Delivery - Docker and CI/CD

Why this step matters

Shipping software reliably is as important as writing code. Docker and CI/CD make builds reproducible and deployments safer.

Containerize the app

Typical Java Dockerfile pattern:

FROM eclipse-temurin:21-jre
WORKDIR /app
COPY target/app.jar app.jar
ENTRYPOINT ["java", "-jar", "app.jar"]

Use lightweight runtime images and keep image size controlled.

CI pipeline essentials

A baseline pipeline should run:

1. lint/format checks
2. unit tests
3. build package
4. optional integration tests
5. image build and publish

Fail fast on quality gates.

CD and deployment strategy

Common strategies:

• rolling update
• blue/green
• canary

Rolling update

You progressively replace old instances with new ones. Traffic keeps flowing during the rollout, without full downtime.

Strengths:

• simple to implement on most platforms
• little extra infrastructure needed

Limitation:

• rollback can be slower because old and new versions are mixed during transition

Blue/Green

You keep two full environments:

• blue = current production version
• green = new candidate version

When green is validated, you switch all traffic at once.

Strengths:

• very fast rollback (switch traffic back to blue)
• reduced deployment risk during cutover

Limitation:

• higher infrastructure cost (double environment)

Canary

You first route a small share of traffic (for example 5%) to the new version. If metrics stay healthy, you increase gradually (20%, 50%, 100%).

Strengths:

• early detection of real-user regressions
• limited blast radius if something fails

Limitation:

• requires strong observability (logs, metrics, alerts) and fine-grained traffic routing

Start simple and add progressive rollout when needed.

Environment management

Keep secrets and env config outside images. Use platform secret stores or CI secret management.

Common mistakes

• deploying directly from developer machines
• skipping test gates before deployment
• mutable image tags without traceability
• no rollback strategy

Takeaway

1. Build once, deploy consistently
2. Automate tests and packaging in CI
3. Use safe rollout strategy in CD
4. Keep deployment observable and reversible