Sidecar Containers

FEATURE STATE: Kubernetes v1.28 [alpha]

Sidecar containers are the secondary containers that run along with the main application container within the same Pod. These containers are used to enhance or to extend the functionality of the main application container by providing additional services, or functionality such as logging, monitoring, security, or data synchronization, without directly altering the primary application code.

Enabling sidecar containers

Starting with Kubernetes 1.28, a feature gate named SidecarContainers allows you to specify a restartPolicy for containers listed in a Pod’s initContainers field. These restartable sidecar containers are independent with other init containers and main application container within the same pod. These can be started, stopped, or restarted without effecting the main application container and other init containers.

Sidecar containers and Pod lifecycle

If an init container is created with its restartPolicy set to Always, it will start and remain running during the entire life of the Pod. This can be helpful for running supporting services separated from the main application containers.

If a readinessProbe is specified for this init container, its result will be used to determine the ready state of the Pod.

Since these containers are defined as init containers, they benefit from the same ordering and sequential guarantees as other init containers, allowing them to be mixed with other init containers into complex Pod initialization flows.

Compared to regular init containers, sidecars defined within initContainers continue to run after they have started. This is important when there is more than one entry inside .spec.initContainers for a Pod. After a sidecar-style init container is running (the kubelet has set the started status for that init container to true), the kubelet then starts the next init container from the ordered .spec.initContainers list. That status either becomes true because there is a process running in the container and no startup probe defined, or as a result of its startupProbe succeeding.

Here’s an example of a Deployment with two containers, one of which is a sidecar:

application/deployment-sidecar.yaml

apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
  labels:
    app: myapp
spec:
  replicas: 1
  selector:
    matchLabels:
      app: myapp
  template:
    metadata:
      labels:
        app: myapp
    spec:
      containers:
        - name: myapp
          image: alpine:latest
          command: ['sh', '-c', 'while true; do echo "logging" >> /opt/logs.txt; sleep 1; done']
          volumeMounts:
            - name: data
              mountPath: /opt
      initContainers:
        - name: logshipper
          image: alpine:latest
          restartPolicy: Always
          command: ['sh', '-c', 'tail -F /opt/logs.txt']
          volumeMounts:
            - name: data
              mountPath: /opt
      volumes:
        - name: data
          emptyDir: {}

This feature is also useful for running Jobs with sidecars, as the sidecar container will not prevent the Job from completing after the main container has finished.

Here’s an example of a Job with two containers, one of which is a sidecar:

application/job/job-sidecar.yaml

apiVersion: batch/v1
kind: Job
metadata:
  name: myjob
spec:
  template:
    spec:
      containers:
        - name: myjob
          image: alpine:latest
          command: ['sh', '-c', 'echo "logging" > /opt/logs.txt']
          volumeMounts:
            - name: data
              mountPath: /opt
      initContainers:
        - name: logshipper
          image: alpine:latest
          restartPolicy: Always
          command: ['sh', '-c', 'tail -F /opt/logs.txt']
          volumeMounts:
            - name: data
              mountPath: /opt
      restartPolicy: Never
      volumes:
        - name: data
          emptyDir: {}

By default, this feature is not available in Kubernetes. To avail this feature, you need to enable the feature gate named SidecarContainers.

Differences from regular containers

Sidecar containers run alongside regular containers in the same pod. However, they do not execute the primary application logic; instead, they provide supporting functionality to the main application.

Sidecar containers have their own independent lifecycles. They can be started, stopped, and restarted independently of regular containers. This means you can update, scale, or maintain sidecar containers without affecting the primary application.

Sidecar containers share the same network and storage namespaces with the primary container This co-location allows them to interact closely and share resources.

Differences from init containers

Sidecar containers work alongside the main container, extending its functionality and providing additional services.

Sidecar containers run concurrently with the main application container. They are active throughout the lifecycle of the pod and can be started and stopped independently of the main container. Unlike init containers, sidecar containers support probes to control their lifecycle.

These containers can interact directly with the main application containers, sharing the same network namespace, filesystem, and environment variables. They work closely together to provide additional functionality.

Resource sharing within containers

Given the order of execution for init, sidecar and app containers, the following rules for resource usage apply:

• The highest of any particular resource request or limit defined on all init containers is the effective init request/limit. If any resource has no resource limit specified this is considered as the highest limit.
• The Pod’s effective request/limit for a resource is the sum of pod overhead and the higher of:
  • the sum of all non-init containers(app and sidecar containers) request/limit for a resource
  • the effective init request/limit for a resource
• Scheduling is done based on effective requests/limits, which means init containers can reserve resources for initialization that are not used during the life of the Pod.
• The QoS (quality of service) tier of the Pod’s effective QoS tier is the QoS tier for all init, sidecar and app containers alike.

Quota and limits are applied based on the effective Pod request and limit.

Pod level control groups (cgroups) are based on the effective Pod request and limit, the same as the scheduler.

What’s next

• Read a blog post on native sidecar containers.
• Read about creating a Pod that has an init container.
• Learn about the types of probes: liveness, readiness, startup probe.
• Learn about pod overhead.