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Sidecars are containers that run alongside the Steps in your Task, providing supporting services like databases, Docker daemons, or SSH servers. This guide shows you how to use sidecars effectively.

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Overview

Sidecars enable you to:
  • Run services that Steps need to communicate with
  • Provide test dependencies like databases or message queues
  • Run Docker-in-Docker for building container images
  • Set up SSH servers or other network services
  • Share resources with Steps through volumes
Sidecars start before Steps execute and run throughout the Task execution.

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Basic Sidecar Configuration

Define sidecars in the Task specification: 
apiVersion: tekton.dev/v1
kind: TaskRun
metadata:
  generateName: sidecar-example-
spec:
  taskSpec:
    steps:
      - name: main
        image: alpine
        script: |
          echo "Main step running"
          # Steps can communicate with sidecar
    sidecars:
      - name: helper
        image: nginx

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Docker-in-Docker Sidecar

A common use case is running Docker commands from Steps: 
apiVersion: tekton.dev/v1
kind: TaskRun
metadata:
  generateName: dind-sidecar-
spec:
  taskSpec:
    steps:
      - name: client
        image: docker
        env:
          # Connect to the sidecar over TCP, with TLS
          - name: DOCKER_HOST
            value: tcp://localhost:2376
          # Verify TLS
          - name: DOCKER_TLS_VERIFY
            value: '1'
          # Use the certs generated by the sidecar daemon
          - name: DOCKER_CERT_PATH
            value: /certs/client
        workingDir: /workspace
        script: |
          #!/usr/bin/env sh
          set -e
          # Run a Docker container
          docker run busybox echo hello

          # Write a Dockerfile and build it
          cat > Dockerfile << EOF
          FROM ubuntu
          RUN apt-get update
          ENTRYPOINT ["echo", "hello"]
          EOF
          docker build -t hello .
          docker images

          # Run the built image
          docker run hello
        volumeMounts:
          - mountPath: /certs/client
            name: dind-certs

    sidecars:
      - name: server
        image: docker:dind
        args:
          - --storage-driver=vfs
          - --userland-proxy=false
          - --debug
        computeResources:
          requests:
            memory: "512Mi"
        securityContext:
          privileged: true
        env:
          # Write generated certs to the path shared with the client
          - name: DOCKER_TLS_CERTDIR
            value: /certs
        volumeMounts:
          - mountPath: /certs/client
            name: dind-certs
        # Wait for the dind daemon to generate the certs
        startupProbe:
          periodSeconds: 1
          failureThreshold: 30
          exec:
            command: ['ls', '/certs/client/ca.pem']
        # Verify Docker daemon is ready to accept commands
        readinessProbe:
          periodSeconds: 2
          exec:
            command: ['docker', 'info']

    volumes:
      - name: dind-certs
        emptyDir: {}
Docker-in-Docker requires privileged mode. Ensure your cluster security policies allow privileged containers.

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Sharing Data with Volumes

Sidecars and Steps can share data through volumes: 
apiVersion: tekton.dev/v1
kind: TaskRun
metadata:
  generateName: shared-volume-
spec:
  taskSpec:
    volumes:
      - name: messages
        emptyDir: {}
    steps:
      - name: write
        image: alpine
        volumeMounts:
          - name: messages
            mountPath: /messages
        script: |
          echo "Hello from step" > /messages/greeting.txt
      - name: read
        image: alpine
        volumeMounts:
          - name: messages
            mountPath: /messages
        script: |
          cat /messages/greeting.txt
    sidecars:
      - name: processor
        image: alpine
        volumeMounts:
          - name: messages
            mountPath: /messages
        command: ['sh', '-c']
        args:
          - |
            while true; do
              if [ -f /messages/greeting.txt ]; then
                echo "Sidecar saw: $(cat /messages/greeting.txt)"
              fi
              sleep 2
            done

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Sidecar Readiness

Use readiness probes to ensure sidecars are ready before Steps start: 
sidecars:
  - name: database
    image: postgres:13
    env:
      - name: POSTGRES_PASSWORD
        value: example
    readinessProbe:
      exec:
        command:
          - pg_isready
          - -U
          - postgres
      periodSeconds: 2
      initialDelaySeconds: 5
Steps wait for sidecars with readiness probes to become ready before executing.

