A sidecar runs alongside your main application container:
Use cases:
- Logging and monitoring agents
- Service mesh proxies (Envoy, Linkerd)
- Configuration management
- Security (mTLS handling)
How it works: Sidecar shares network namespace with main container. Intercepts traffic transparently.
Benefits: Main application stays simple. Cross-cutting concerns handled separately. Language-agnostic.
Example: Istio injects Envoy sidecars to handle service-to-service communication, retries, and observability.