Kubernetes Multicontainer Pods: An Overview

By Agata Skorupka (The Scale Factory) |

As cloud-native architectures continue to evolve, Kubernetes has become the go-to platform for deploying complex, distributed systems. One of the most powerful yet nuanced design patterns in this ecosystem is the sidecar pattern—a technique that allows developers to extend application functionality without diving deep into source code.

The origins of the sidecar pattern

Think of a sidecar like a trusty companion motorcycle attachment. Historically, IT infrastructures have always used auxiliary services to handle critical tasks. Before containers, we relied on background processes and helper daemons to manage logging, monitoring, and networking. The microservices revolution transformed this approach, making sidecars a structured and intentional architectural choice. With the rise of microservices, the sidecar pattern became more clearly defined, allowing developers to 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 can elegantly handle observability, security, and traffic management in distributed systems.

Kubernetes implementation

In Kubernetes, sidecar containers operate within the same Pod as the main application, enabling communication and resource sharing. Does this sound just like defining multiple containers along each other inside the Pod? It actually does, and this is how sidecar containers had to be implemented before Kubernetes v1.29.0, which introduced native support for sidecars. Sidecar containers can now be defined within a Pod manifest using the spec.initContainers field. What makes it a sidecar container is that you specify it with restartPolicy: Always. You can see an example of this below, which is a partial snippet of the full Kubernetes manifest:

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

That field name, spec.initContainers may sound confusing. How come when you want to define a sidecar container, you have to put an entry in the spec.initContainers array? spec.initContainers are run to completion just before main application starts, so they’re one-off, whereas sidecars often run in parallel to the main app container. It’s the spec.initContainers with restartPolicy:Always which differs classic init containers from Kubernetes-native sidecar containers and ensures they are always up.

When to embrace (or avoid) sidecars

While 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 such as built-in libraries or shared infrastructure should be considered first.

Deploy a sidecar when:

  1. You need to extend application functionality without touching the original code
  2. Implementing cross-cutting concerns like logging, monitoring or security
  3. Working with legacy applications requiring modern networking capabilities
  4. Designing microservices that demand independent scaling and updates

Proceed with caution if:

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

Four essential multi-container patterns

Init container pattern

The Init container pattern is used to execute (often critical) setup tasks before the main application container starts. Unlike regular containers, init containers run to completion and then terminate, ensuring that preconditions for the main application are met.

Ideal for:

  1. Preparing configurations
  2. Loading secrets
  3. Verifying dependency availability
  4. Running database migrations

The init container ensures your application starts in a predictable, controlled environment without code modifications.

Ambassador pattern

An ambassador container provides Pod-local helper services that expose a simple way to access a network service. Commonly, ambassador containers send network requests on behalf of a an application container and take care of challenges such as service discovery, peer identity verification, or encryption in transit.

Perfect when you need to:

  1. Offload client connectivity concerns
  2. Implement language-agnostic networking features
  3. Add security layers like TLS
  4. Create robust circuit breakers and retry mechanisms

Configuration helper

A configuration helper sidecar provides configuration updates to an application dynamically, ensuring it always has access to the latest settings without disrupting the service. Often the helper needs to provide an initial configuration before the application would be able to start successfully.

Use cases:

  1. Fetching environment variables and secrets
  2. Polling configuration changes
  3. Decoupling configuration management from application logic

Adapter pattern

An adapter (or sometimes façade) container enables interoperability between the main application container and external services. It does this by translating data formats, protocols, or APIs.

Strengths:

  1. Transforming legacy data formats
  2. Bridging communication protocols
  3. Facilitating integration between mismatched services

Wrap-up

While sidecar patterns offer tremendous flexibility, they're not a silver bullet. Each added sidecar introduces complexity, consumes resources, and potentially increases operational overhead. Always evaluate simpler alternatives first. The key is strategic implementation: use sidecars as precision tools to solve specific architectural challenges, not as a default approach. When used correctly, they can improve security, networking, and configuration management in containerized environments. Choose wisely, implement carefully, and let your sidecars elevate your container ecosystem.