Deployment
• Use Case: Best suited for stateless applications where the identity of individual pods doesn't matter.
• Features:
- Ensures the specified number of replicas (pods) are running at any given time.
- Handles rolling updates and rollbacks.
- Pods are identical and can be created and destroyed in any order.
• Example: A web server application like nginx or a frontend service where you want multiple instances for load balancing.
YAML Configuration
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx:1.14.2
StatefulSet
• Use Case: Used for stateful applications where each pod requires a stable identity or persistent storage.
• Features:
- Pods are created in order and maintain a unique, stable identity (like pod-0, pod-1).
- Useful for databases and applications that require persistent storage, where the identity and storage must remain consistent.
- Supports the retention of network identities and the ordering of deployment.
• Example: A MySQL database where each instance (pod) requires a unique identity and must maintain its own data.
YAML Configuration
apiVersion: apps/v1
kind: StatefulSet
metadata:
name: mysql-statefulset
spec:
serviceName: "mysql"
replicas: 3
selector:
matchLabels:
app: mysql
template:
metadata:
labels:
app: mysql
spec:
containers:
- name: mysql
image: mysql:5.7
volumeMounts:
- name: mysql-persistent-storage
mountPath: /var/lib/mysql
volumeClaimTemplates:
- metadata:
name: mysql-persistent-storage
spec:
accessModes: ["ReadWriteOnce"]
resources:
requests:
storage: 1Gi
DaemonSet
• Use Case: Ensures that a copy of a pod runs on all (or some) nodes in a cluster. Ideal for system-level applications.
• Features:
- Ensures that one pod is created on every node (or specific nodes) in the cluster.
- Used for tasks like logging, monitoring, or other node-wide services.
• Example: A fluentd logging agent where each node requires its own logging instance, or Prometheus node exporter for monitoring.
YAML Configuration
apiVersion: apps/v1
kind: DaemonSet
metadata:
name: fluentd-daemonset
spec:
selector:
matchLabels:
name: fluentd
template:
metadata:
labels:
name: fluentd
spec:
containers:
- name: fluentd
image: fluent/fluentd
Key Differences
| Feature | Deployment | StatefulSet | DaemonSet |
|---|---|---|---|
| Purpose | For stateless applications | For stateful applications | To run on every node |
| Pod Identity | No stable identity | Stable, unique identity | No identity, runs on every node |
| Pod Ordering | No specific order | Pods are created in sequence | One pod per node |
| Scaling | Horizontal scaling, flexible | Scales based on unique pods | No scaling, fixed per node |
| Examples | Web servers, microservices | Databases, Zookeeper | Monitoring, logging agents |
When to Use
- Deployment: When your application is stateless, like an Nginx web server.
- StatefulSet: When your application needs persistent storage and stable identity, like a database.
- DaemonSet: When you want a service like monitoring or logging to run on all nodes in your cluster.
