Sidecar Containers
Kubernetes v1.29 [beta]
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 primary app container by providing additional services, or functionality such as logging, monitoring, security, or data synchronization, without directly altering the primary application code.
Typically, you only have one app container in a Pod. For example, if you have a web application that requires a local webserver, the local webserver is a sidecar and the web application itself is the app container.
Sidecar containers in Kubernetes
Kubernetes implements sidecar containers as a special case of init containers; sidecar containers remain running after Pod startup. This document uses the term regular init containers to clearly refer to containers that only run during Pod startup.
Provided that your cluster has the SidecarContainers
feature gate enabled
(the feature is active by default since Kubernetes v1.29), you can specify a restartPolicy
for containers listed in a Pod's initContainers
field.
These restartable sidecar containers are independent from other init containers and from
the main application container(s) within the same pod.
These can be started, stopped, or restarted without effecting the main application container
and other init containers.
You can also run a Pod with multiple containers that are not marked as init or sidecar
containers. This is appropriate if the containers within the Pod are required for the
Pod to work overall, but you don't need to control which containers start or stop first.
You could also do this if you need to support older versions of Kubernetes that don't
support a container-level restartPolicy
field.
Example application
Here's an example of a Deployment with two containers, one of which is a sidecar:
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: {}
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 regular init containers, allowing you to mix sidecar containers with regular init containers for 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.
Upon Pod termination, the kubelet postpones terminating sidecar containers until the main application container has fully stopped. The sidecar containers are then shut down in the opposite order of their appearance in the Pod specification. This approach ensures that the sidecars remain operational, supporting other containers within the Pod, until their service is no longer required.
Jobs with sidecar containers
If you define a Job that uses sidecar using Kubernetes-style init containers, the sidecar container in each Pod does 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:
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: {}
Differences from application containers
Sidecar containers run alongside app 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 app 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.
Sidecar containers can interact directly with the main application containers, because like init containers they always share the same network, and can optionally also share volumes (filesystems).
Init containers stop before the main containers start up, so init containers cannot
exchange messages with the app container in a Pod. Any data passing is one-way
(for example, an init container can put information inside an emptyDir
volume).
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.
Sidecar containers and Linux cgroups
On Linux, resource allocations for Pod level control groups (cgroups) are based on the effective Pod request and limit, the same as the scheduler.
What's next
- Learn how to Adopt Sidecar Containers
- 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.