Sidecars have been a part of Kubernetes since its early days. They were first described in a blog post in 2015 as additional containers that can be used to extend and enhance the functionality of the main container. Sidecars are now a common deployment pattern in Kubernetes, and they are often used for tasks such as network proxying and logging. Until recently, sidecars were not natively supported by Kubernetes, which led to some usability challenges. This enhancement aims to address these challenges by providing native support for sidecars.
Sidecar containers in Kubernetes 1.28 are init containers that have the
restartPolicy field set to
Always. This means that the container will be restarted if it exits. This makes them ideal for the sidecar deployment pattern, where a container is used to provide auxiliary services to the main container.
Here are some of the benefits of using sidecar containers in Kubernetes 1.28:
- Well-defined startup order: Init containers have a well-defined startup order, regardless of whether you set a
restartPolicy. This means that you can ensure that your sidecar starts before any container declarations that come after the sidecar declaration in your manifest.
- No lifetime extension: Sidecar containers don’t extend the lifetime of the Pod, so you can use them in short-lived Pods with no changes to the Pod lifecycle.
- Improved resilience: Sidecar containers are restarted on exit, which improves resilience and lets you use sidecars to provide services that your main containers can more reliably consume.
Here are some examples of when you might find sidecar containers useful:
- Batch or AI/ML workloads: These workloads often benefit from the well-defined startup order and improved resilience that sidecar containers provide.
- Network proxies: Sidecar containers can be used to provide network proxies for your Pods, such as Envoy or Istio.
- Log collection: Sidecar containers can be used to collect logs from your Pods, such as fluentd or ELK.
- Jobs: Sidecar containers can be used in Jobs for any purpose, such as monitoring or debugging.
Sidecar containers and init containers are both used in Kubernetes to extend the functionality of pods. However, they have different purposes and should be used in different situations. Sidecar containers are run alongside the main container in a pod. They share the same network and storage resources, and they can communicate with each other using localhost. Sidecar containers are typically used to provide auxiliary services to the main container, such as logging, monitoring, or load balancing.
Init containers run before the main container in a pod. They are used to perform tasks that need to be completed before the main container can start, such as downloading dependencies or setting up environment variables. Init containers do not share the same network and storage resources as the main container, and they cannot communicate with each other using localhost.
You should use sidecar containers when you need to provide auxiliary services to the main container. For example, you could use a sidecar container to provide logging, monitoring, or load balancing for your application. Sidecar containers are also a good choice when you need to isolate the main container from the rest of the pod.
You should use init containers when you need to perform tasks that need to be completed before the main container can start. For example, you could use an init container to download dependencies or set up environment variables for your application. Init containers are also a good choice when you need to ensure that the main container starts successfully.
The best choice for you will depend on your specific needs. If you need to provide auxiliary services to the main container, then you should use sidecar containers. If you need to perform tasks that need to be completed before the main container can start, then you should use init containers.
Here is a table that summarizes the key differences between sidecar containers and init containers:
Here’s an example YAML file that shows how to use sidecar containers and init containers:
- name: main
- name: sidecar
command: ["sleep", "3600"]
- name: init
command: ["echo", "Hello from init container"]
This YAML file defines a pod with two containers: a main container and a sidecar container. The main container is responsible for running the nginx web server. The sidecar container is responsible for sleeping for 3600 seconds. The init container is responsible for printing the message “Hello from init container” to the console.
The main container and the sidecar container share the same network and storage resources. However, the init container does not share the same network and storage resources as the main container.
The init container runs before the main container. This means that the init container will start first and will print the message “Hello from init container” to the console before the main container starts.
I hope this blog post helps you to understand the difference between sidecar containers and init containers. If you have any questions, please feel free to ask me.