In Kubernetes, Prometheus configuration changes traditionally require a pod restart to take effect. However, using a configuration reload method, you can avoid downtime and ensure smooth configuration updates.

The Problem

When running Prometheus in Kubernetes, configuration changes to the Prometheus ConfigMap typically require a pod restart to load the new configuration. While configuration changes might not happen often, the downtime from restarting the pod can be undesirable.

The Solution

Starting with Prometheus 2.0, you can enable HTTP reloads using the --web.enable-lifecycle flag and integrate the configmap-reload container to trigger a reload when the ConfigMap is updated.

The configmap-reload is a simple binary that watches for changes in the Kubernetes ConfigMap and triggers a reload in Prometheus when needed. Here’s how you can set it up.

Step-by-Step Implementation

1. Update Your Prometheus Deployment

First, update your Prometheus Deployment configuration to enable HTTP reloads by adding the --web.enable-lifecycle flag. You also need to add the prometheus-server-configmap-reload container to handle the reloads.

Here is an example Kubernetes Deployment configuration with the necessary changes. Most of the non-relevant parts of the deployment have been omitted for simplicity:

---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: prometheus-server
  namespace: monitoring
spec:
  template:
    spec:
      volumes:
        - name: config-volume
          configMap:
            name: prometheus-server
            defaultMode: 420
      containers:
        - name: prometheus-server-configmap-reload
          image: jimmidyson/configmap-reload:v0.5.0
          imagePullPolicy: IfNotPresent
          args:
            - '--volume-dir=/etc/config'
            - '--webhook-url=http://127.0.0.1:9090/-/reload'
          volumeMounts:
            - name: config-volume
              readOnly: true
              mountPath: /etc/config
        - name: prometheus-server
          image: quay.io/prometheus/prometheus:v2.37.0
          imagePullPolicy: IfNotPresent
          args:
            - '--config.file=/etc/config/prometheus.yml'
            - '--enable-feature=expand-external-labels'
            - '--storage.tsdb.path=/data'
            - '--storage.tsdb.retention.time=14d'
            - '--web.enable-lifecycle'

2. Deploy the Updated Configuration

After making these changes, deploy the updated configuration to your Kubernetes cluster. The prometheus-server-configmap-reload container will monitor changes in the Prometheus ConfigMap and automatically reload the configuration without restarting the pod.

Final Thoughts

By adding the --web.enable-lifecycle flag and the configmap-reload container, you can make your Prometheus configuration dynamic and reload changes seamlessly, avoiding downtime and improving overall efficiency.

Reload Prometheus ConfigMap without Restarting the Kubernetes Pod

Utilizing the HTTP reload endpoint and the configmap-reload container, you can make Prometheus reload its configuration without restarting the pod.