Sidecars in Kubernetes: The Key to Extending Your Applications

Kubernetes has established itself as the preferred platform for deploying complex, distributed systems. One of the most powerful yet subtle design patterns in this ecosystem is the sidecar pattern. It allows developers to extend the functionality of applications without delving deeply into the source code.

The Origins of the Sidecar Pattern

Imagine a sidecar as a loyal companion on a motorcycle. Historically, IT infrastructures have always used auxiliary services to perform critical tasks. Before containers, we worked with background processes and auxiliary services to manage logging, monitoring, and network administration. The microservices revolution transformed this approach, making the sidecar pattern a structured and purposeful architectural choice. With the rise of microservices, the sidecar pattern became more clearly defined. Developers can offload specific responsibilities from the main service without altering its code. Service meshes like Istio and Linkerd have popularized sidecar proxies, demonstrating how these companion containers elegantly handle observability, security, and traffic management in distributed systems.

Implementation in Kubernetes

In Kubernetes, sidecar containers operate within the same pod as the main application, enabling communication and resource sharing. Doesn’t that sound like simply defining multiple containers side by side in the pod? It does indeed, and that’s how sidecar containers had to be implemented before Kubernetes v1.29.0 introduced native support for sidecars. Sidecar containers can now be defined in the pod manifest under the spec.initContainers field. What makes it a sidecar container is specifying restartPolicy: Always. Here’s an example showing part of a complete Kubernetes manifest:

initContainers:
  - name: logshipper
    image: alpine:latest
    restartPolicy: Always
    command: ['sh', '-c', 'tail -F /opt/logs.txt']
    volumeMounts:
    - name: data
      mountPath: /opt

The field name spec.initContainers might seem confusing. How is it that when defining a sidecar container, you need to make an entry in the spec.initContainers array? spec.initContainers are executed to completion before the main application starts, making them one-time, while sidecars often run in parallel with the main application. It is the spec.initContainers with restartPolicy: Always that distinguishes classic init containers from Kubernetes-native sidecar containers, ensuring they are always active.

When to Use (or Avoid) Sidecars

Although the sidecar pattern can be useful in many cases, it is generally not the preferred approach unless the use case justifies it. Adding a sidecar increases complexity, resource consumption, and potential network latency. Instead, simpler alternatives like integrated libraries or shared infrastructures should be considered first.

Deploy a sidecar when:

1. You want to extend the functionality of an application without changing the original code.
2. You are implementing cross-cutting concerns like logging, monitoring, or security.
3. You are working with legacy applications that require modern network capabilities.
4. You are designing microservices that require independent scaling and updating.

Exercise caution when:

1. Resource efficiency is your main concern.
2. Minimal network latency is crucial.
3. Simpler alternatives exist.
4. You want to minimize complexity in troubleshooting.

Four Essential Multi-Container Patterns

In the world of Kubernetes, there are several proven multi-container patterns that can help in designing your applications. They are not only practical but also crucial for the effectiveness of your container architecture. ayedo supports you in successfully implementing these concepts in your Kubernetes environment!

Source: Kubernetes Blog