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Startup Probes

Use startup probes for sidecars that need extra time to initialize: 
sidecars:
  - name: slow-service
    image: my-slow-service
    startupProbe:
      periodSeconds: 1
      failureThreshold: 30  # Allow up to 30 seconds for startup
      exec:
        command: ['cat', '/tmp/ready']
    readinessProbe:
      periodSeconds: 2
      exec:
        command: ['curl', 'localhost:8080/health']

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SSH Server Sidecar

Run an SSH server for Git operations: 
apiVersion: tekton.dev/v1
kind: TaskRun
metadata:
  name: authenticating-git-commands
spec:
  serviceAccountName: ssh-key-service-account
  taskSpec:
    volumes:
      - name: messages
        emptyDir: {}
    sidecars:
      - name: server
        image: alpine/git:v2.26.2
        securityContext:
          runAsUser: 0
        volumeMounts:
          - name: messages
            mountPath: /messages
        script: |
          #!/usr/bin/env ash

          # Generate a private host key
          ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key
          chmod 0600 /etc/ssh/ssh_host_rsa_key*
          HOST_PUBLIC_KEY=$(cat /etc/ssh/ssh_host_rsa_key.pub | awk '{ print $2 }')
          echo "localhost ssh-rsa $HOST_PUBLIC_KEY" > /messages/known_hosts

          # Wait for Steps to supply the server a public key
          while [ ! -f /messages/authorized_keys ] ; do
            sleep 1
          done

          # Configure SSH to allow login
          mkdir /root/.ssh
          cp /messages/authorized_keys /root/.ssh/
          sed -i s/root:!/'root:*'/g /etc/shadow

          # Create a git repo
          cd /root/
          mkdir repo
          cd repo
          git init . --bare

          # Start the sshd server
          /usr/sbin/sshd -E /var/log/sshd
          touch /messages/sshd-ready
          tail -f /var/log/sshd
    steps:
      - name: setup
        image: alpine/git:v2.26.2
        volumeMounts:
          - name: messages
            mountPath: /messages
        script: |
          # Generate authorized_keys from credentials
          ssh-keygen -y -f $(credentials.path)/.ssh/id_* > /messages/authorized_keys

          # Wait for sshd to start
          while [ ! -f /messages/sshd-ready ] ; do
            sleep 1
          done
      - name: git-clone-and-push
        image: alpine/git:v2.26.2
        volumeMounts:
          - name: messages
            mountPath: /messages
        script: |
          ln -s $(credentials.path)/.ssh /root/.ssh
          cp /messages/known_hosts /root/.ssh/
          git clone root@localhost:/root/repo ./repo
          cd repo
          echo "Hello, world!" > README
          git add README
          git commit -m "Test commit"
          git push origin master

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Workspaces in Sidecars

Sidecars can access Task workspaces: 
apiVersion: tekton.dev/v1
kind: Task
metadata:
  name: sidecar-workspace-example
spec:
  workspaces:
    - name: source
  sidecars:
    - name: file-server
      image: python:3-alpine
      workingDir: $(workspaces.source.path)
      command: ['python', '-m', 'http.server', '8000']
  steps:
    - name: download
      image: alpine
      script: |
        echo "File server available at localhost:8000"
        # Step can access files via HTTP from sidecar

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Variable Substitution in Sidecars

