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content/en/docs/concepts/workloads/pods/sidecar-containers.md
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| --- | ||
| title: Sidecar Containers | ||
| content_type: concept | ||
| weight: 50 | ||
| --- | ||
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| <!-- overview --> | ||
| {{< feature-state for_k8s_version="v1.28" state="alpha" >}} | ||
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| Sidecar containers are the secondary containers that run along with the main | ||
| application container within the same {{< glossary_tooltip text="Pod" term_id="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. | ||
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| <!-- body --> | ||
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| ## Enabling sidecar containers | ||
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| Starting with Kubernetes 1.28, a | ||
| [feature gate](/docs/reference/command-line-tools-reference/feature-gates/) 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](/docs/concepts/workloads/pods/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. | ||
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| ## Sidecar containers and Pod lifecycle | ||
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| 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. | ||
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| If a `readinessProbe` is specified for this init container, its result will be used | ||
| to determine the `ready` state of the Pod. | ||
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| 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. | ||
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| 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. | ||
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| Here's an example of a Deployment with two containers, one of which is a sidecar: | ||
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| {{% code_sample language="yaml" file="application/deployment-sidecar.yaml" %}} | ||
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| 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. | ||
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| Here's an example of a Job with two containers, one of which is a sidecar: | ||
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| {{% code_sample language="yaml" file="application/job/job-sidecar.yaml" %}} | ||
| By default, this feature is not available in Kubernetes. To avail this feature, you | ||
| need to enable the [feature gate](/docs/reference/command-line-tools-reference/feature-gates/) | ||
| named `SidecarContainers`. | ||
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| ## Differences from regular containers | ||
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| 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. | ||
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| 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. | ||
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| Sidecar containers share the same network and storage namespaces with the primary | ||
| container This co-location allows them to interact closely and share resources. | ||
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| ## Differences from init containers | ||
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| Sidecar containers work alongside the main container, extending its functionality and | ||
| providing additional services. | ||
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| 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](/docs/concepts/workloads/pods/init-containers/), | ||
| sidecar containers support [probes](/docs/concepts/workloads/pods/pod-lifecycle/#types-of-probe) to control their lifecycle. | ||
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| 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. | ||
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| ## Resource sharing within containers | ||
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| {{< comment >}} | ||
| This section is also present in the [init containers](/docs/concepts/workloads/pods/init-containers/) page. | ||
| If you're editing this section, change both places. | ||
| {{< /comment >}} | ||
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| Given the order of execution for init, sidecar and app containers, the following rules | ||
| for resource usage apply: | ||
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| * 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 higher of: | ||
| * the sum of all app 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 init containers and app containers alike. | ||
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| Quota and limits are applied based on the effective Pod request and | ||
| limit. | ||
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| Pod level control groups (cgroups) are based on the effective Pod request and | ||
| limit, the same as the scheduler. | ||
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| ## {{% heading "whatsnext" %}} | ||
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| * Read a blog post on [native sidecar containers](/blog/2023/08/25/native-sidecar-containers/). | ||
| * Read about [creating a Pod that has an init container](/docs/tasks/configure-pod-container/configure-pod-initialization/#create-a-pod-that-has-an-init-container). | ||
| * Learn about the [types of probes](/docs/concepts/workloads/pods/pod-lifecycle/#types-of-probe): liveness, readiness, startup probe. |