Open Source CI/CD Tools

What CI/CD Pipelines Actually Do

A CI/CD pipeline is a sequence of automated stages that take code from a developer's commit to a running production deployment. The continuous integration (CI) portion typically includes checking out the code, installing dependencies, running a build step (compilation, transpilation, or bundling), executing unit tests and integration tests, running static analysis or linting, and producing a build artifact such as a compiled binary, Docker image, or deployable package.

The continuous delivery (CD) portion takes that artifact and deploys it through a sequence of environments: typically staging, pre-production, and production. Between environments, the pipeline may run additional tests, require manual approval gates, perform database migrations, update configuration, and verify health checks after deployment. The goal is to make releases routine and low-risk rather than stressful events that require coordination across teams.

The distinction between continuous delivery and continuous deployment is worth noting. Continuous delivery means every code change is automatically built, tested, and prepared for release, but a human decides when to push to production. Continuous deployment removes that final manual step, automatically deploying every change that passes all pipeline stages. Most organizations start with continuous delivery and graduate to continuous deployment as their test coverage and confidence grow.

Jenkins

Jenkins is the oldest widely-used open source CI/CD server, with roots going back to 2004 when it was released as Hudson. After an ownership dispute with Oracle, the project was forked and renamed Jenkins in 2011. It runs on the Java Virtual Machine and is available as a WAR file, Docker image, or native package for most Linux distributions.

Jenkins' defining feature is its plugin ecosystem. With over 1,800 plugins maintained by the community, Jenkins integrates with virtually every version control system, build tool, testing framework, cloud provider, notification service, and deployment target imaginable. If a tool exists in the DevOps space, there is almost certainly a Jenkins plugin for it.

Jenkins Pipeline, introduced in 2016, allows you to define entire build and deployment workflows as code in a Jenkinsfile stored in your repository. Pipelines use a Groovy-based DSL with two syntax options: Declarative Pipeline for structured, opinionated configurations, and Scripted Pipeline for maximum flexibility with full Groovy scripting capabilities. Shared Libraries let teams create reusable pipeline components that can be version-controlled and maintained separately.

The main drawback of Jenkins is operational overhead. Plugin compatibility issues, security vulnerabilities requiring prompt updates, Java heap sizing, and build agent management all demand ongoing attention. Jenkins does not include built-in container orchestration, user management beyond basic LDAP/SAML integration, or a container registry, so these capabilities must come from plugins or external tools. For teams with dedicated DevOps engineers, Jenkins' flexibility outweighs its maintenance burden. For smaller teams, simpler alternatives may be more practical.

GitLab CI/CD

GitLab CI/CD is built directly into the GitLab source code management platform, providing a unified interface for code hosting, merge requests, issue tracking, and automated pipelines. Pipelines are defined in a .gitlab-ci.yml file at the root of each repository using a YAML-based syntax that is more approachable than Jenkins' Groovy DSL.

A GitLab CI pipeline consists of stages (such as build, test, and deploy), each containing one or more jobs that can run in parallel. Jobs execute in isolated environments called runners, which can be Docker containers, virtual machines, Kubernetes pods, or bare-metal servers. GitLab provides shared runners on its SaaS platform, and self-hosted GitLab instances can register their own runners.

GitLab CI includes features that require plugins or external tools in Jenkins: a built-in container registry for storing Docker images, artifact management for passing build outputs between stages, environment management for tracking deployments, a secrets management integration, and merge request pipelines that run tests specifically on proposed changes before they are merged.

The Auto DevOps feature can automatically detect the application's language and framework, then generate a pipeline that builds a Docker image, runs tests, performs security scanning, and deploys to a Kubernetes cluster, all without writing a .gitlab-ci.yml file manually. While Auto DevOps does not suit every project, it demonstrates how tightly integrated GitLab's pipeline system is with the rest of the platform.

The self-hosted GitLab Community Edition includes the full CI/CD engine at no cost. Some advanced features like security scanning dashboards, compliance pipelines, and advanced SAST/DAST tools are only available in the paid tiers, but the core pipeline functionality is fully open source.

Tekton

Tekton is a Kubernetes-native CI/CD framework that defines pipeline components as Kubernetes custom resources. Instead of running on a separate server like Jenkins, Tekton runs inside an existing Kubernetes cluster, using Kubernetes itself for scheduling, scaling, and resource management.

