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Sidecar Pattern
Definition
The Sidecar Pattern deploys components of an application as separate containers or processes alongside the main application. Like a motorcycle sidecar, these helper components are attached to the parent application, providing supporting features while maintaining separation of concerns.
When to Use It
- Adding cross-cutting concerns without modifying the main app
- Applications written in different languages need common functionality
- You want to isolate auxiliary features
- Implementing service mesh capabilities
- Monitoring, logging, or configuration management
- Legacy applications that can't be modified
Pros & Cons
✓ Pros
- Separation of concerns
- Technology independence
- Reusable across applications
- Independent deployment and updates
- Enhanced observability
- No code changes to main app
✗ Cons
- Increased resource consumption
- Additional complexity
- Inter-process communication overhead
- More containers to manage
- Latency for local communication
- Tighter deployment coupling
Architecture Diagram
Kubernetes Pod with Sidecar
============================
┌─────────────────────────────────────────────────────┐
│ Pod │
│ │
│ ┌─────────────────────┐ ┌──────────────────────┐│
│ │ │ │ ││
│ │ Main Application │ │ Sidecar Container ││
│ │ Container │ │ ││
│ │ │ │ • Logging Agent ││
│ │ • Business Logic │◄─┤ • Metrics Collector ││
│ │ • API Endpoints │ │ • Config Manager ││
│ │ • Core Features │ │ • Security Proxy ││
│ │ │ │ ││
│ └──────────┬──────────┘ └──────────┬───────────┘│
│ │ │ │
│ │ Shared Resources: │ │
│ │ • Network Namespace │ │
│ │ • Storage Volumes │ │
│ │ • IPC │ │
│ │ │ │
└─────────────┼────────────────────────┼─────────────┘
│ │
▼ ▼
┌──────────┐ ┌──────────────┐
│ Network │ │ Log Storage │
└──────────┘ └──────────────┘
Service Mesh Sidecar (Envoy Proxy):
────────────────────────────────────
┌─────────────────────────────────┐
│ Service A │
│ │
│ ┌──────────┐ ┌───────────┐ │
│ │ App │◄──►│ Envoy │ │
│ │Container │ │ Sidecar │ │
│ └──────────┘ └─────┬─────┘ │
└──────────────────────────┼───────┘
│
│ mTLS, Tracing
│ Load Balancing
│
┌──────────────────────────┼───────┐
│ Service B ▼ │
│ │
│ ┌──────────┐ ┌───────────┐ │
│ │ App │◄──►│ Envoy │ │
│ │Container │ │ Sidecar │ │
│ └──────────┘ └───────────┘ │
└───────────────────────────────────┘
Common Sidecar Use Cases:
──────────────────────────
1. Logging Sidecar
┌─────────┐ Logs ┌──────────┐
│ App │───────►│Fluentd │─► Log System
└─────────┘ └──────────┘
2. Monitoring Sidecar
┌─────────┐ Metrics ┌──────────┐
│ App │────────►│Prometheus│─► Grafana
└─────────┘ └──────────┘
3. Security Sidecar
┌─────────┐ Auth ┌──────────┐
│ App │◄───────►│OAuth2 │─► Identity
└─────────┘ │ Proxy │ Provider
└──────────┘
4. Configuration Sidecar
┌─────────┐ Config ┌──────────┐
│ App │◄────────│ Consul │◄─ Config Store
└─────────┘ │ Agent │
└──────────┘
Benefits:
• Cross-cutting concerns externalized
• Main app stays focused on business logic
• Sidecars reusable across services
• Independent scaling and updates