K8s by Example: Adapter Pattern

The adapter pattern transforms an application’s interface to match a standard expected by external systems. Unlike sidecars that add features, adapters normalize existing interfaces. Use for: exposing Prometheus metrics from apps that don’t support it, converting log formats, standardizing health check endpoints.
adapter-redis-prometheus.yaml

Redis doesn’t expose Prometheus metrics natively. The redis_exporter adapter queries Redis and exposes metrics in Prometheus format on port 9121. Prometheus scrapes the adapter, not Redis directly.

 
apiVersion: v1
kind: Pod
metadata:
  name: redis-with-metrics
  labels:
    app: redis
  annotations:
    prometheus.io/scrape: "true"
    prometheus.io/port: "9121"
spec:
  containers:
    - name: redis
      image: redis:7-alpine
      ports:
        - containerPort: 6379

The adapter container connects to Redis on localhost:6379 (shared network namespace) and translates Redis INFO command output into Prometheus metrics format.

 
    - name: redis-exporter
      image: oliver006/redis_exporter:v1.55.0
      ports:
        - containerPort: 9121
          name: metrics
      env:
        - name: REDIS_ADDR
          value: "localhost:6379"
adapter-mysql-prometheus.yaml

MySQL metrics adapter exposes database performance metrics (queries/sec, connections, replication lag) in Prometheus format. Requires read-only database credentials.

 
apiVersion: v1
kind: Pod
metadata:
  name: mysql-with-metrics
  annotations:
    prometheus.io/scrape: "true"
    prometheus.io/port: "9104"
spec:
  containers:
    - name: mysql
      image: mysql:8.0
      env:
        - name: MYSQL_ROOT_PASSWORD
          valueFrom:
            secretKeyRef:
              name: mysql-secret
              key: password
      ports:
        - containerPort: 3306
    - name: mysqld-exporter
      image: prom/mysqld-exporter:v0.15.1
      ports:
        - containerPort: 9104
      env:
        - name: DATA_SOURCE_NAME
          value: "exporter:password@(localhost:3306)/"
adapter-nginx-prometheus.yaml

Nginx exports basic metrics via stub_status module. The nginx-prometheus-exporter adapter parses this output and converts it to Prometheus format with proper metric names and labels.

 
apiVersion: v1
kind: Pod
metadata:
  name: nginx-with-metrics
  annotations:
    prometheus.io/scrape: "true"
    prometheus.io/port: "9113"
spec:
  containers:
    - name: nginx
      image: nginx:alpine
      ports:
        - containerPort: 80
      volumeMounts:
        - name: nginx-conf
          mountPath: /etc/nginx/conf.d
    - name: nginx-exporter
      image: nginx/nginx-prometheus-exporter:1.0
      args:
        - -nginx.scrape-uri=http://localhost:8080/stub_status
      ports:
        - containerPort: 9113
  volumes:
    - name: nginx-conf
      configMap:
        name: nginx-status-config
nginx-status-config.yaml

Enable nginx stub_status on a separate port (8080) for the exporter to scrape. This endpoint is internal to the Pod and not exposed externally.

 
apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-status-config
data:
  status.conf: |
    server {
      listen 8080;
      location /stub_status {
        stub_status;
        allow 127.0.0.1;
        deny all;
      }
    }
adapter-log-format.yaml

A log format adapter transforms application-specific log formats into a standardized JSON format. The adapter reads logs from a shared volume and outputs normalized JSON to stdout.

 
apiVersion: v1
kind: Pod
metadata:
  name: app-with-log-adapter
spec:
  containers:
    - name: legacy-app
      image: legacy-app:v1
      volumeMounts:
        - name: logs
          mountPath: /var/log/app
    - name: log-adapter
      image: fluent/fluentd:v1.16
      volumeMounts:
        - name: logs
          mountPath: /var/log/app
          readOnly: true
        - name: fluentd-config
          mountPath: /fluentd/etc
  volumes:
    - name: logs
      emptyDir: {}
    - name: fluentd-config
      configMap:
        name: fluentd-log-adapter
adapter-health-check.yaml

A health check adapter provides a standardized HTTP health endpoint for apps that don’t have one. The adapter runs queries against the app and exposes the result on a standard endpoint.

 
apiVersion: v1
kind: Pod
metadata:
  name: db-with-health-adapter
spec:
  containers:
    - name: mysql
      image: mysql:8.0
      ports:
        - containerPort: 3306
    - name: health-adapter
      image: my-health-adapter:v1
      ports:
        - containerPort: 8080
      livenessProbe:
        httpGet:
          path: /healthz
          port: 8080
        initialDelaySeconds: 30
      env:
        - name: CHECK_QUERY
          value: "SELECT 1"
        - name: DB_HOST
          value: "localhost"
terminal

Verify the adapter is working by checking its metrics endpoint. Prometheus will scrape this endpoint automatically based on the annotations. The adapter translates Redis-specific data to standard metrics.

 
$ kubectl port-forward redis-with-metrics 9121:9121 &
Forwarding from 127.0.0.1:9121 -> 9121

$ curl localhost:9121/metrics | head -20
# HELP redis_up Information about the Redis instance
# TYPE redis_up gauge
redis_up 1
# HELP redis_connected_clients Number of client connections
# TYPE redis_connected_clients gauge
redis_connected_clients 2
# HELP redis_commands_processed_total Total commands processed
# TYPE redis_commands_processed_total counter
redis_commands_processed_total 1542

$ kubectl logs redis-with-metrics -c redis-exporter
INFO Listening on :9121
INFO Redis connection established

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