Sidecar Pattern

Deploys auxiliary services alongside the main application, sharing lifecycle. In Kubernetes, sidecars run in the same pod and share a network namespace (communicate via localhost). In Docker Compose, sidecars share a Docker network (communicate via service name). Common in microservices for cross-cutting concerns.

When to Use

• Adding logging, monitoring, or security to microservices without code changes
• Service mesh (Envoy, Istio) for traffic management and observability
• Centralizing TLS termination or auth proxy across services
• Configuration management that updates dynamically

Don't use for:

• Monolithic applications (use middleware instead)
• Single simple services where overhead isn't justified
• Tight coupling between sidecar and main app logic