What are Kubernetes Sidecar Containers?

Question

Accepted Answer

**Sidecar Containers** — auxiliary containers running alongside the main container in a Pod.

```yaml
apiVersion: apps/v1
kind: Deployment
metadata:
  name: myapp
spec:
  template:
    spec:
      containers:
      - name: app
        image: myapp:latest
        ports:
        - containerPort: 8080
      - name: log-sidecar
        image: fluent/fluent-bit:latest
        volumeMounts:
        - name: shared-logs
          mountPath: /var/log/app
      - name: envoy-sidecar
        image: envoyproxy/envoy:latest
        ports:
        - containerPort: 9901
      - name: vault-agent
        image: hashicorp/vault:latest
        lifecycle:
          postStart:
            exec:
              command: ["vault", "agent", "-config=/etc/vault/config.hcl"]
      initContainers:
      - name: init-sidecar
        image: busybox:1.36
        restartPolicy: Always
        command: ["sh", "-c", "while true; do sleep 3600; done"]
```

**Use cases:**
- Log shipping (Fluentd, Fluent Bit).
- Service mesh proxy (Envoy, Istio).
- Secret management (Vault Agent).
- Monitoring agent (Datadog, New Relic).

**vs Init Containers:**
- Init: run before main, to completion.
- Sidecar: run alongside main, long-lived.

DevOps: What are Kubernetes Sidecar Containers?