Implementing Sticky Session Through Load Balancing

Concepts

Session persistence is one of the most common while complex problems in load balancing.

Session persistence is also called sticky sessions. After the sticky session function is enabled, requests from the same client are distributed to the same backend ECS by the load balancer for better continuity.

In load balancing and sticky session, connection and session are two key concepts. When only load balancing is concerned, session and connection refer to the same thing.

Simply put, if a user needs to log in, it can be regarded as a session; otherwise, a connection.

The sticky session mechanism fundamentally conflicts with the basic functions of load balancing. A load balancer forwards requests from clients to multiple backend servers to avoid overload on a single server. However, sticky session requires that some requests be forwarded to the same server for processing. Therefore, select a proper sticky session mechanism based on the application environment.

Layer-4 Load Balancing (Service)

In layer-4 load balancing, source IP address-based sticky session (Hash routing based on the client IP address) can be enabled. To enable source IP address-based sticky session on Services, the following conditions must be met:

Layer-7 Load Balancing (Ingress)

In layer-7 load balancing, sticky session based on HTTP cookies and app cookies can be enabled. To enable such sticky session, the following conditions must be met:

1. The application (workload) corresponding to the ingress is enabled with workload anti-affinity.
2. Node affinity is enabled for the Service corresponding to the ingress.

Procedure

1. Create an Nginx workload.
   Set the number of pods to 3 and set the podAntiAffinity.

   kind: Deployment
   apiVersion: apps/v1
   metadata:
     name: nginx
     namespace: default
   spec:
     replicas: 3
     selector:
       matchLabels:
         app: nginx
     template:
       metadata:
         labels:
           app: nginx
       spec:
         containers:
           - name: container-0
             image: 'nginx:perl'
             resources:
               limits:
                 cpu: 250m
                 memory: 512Mi
               requests:
                 cpu: 250m
                 memory: 512Mi
         imagePullSecrets:
           - name: default-secret
         affinity:
           podAntiAffinity:                   # Pod anti-affinity.
             requiredDuringSchedulingIgnoredDuringExecution:
               - labelSelector:
                   matchExpressions:
                     - key: app
                       operator: In
                       values:
                         - nginx
                 topologyKey: kubernetes.io/hostname

2. Creating a NodePort Service
   Configure the sticky session in a Service. An ingress can connect to multiple Services, and each Service can have different sticky sessions.

   apiVersion: v1
   kind: Service
   metadata:
     name: nginx
     namespace: default
     annotations:
       kubernetes.io/elb.lb-algorithm: ROUND_ROBIN
       kubernetes.io/elb.session-affinity-mode: HTTP_COOKIE      # HTTP cookie type.
       kubernetes.io/elb.session-affinity-option: '{"persistence_timeout":"1440"}'   # Session stickiness duration, in minutes. The value ranges from 1 to 1440.
   spec:
     selector:
       app: nginx
     ports:
       - name: cce-service-0
         protocol: TCP
         port: 80
         targetPort: 80
         nodePort: 32633            # Node port number.
     type: NodePort
     externalTrafficPolicy: Local   # Node-level forwarding.

   You can also select APP_COOKIE.

   apiVersion: v1
   kind: Service
   metadata:
     name: nginx
     namespace: default
     annotations:
       kubernetes.io/elb.lb-algorithm: ROUND_ROBIN
       kubernetes.io/elb.session-affinity-mode: APP_COOKIE     # Select APP_COOKIE.
       kubernetes.io/elb.session-affinity-option: '{"app_cookie_name":"test"}'  # Application cookie name.
   ...

3. Create an ingress and associate it with a Service.

   apiVersion: networking.k8s.io/v1
   kind: Ingress 
   metadata: 
     name: ingress-test
     namespace: default
     annotations: 
       kubernetes.io/elb.class: union
       kubernetes.io/elb.port: '80'
       kubernetes.io/elb.autocreate: 
         '{
             "type":"public",
             "bandwidth_name":"cce-bandwidth-test",
             "bandwidth_chargemode":"traffic",
             "bandwidth_size":1,
             "bandwidth_sharetype":"PER",
             "eip_type":"5_bgp"
           }'
   spec:
     rules: 
     - host: 'www.example.com'
       http: 
         paths: 
         - path: '/'
           backend: 
             service:
               name: nginx     #Service name
               port: 
                 number: 80
           property:
             ingress.beta.kubernetes.io/url-match-mode: STARTS_WITH
           pathType: ImplementationSpecific
     ingressClassName: cce

4. Log in to the ELB console, access the load balancer details page, and check whether the sticky session feature is enabled.