Master GitLab CI/CD with Kubernetes: Deploy a Flask App End‑to‑End

Service Background

Deploy a Flask web service using GitLab CI to perform code‑style checks, unit tests, packaging, and release into a Kubernetes environment. The .gitlab-ci.yml file configures branch‑ and tag‑based pipelines for automated builds.

Dockerfile for the Flask Service

FROM python:3.4

COPY . /skeleton
WORKDIR /skeleton
RUN pip install -r requirements.txt -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com  # configure pip source
EXPOSE 5000
ENTRYPOINT ["sh","scripts/dev.sh"]

Define .gitlab-ci.yml

stages:  # five stages executed sequentially
  - style
  - test
  - release
  - review
  - deploy

pep8:  # code‑style job
  image: python:2.7
  stage: style
  script:
    - pip install -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com tox
    - tox -e pep8

unittest-py2.7:
  image: python:2.7
  stage: test
  script:
    - pip install -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com tox
    - tox -e py27

unittest-py3.4:
  image: python:3.4
  stage: test
  script:
    - pip install -i http://pypi.douban.com/simple/ --trusted-host pypi.douban.com tox
    - tox -e py34

buildimage:
  image: docker:latest
  variables:
    DOCKER_DRIVER: overlay
    DOCKER_HOST: tcp://localhost:2375
  services:
    - name: docker:17.03-dind
      command:
        - "--registry-mirror=https://*****.mirror.aliyuncs.com"
  stage: release
  script:
    - docker login -u "${CI_REGISTRY_USER}" -p "${CI_REGISTRY_PASSWORD}" ${CI_REGISTRY_REPO_URL}
    - docker build -t "${CI_REGISTRY_IMAGE}:latest" -f ./Dockerfile .
    - docker tag "${CI_REGISTRY_IMAGE}:latest" "${CI_REGISTRY_REPO_URL}/${CI_REGISTRY_NAMESPACE}/${CI_REGISTRY_IMAGE}:${CI_COMMIT_REF_NAME}"
    - test ! -z "${CI_COMMIT_TAG}" && docker push "${CI_REGISTRY_REPO_URL}/${CI_REGISTRY_NAMESPACE}/${CI_REGISTRY_IMAGE}:latest"
    - docker push "${CI_REGISTRY_REPO_URL}/${CI_REGISTRY_NAMESPACE}/${CI_REGISTRY_IMAGE}:${CI_COMMIT_REF_NAME}"

deploy_review:
  image: bitnami/kubectl
  stage: review
  only:
    - branches
  except:
    - tags
  environment:
    name: dev
    url: https://dev-gitlab-k8s-demo.****.cn-beijing.alicontainer.com
    on_stop: stop_review
  script:
    - kubectl version
    - cd manifests/
    - sed -i "s/__CI_ENVIRONMENT_SLUG__/${CI_ENVIRONMENT_SLUG}/" deployment.yaml ingress.yaml service.yaml
    - sed -i "s/__VERSION__/${CI_COMMIT_REF_NAME}/" deployment.yaml ingress.yaml service.yaml
    - |
      if kubectl apply -f deployment.yaml | grep -q unchanged; then
        echo "=> Patching deployment to force image update."
        kubectl patch -f deployment.yaml -p '{"spec":{"template":{"metadata":{"annotations":{"ci-last-updated":"$(date +'%s')"}}}}}'
      else
        echo "=> Deployment apply has changed the object, no need to force image update."
      fi
    - kubectl apply -f service.yaml || true
    - kubectl apply -f ingress.yaml
    - kubectl rollout status -f deployment.yaml
    - kubectl get all,ing -n devops
  when: manual

stop_review:
  image: bitnami/kubectl
  stage: review
  variables:
    GIT_STRATEGY: none
  when: manual
  only:
    - branches
  except:
    - master
    - tags
  environment:
    name: dev
    action: stop
  script:
    - kubectl version
    - kubectl delete ing -l ref=${CI_ENVIRONMENT_SLUG}
    - kubectl delete all -l ref=${CI_ENVIRONMENT_SLUG}

deploy:
  image: bitnami/kubectl
  stage: deploy
  environment:
    name: live
    url: https://${CI_ENVIRONMENT_SLUG}.****.cn-beijing.alicontainer.com
  only:
    - tags
  when: manual
  script:
    - kubectl version
    - cd manifests/
    - sed -i "s/__CI_ENVIRONMENT_SLUG__/${CI_ENVIRONMENT_SLUG}/" deployment.yaml ingress.yaml service.yaml
    - sed -i "s/__VERSION__/${CI_COMMIT_REF_NAME}/" deployment.yaml ingress.yaml service.yaml
    - kubectl apply -f deployment.yaml service.yaml ingress.yaml
    - kubectl rollout status -f deployment.yaml
    - kubectl get all,ing -l ref=${CI_ENVIRONMENT_SLUG}