Sidecars support variable substitution: 
sidecars:
  - name: database
    image: $(params.database-image)
    env:
      - name: DB_NAME
        value: $(params.database-name)
      - name: WORKSPACE_PATH
        value: $(workspaces.data.path)
Supported fields:
  • spec.sidecars[].name
  • spec.sidecars[].image
  • spec.sidecars[].imagePullPolicy
  • spec.sidecars[].env.value
  • spec.sidecars[].command
  • spec.sidecars[].args
  • spec.sidecars[].script
  • spec.sidecars[].volumeMounts

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Sidecar Lifecycle

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Startup Order

  1. Sidecars start first
  2. Sidecars with readiness probes must become ready
  3. Steps execute in order
  4. Sidecars continue running during Step execution

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Shutdown

Sidecars automatically stop when:
  • All Steps complete successfully
  • A Step fails and the Task stops
  • The TaskRun times out
Tekton sends a SIGTERM to sidecars to request graceful shutdown. If sidecars don’t exit within the grace period, they receive SIGKILL.

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Resource Management

Specify resource requests and limits for sidecars: 
sidecars:
  - name: memory-intensive
    image: my-service
    computeResources:
      requests:
        memory: "512Mi"
        cpu: "500m"
      limits:
        memory: "1Gi"
        cpu: "1000m"

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Best Practices

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Use Readiness Probes

Always configure readiness probes for sidecars that Steps depend on. This prevents race conditions where Steps try to connect before the sidecar is ready.
sidecars:
  - name: api-server
    image: my-api
    readinessProbe:
      httpGet:
        path: /health
        port: 8080
      periodSeconds: 2

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Minimize Sidecar Size

# Good: Lightweight sidecar
sidecars:
  - name: cache
    image: redis:alpine

# Avoid: Unnecessarily large images
sidecars:
  - name: cache
    image: ubuntu-with-redis

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Share Data via Volumes

Use volumes instead of network communication when possible: 
volumes:
  - name: shared-data
    emptyDir: {}
steps:
  - name: producer
    volumeMounts:
      - name: shared-data
        mountPath: /data
sidecars:
  - name: consumer
    volumeMounts:
      - name: shared-data
        mountPath: /data

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Set Security Contexts

sidecars:
  - name: secure-service
    image: my-service
    securityContext:
      runAsNonRoot: true
      runAsUser: 1000
      allowPrivilegeEscalation: false
      capabilities:
        drop:
          - ALL

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Common Patterns

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Database for Testing

sidecars:
  - name: postgres
    image: postgres:13
    env:
      - name: POSTGRES_PASSWORD
        value: test
      - name: POSTGRES_DB
        value: testdb
    readinessProbe:
      exec:
        command: ['pg_isready', '-U', 'postgres']
      periodSeconds: 2

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Service Mesh Proxy

sidecars:
  - name: envoy
    image: envoyproxy/envoy:v1.20.0
    args:
      - -c
      - /config/envoy.yaml
    volumeMounts:
      - name: envoy-config
        mountPath: /config

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Message Queue

sidecars:
  - name: rabbitmq
    image: rabbitmq:3-alpine
    env:
      - name: RABBITMQ_DEFAULT_USER
        value: guest
      - name: RABBITMQ_DEFAULT_PASS
        value: guest
    readinessProbe:
      exec:
        command: ['rabbitmq-diagnostics', 'ping']
      periodSeconds: 5
Check these common issues:
  1. Image pull errors: Verify the image exists and is accessible
  2. Resource constraints: Ensure the cluster has sufficient resources
  3. Security policies: Check if security contexts or policies block the sidecar
  4. Volume mount conflicts: Verify volume mounts don’t conflict with Step mounts
View sidecar logs:
kubectl logs <pod-name> -c <sidecar-name>
Steps communicate with sidecars via:
  1. Localhost networking: Sidecars listen on localhost or 127.0.0.1
  2. Shared volumes: Exchange files through mounted volumes
  3. Environment variables: Pass configuration via env vars
Example:
steps:
  - name: client
    script: |
      # Connect to sidecar on localhost
      curl http://localhost:8080/api

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