Template Variables Reference

Template variables enable dynamic configuration based on deployment context. They are essential for creating flexible, reusable m2p configurations.

Available Variables

Variable	Description	Example Output
@{ obj.key }@	Unique package key	myapp-abc123
@{ obj.fqdn_hostname }@	Main FQDN	myapp.example.com
@{ package_release.name }@	Package name	myapp
@{ package_release.main_fqdn }@	Full main FQDN	myapp.example.com

Service Naming Convention

Muppy creates services with the pattern: {key}-{part_code}-svc

parts:
  api:        # Creates: {key}-api-svc
  database:   # Creates: {key}-database-svc
  cache:      # Creates: {key}-cache-svc

Usage Rules

1. Template variables ONLY work inside quoted strings

✅ value: "mongodb://@{ obj.key }@-mongo-svc:27017"
❌ value: mongodb://@{ obj.key }@-mongo-svc:27017

2. Use for ALL inter-service communication

• Database connections
• API endpoints
• Cache connections
• Message queues

3. Special Cases

• MongoDB replica set members
• Redis sentinel configurations
• Kubernetes probes with service references

Common Patterns

Database Connections

# PostgreSQL
- name: DATABASE_URL
  value: "postgresql://user:pass@@{ obj.key }@-postgres-svc:5432/dbname"

# MongoDB
- name: MONGO_URI
  value: "mongodb://@{ obj.key }@-mongo-svc:27017/dbname"

# MongoDB with replica set
- name: MONGO_URI
  value: "mongodb://@{ obj.key }@-mongo-svc:27017/dbname?replicaSet=rs0"

# Redis
- name: REDIS_URL
  value: "redis://@{ obj.key }@-redis-svc:6379/0"

API Communications

# Internal API communication
- name: INTERNAL_API_URL
  value: "http://@{ obj.key }@-api-svc:8080"

# External/public URL
- name: PUBLIC_URL
  value: "https://@{ obj.fqdn_hostname }@"

# Frontend to backend
- name: BACKEND_URL
  value: "http://@{ obj.key }@-backend-svc:3000/api"

Microservices Architecture

# Service-to-service communication
- name: AUTH_SERVICE_URL
  value: "http://@{ obj.key }@-auth-svc:4000"

- name: NOTIFICATION_SERVICE_URL
  value: "http://@{ obj.key }@-notifier-svc:5000"

- name: PAYMENT_SERVICE_URL
  value: "http://@{ obj.key }@-payment-svc:6000"

Special Cases

MongoDB Replica Set Initialization

When using MongoDB with replica sets, the probe commands also need template variables:

probes:
  readiness:
    enabled: true
    type: exec
    command:
      - bash
      - "-c"
      - |
        echo "try { rs.status() } catch (err) { rs.initiate({_id:'rs0',members:[{_id:0,host:'@{ obj.key }@-mongodb-svc:27017'}]}) }" | mongosh --port 27017 --quiet

Redis with Sentinel

- name: REDIS_SENTINEL_URL
  value: "redis-sentinel://@{ obj.key }@-redis-sentinel-svc:26379/mymaster"

Kubernetes Health Checks

probes:
  readiness:
    enabled: true
    type: httpGet
    path: /health
    port: 8080
    httpHeaders:
      - name: Host
        value: "@{ obj.fqdn_hostname }@"

Troubleshooting

Common Errors

Error: Pods start but can't connect to other services Cause: Hardcoded service names without template variables Solution: Replace mongodb://mongodb:27017 with "mongodb://@{ obj.key }@-mongodb-svc:27017"

Error: Template variable not working Cause: Missing quotes around the value Solution: Always wrap values containing template variables in quotes

Error: Wrong service name format Cause: Incorrect template variable usage Solution: Use exact pattern @{ obj.key }@-{part_name}-svc

Debugging Tips

1. Check rendered values: Download the final Helm values from Muppy to see how variables were resolved
2. Verify service names: Use kubectl get services -n {namespace} to see actual service names
3. Test connections: Use debug mode to shell into containers and test connectivity manually

Best Practices

1. Always use template variables for service discovery
2. Quote all template variable values to prevent YAML parsing issues
3. Test locally with helm template --debug before deploying
4. Use descriptive part names as they become part of service names
5. Keep part codes ≤ 8 characters to avoid long service names

Migration from Hardcoded Names

If you have existing configurations with hardcoded service names:

Step 1: Identify hardcoded references

Look for patterns like: - mongodb://mongodb: - redis://redis: - http://backend: - postgres://db:

Step 2: Replace with template variables

# Before
- name: DB_URL
  value: postgresql://postgres:5432/myapp

# After  
- name: DB_URL
  value: "postgresql://@{ obj.key }@-postgres-svc:5432/myapp"

Step 3: Update and test

1. Update Parts Config in Muppy
2. Click "Sync 'Parts Config.'"
3. Check pod logs for successful connections

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💡 Remember: Template variables are what make m2p deployments truly portable and enable multiple instances in the same namespace!