Kubernetes Resource Declarations

Deployment (deployment.yaml)

apiVersion: apps/v1
kind: Deployment
metadata:
  name: gitlab-k8s-demo-__CI_ENVIRONMENT_SLUG__
  namespace: devops
  labels:
    app: gitlab-k8s-demo
    ref: __CI_ENVIRONMENT_SLUG__
    track: stable
spec:
  replicas: 2
  selector:
    matchLabels:
      app: gitlab-k8s-demo
      ref: __CI_ENVIRONMENT_SLUG__
  template:
    metadata:
      labels:
        app: gitlab-k8s-demo
        ref: __CI_ENVIRONMENT_SLUG__
        track: stable
    spec:
      imagePullSecrets:
        - name: myregistry
      containers:
        - name: app
          image: registry.cn-beijing.aliyuncs.com/*****/gitlab-ci-flaskapp-test:__VERSION__
          imagePullPolicy: Always
          ports:
            - name: web
              protocol: TCP
              containerPort: 5000
          livenessProbe:
            httpGet:
              path: /
              port: 5000
            initialDelaySeconds: 3
            timeoutSeconds: 2
          readinessProbe:
            httpGet:
              path: /
              port: 5000
            initialDelaySeconds: 3
            timeoutSeconds: 2

Service (service.yaml)

apiVersion: v1
kind: Service
metadata:
  name: gitlab-k8s-demo-__CI_ENVIRONMENT_SLUG__
  namespace: devops
  labels:
    app: gitlab-k8s-demo
    ref: __CI_ENVIRONMENT_SLUG__
  annotations:
    prometheus.io/scrape: "true"
    prometheus.io/port: "5000"
    prometheus.io/scheme: "http"
    prometheus.io/path: "/"
spec:
  type: ClusterIP
  ports:
    - name: http-metrics
      port: 5000
      protocol: TCP
  selector:
    app: gitlab-k8s-demo
    ref: __CI_ENVIRONMENT_SLUG__

Ingress (ingress.yaml)

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: gitlab-k8s-demo-__CI_ENVIRONMENT_SLUG__
  namespace: devops
  labels:
    app: gitlab-k8s-demo
    ref: __CI_ENVIRONMENT_SLUG__
  annotations:
    nginx.ingress.kubernetes.io/service-weight: ''
spec:
  rules:
    - host: __CI_ENVIRONMENT_SLUG__-gitlab-k8s-demo.****.cn-beijing.alicontainer.com
      http:
        paths:
          - path: /
            backend:
              serviceName: gitlab-k8s-demo-__CI_ENVIRONMENT_SLUG__
              servicePort: 5000

Configure Runner Environment Variables

Runner variables can be defined at the project level or group level in the GitLab UI. After setting them, they are referenced in the .gitlab-ci.yml pipeline.

Pipeline Execution Overview

When code is pushed to the master branch, GitLab automatically creates a pipeline that runs the defined jobs. The pipeline view shows stages such as style , test , release , etc.

For feature branches (e.g., feature-01), only the jobs allowed by only/except run. Review jobs require manual triggering, and stop_review can be used to delete the temporary environment.

Tag‑Based Deployments

Pushing a new tag (e.g., v2.0) triggers a pipeline that runs the deploy job, indicating a stable version ready for production.

git tag v2.0
🐳 👉 git push origin --tags

After deployment, the service can be accessed via the generated URL and the Flask login page should appear, confirming a successful CI/CD workflow.

References

GitLab CI YAML reference: https://docs.gitlab.com/ee/ci/yaml/README.html

Runner variables documentation: https://docs.gitlab.com/ee/ci/variables/README.html