Jenkins to GitLab CI Migration with Zero Downtime
Led enterprise-wide migration from Jenkins to GitLab CI for 50+ pipelines serving 80+ developers. Achieved zero-downtime migration by running systems in parallel, improved deployment frequency by 3x, and maintained 100% deployment success rate throughout transition.
DevOps Platform Engineer
Jenkins GitLab CI YAML Groovy Docker GitLab Runner AWS EC2 Spot Instances Python Bash
The Problem
Our organization relied on a 7-year-old Jenkins installation with 200+ freestyle jobs created by various engineers over time, resulting in a maintenance nightmare. The Jenkins server had become a single point of failure, crashing weekly and requiring manual restarts. Plugin version conflicts caused unpredictable behavior, and we ran 30+ outdated plugins with known security vulnerabilities. Job configurations were stored as XML files with zero version control, making auditing impossible. The team spent 15-20 hours per week troubleshooting Jenkins issues rather than delivering features. Build agents were snowflake servers configured manually with inconsistent tooling. We had zero pipeline-as-code, and the Jenkins server cost $800/month in EC2 charges. Leadership demanded migration to GitLab CI for better maintainability and reduced operational overhead, but we couldn't afford downtime during the transition.
The Solution
**Migration Strategy & Planning**: Conducted comprehensive Jenkins job audit categorizing 200+ jobs by complexity, dependencies, and usage frequency. Identified 75 active jobs (125 were obsolete). Created detailed migration plan with four phases over 8 weeks: analysis (2 weeks), pilot migration (2 weeks), bulk migration (3 weeks), and validation (1 week). Prioritized critical production pipelines for early migration.
**Parallel Running Approach**: Implemented dual-running strategy where both Jenkins and GitLab CI executed builds simultaneously for 3 weeks, comparing results to ensure parity. Created automated comparison scripts detecting discrepancies in build artifacts, test results, and deployment outcomes. This approach gave the team confidence before cutting over completely while maintaining zero downtime.
**Pipeline Conversion**: Developed automated converter tool transforming Jenkinsfile Groovy scripts to GitLab CI YAML with 70% accuracy, requiring only manual touchups. Migrated job configurations to GitLab CI YAML files stored in repository .gitlab-ci.yml, enabling version control and code review. Converted Jenkins shared libraries to reusable GitLab CI templates and includes. Migrated credentials from Jenkins credential store to GitLab CI/CD variables with proper masking.
**Infrastructure Modernization**: Replaced snowflake Jenkins agents with GitLab Runners using Docker executors and standardized images. Configured autoscaling GitLab Runner fleet on AWS EC2 Spot Instances, reducing compute costs by 60%. Implemented GitLab Runner caching strategy dramatically improving build performance.
**Parallel Running Approach**: Implemented dual-running strategy where both Jenkins and GitLab CI executed builds simultaneously for 3 weeks, comparing results to ensure parity. Created automated comparison scripts detecting discrepancies in build artifacts, test results, and deployment outcomes. This approach gave the team confidence before cutting over completely while maintaining zero downtime.
**Pipeline Conversion**: Developed automated converter tool transforming Jenkinsfile Groovy scripts to GitLab CI YAML with 70% accuracy, requiring only manual touchups. Migrated job configurations to GitLab CI YAML files stored in repository .gitlab-ci.yml, enabling version control and code review. Converted Jenkins shared libraries to reusable GitLab CI templates and includes. Migrated credentials from Jenkins credential store to GitLab CI/CD variables with proper masking.
**Infrastructure Modernization**: Replaced snowflake Jenkins agents with GitLab Runners using Docker executors and standardized images. Configured autoscaling GitLab Runner fleet on AWS EC2 Spot Instances, reducing compute costs by 60%. Implemented GitLab Runner caching strategy dramatically improving build performance.
Key Highlights
- Successfully migrated 75 production pipelines with zero downtime
- Reduced CI/CD operational maintenance from 15 hours/week to 2 hours/week
- Improved average build time by 40% with GitLab caching and parallel jobs
- Eliminated weekly Jenkins crashes improving system reliability to 99.9%
- Reduced CI/CD infrastructure costs from $800/month to $320/month (60% savings)
- Achieved 100% pipeline-as-code with full Git version control
- Removed 30+ outdated Jenkins plugins eliminating security vulnerabilities
- Automated 70% of pipeline conversion saving 80+ hours of manual work
- Configured autoscaling runners handling 3x peak load efficiently
- Enabled self-service pipeline creation for developers with templates
- Completed entire migration 2 weeks ahead of schedule
- Improved deployment frequency by 45% due to better CI/CD reliability
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