The core abstractions in Tekton are Steps (individual container commands), Tasks (ordered sequences of Steps), Pipelines (graphs of Tasks with dependencies and data passing), and Triggers (event listeners that start PipelineRuns in response to Git webhooks or other events). Each of these is a Kubernetes custom resource defined in YAML.

Tekton's strength is its composability. Tasks are reusable units that can be shared through the Tekton Hub, a catalog of community-contributed tasks for common operations like Git clone, Docker build, Helm deploy, and Slack notification. Pipelines compose these tasks into complete workflows, and different pipelines can share the same tasks without duplication.

The trade-off is that Tekton requires a running Kubernetes cluster and fluency with Kubernetes concepts. It does not include a web dashboard by default (the Tekton Dashboard is a separate installation), and the YAML configuration for complex pipelines can be verbose. Tekton is best suited for organizations that have already adopted Kubernetes and want their CI/CD infrastructure to be managed by the same platform that runs their applications.

Argo CD

Argo CD is a declarative, GitOps continuous delivery tool for Kubernetes. Rather than pushing deployments to clusters (the traditional CD model), Argo CD pulls the desired state from a Git repository and continuously reconciles the live cluster state to match. If someone manually changes a Kubernetes resource, Argo CD detects the drift and can automatically revert it.

An Argo CD Application resource points to a Git repository path containing Kubernetes manifests, Helm charts, or Kustomize configurations. Argo CD monitors the repository for changes and automatically syncs them to the target cluster. The web dashboard provides a visual representation of every application's sync status, health status, and resource tree, making it straightforward to see whether deployments are current.

Argo CD supports multi-cluster deployments, allowing a single Argo CD instance to manage applications across development, staging, and production clusters. ApplicationSets can generate multiple Application resources from templates, enabling patterns like deploying the same application to multiple regions or creating per-branch preview environments.

Argo CD focuses exclusively on the delivery side. For the CI portion (building and testing code), it pairs naturally with Tekton, Jenkins, GitLab CI, or any other CI tool that produces artifacts and commits manifests to a Git repository. The combination of a CI tool plus Argo CD for CD is a common and well-documented pattern in the Kubernetes ecosystem.

Drone and Woodpecker

Drone (now Harness Open Source) is a container-native CI/CD platform where every pipeline step executes in its own Docker container. Pipelines are defined in a .drone.yml file using a simple YAML syntax that specifies the container image and commands for each step. This container-per-step model ensures that builds are reproducible and isolated, since each step starts from a clean container image.

Drone supports multiple version control platforms (GitHub, GitLab, Bitbucket, Gitea) and has a straightforward setup process. Its plugin model is also container-based: a Drone plugin is simply a Docker image that accepts configuration through environment variables and performs an action like deploying to Kubernetes, uploading to S3, or sending a Slack notification.

Woodpecker CI is a community fork of Drone, created when Drone's licensing and development direction changed after its acquisition by Harness. Woodpecker maintains the same container-native architecture and YAML-based pipeline syntax but is developed by a community-governed team. It supports multiple backends including Docker, local execution, and Kubernetes, and has an active community adding features and platform integrations.

Both Drone and Woodpecker are excellent choices for teams that want a simple, container-native CI/CD system without the complexity of Jenkins or the Kubernetes dependency of Tekton. They are particularly well-suited for small to mid-size teams running containerized applications.

Choosing the Right CI/CD Tool

The right choice depends on your existing infrastructure and team expertise. If you already run GitLab for source control, GitLab CI is the most natural choice since it requires no additional infrastructure and integrates deeply with your existing workflows. If you run Kubernetes and want native integration, Tekton for CI and Argo CD for CD is the cloud-native standard. If you need maximum flexibility and customization, Jenkins has the deepest plugin ecosystem. If you want simplicity and fast setup, Drone or Woodpecker deliver container-native pipelines with minimal configuration.

Consider also how many tools you want to operate. GitLab CI combines source control and CI/CD in one platform, reducing operational overhead. Jenkins, Tekton, and Drone are standalone CI/CD tools that need to be integrated with your Git platform, artifact storage, and deployment targets separately. For organizations with dedicated platform engineering teams, the additional flexibility of standalone tools is valuable. For smaller teams, the convenience of an integrated platform often wins.