability and operational effectiveness
• API & Integration Migration
  • Evaluate transitions such as:
    • REST to GraphQL
    • to gRPC
    • SOAP or legacy APIs to modern interfaces
  • Assess backward compatibility and downstream integration impact
  • Review API contracts, dependency changes, and migration sequencing
  • Identify compatibility risks and behavioural differences
  • Evaluate strategies for maintaining continuity during interface transitions
• Authentication & Identity Modernisation
  • Review migrations from legacy authentication systems to:
    • OAuth/OIDC
    • SAML/SSO
  • Assess security, compatibility, authorisation, and integration implications
  • Identify authentication or access-control regressions
  • Review migration sequencing and transition strategies
  • Evaluate implementation quality and production readiness
• Repository, Dependency & Toolchain Migration
  • Evaluate polyrepo to monorepo transitions using tools such as:
    • Bazel
    • Nx
    • Turborepo
  • Review replacement of deprecated or end-of-life libraries, APIs, runtimes, and vendor SDKs
  • Assess dependency graphs and compatibility across large software ecosystems
  • Identify upgrade risks created by transitive dependencies or legacy integrations
  • Evaluate migration strategies for maintainability and long-term sustainability
• Testing, Validation & Production Safety
  • Develop and review testing strategies for complex migrations
  • Evaluate unit, integration, browser, and end-to-end testing migrations
  • Identify migration regressions and behavioural differences
  • Assess compatibility, observability, staged rollout, and rollback strategies
  • Evaluate whether proposed migration solutions are sufficiently validated for production use
• Technical Evaluation
  • Review migration solutions for correctness, completeness, maintainability, and production readiness
  • Analyse unfamiliar codebases and diagnose subtle migration failures
  • Evaluate proposed technical decisions against realistic enterprise constraints
  • Identify missing dependencies, edge cases, regression risks, and unsafe assumptions
  • Provide clear, technically rigorous feedback on migration approaches and implementations

Qualifications

• 5+ years of professional software engineering experience, ideally at Senior, Staff, Principal, Lead, or equivalent level
• Significant hands-on experience executing large-scale production migrations
• Personally owned or made substantial technical contributions to complex modernisation initiatives
• Strong experience working with mature systems involving technical debt, complex dependencies, legacy integrations, and significant regression risk
• Strong knowledge of compatibility, testing, observability, staged rollouts, rollback strategies, and behavioural equivalence
• Ability to reason deeply about unfamiliar codebases and diagnose subtle migration failures
• Experience with one or more major migration categories including languages, frameworks, databases, APIs, infrastructure, CI/CD, authentication, repositories, testing, or dependencies
• Particularly valuable experience modernising systems within financial services, fintech/payments, trading, insurance, healthcare, telecom, large technology companies, or other high-reliability environments
• For mainframe modernisation work, experience with COBOL, z/OS, CICS, JCL, DB2, VSAM, batch processing, and COBOL-to-Java/Python migration is particularly valuable

Engagement Details

• Independent contractor engagement
• Fully remote
• Flexible scheduling based on project requirements
• Talent-network opportunity with potential for ongoing project work
• Work focuses on technically challenging migration problems modelled after large-scale production systems
• Projects may be extended, shortened, or concluded based on project needs and performance
• Work must be completed without using confidential or proprietary information belonging to any employer, client, institution, or other third party
• H1-B and STEM OPT support is unavailable for this engagement

About the Platform

This opportunity is available through 24-MAG LLC. We connect experienced professionals with remote consulting opportunities across technical, evaluation, and project-based workstreams.

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