Traffic Shifting

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Traffic splitting and shifting are powerful features that enable operators to dynamically shift traffic to different backend Services. This can be used to implement A/B experiments, red/green deploys, canary rollouts, fault injection and more.

Linkerd supports two different ways to configure traffic shifting: you can use the Linkerd SMI extension and TrafficSplit resources, or you can use HTTPRoute resources which Linkerd natively supports. While certain integrations such as Flagger rely on the SMI and TrafficSplit approach, using HTTPRoute is the preferred method going forward.

Linkerd Production Tip

This page contains best-effort instructions by the open source community. Production users with mission-critical applications should familiarize themselves with Linkerd production resources and/or connect with a commercial Linkerd provider.

Prerequisites

To use this guide, you’ll need a Kubernetes cluster running:

Linkerd and Linkerd-Viz. If you haven’t installed these yet, follow the Installing Linkerd Guide.

Set up the demo

We will set up a minimal demo which involves a load generator and two backends called v1 and v2 respectively. You could imagine that these represent two different versions of a service and that we would like to test v2 on a small sample of traffic before rolling it out completely.

For load generation we’ll use Slow-Cooker and for the backends we’ll use BB.

To add these components to your cluster and include them in the Linkerd data plane, run:

cat <<EOF | linkerd inject - | kubectl apply -f -
---
apiVersion: v1
kind: Namespace
metadata:
  name: traffic-shift-demo
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: v1
  namespace: traffic-shift-demo
spec:
  replicas: 1
  selector:
    matchLabels:
      app: bb
      version: v1
  template:
    metadata:
      labels:
        app: bb
        version: v1
    spec:
      containers:
      - name: terminus
        image: buoyantio/bb:v0.0.6
        args:
        - terminus
        - "--h1-server-port=8080"
        - "--response-text=v1"
        ports:
        - containerPort: 8080
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: v2
  namespace: traffic-shift-demo
spec:
  replicas: 1
  selector:
    matchLabels:
      app: bb
      version: v2
  template:
    metadata:
      labels:
        app: bb
        version: v2
    spec:
      containers:
      - name: terminus
        image: buoyantio/bb:v0.0.6
        args:
        - terminus
        - "--h1-server-port=8080"
        - "--response-text=v2"
        ports:
        - containerPort: 8080
---
apiVersion: v1
kind: Service
metadata:
  name: bb
  namespace: traffic-shift-demo
spec:
  ports:
  - name: http
    port: 8080
    targetPort: 8080
  selector:
    app: bb
    version: v1
---
apiVersion: v1
kind: Service
metadata:
  name: bb-v2
  namespace: traffic-shift-demo
spec:
  ports:
  - name: http
    port: 8080
    targetPort: 8080
  selector:
    app: bb
    version: v2
---
apiVersion: apps/v1
kind: Deployment
metadata:
  name: slow-cooker
  namespace: traffic-shift-demo
spec:
  replicas: 1
  selector:
    matchLabels:
      app: slow-cooker
  template:
    metadata:
      labels:
        app: slow-cooker
    spec:
      containers:
      - args:
        - -c
        - |
          sleep 5 # wait for pods to start
          /slow_cooker/slow_cooker --qps 10 http://bb:8080
        command:
        - /bin/sh
        image: buoyantio/slow_cooker:1.3.0
        name: slow-cooker
EOF

We can see that slow-cooker is sending traffic to the v1 backend:

> linkerd viz -n traffic-shift-demo stat --from deploy/slow-cooker deploy
NAME   MESHED   SUCCESS       RPS   LATENCY_P50   LATENCY_P95   LATENCY_P99   TCP_CONN
v1        1/1   100.00%   10.1rps           1ms           1ms           8ms          1

Shifting Traffic

Now let’s create an HTTPRoute and split 10% of traffic to the v2 backend:

cat <<EOF | kubectl apply -f -
---
apiVersion: policy.linkerd.io/v1beta2
kind: HTTPRoute
metadata:
  name: bb-route
  namespace: traffic-shift-demo
spec:
  parentRefs:
    - name: bb
      kind: Service
      group: core
      port: 8080
  rules:
    - backendRefs:
      - name: bb
        port: 8080
        weight: 90
      - name: bb-v2
        port: 8080
        weight: 10
EOF

Notice in this HTTPRoute, the parentRef is the bb Service resource that slow-cooker is talking to. This means that whenever a meshed client talks to the bb Service, it will use this HTTPRoute. You may also notice that the bb Service appears again in the list of backendRefs with a weight of 90. This means that 90% of traffic sent to the bb Service will continue on to the endpoints of that Service. The other 10% of requests will get routed to the bb-v2 Service.

We can see this by looking at the traffic stats (keep in mind that the stat command looks at metrics over a 1 minute window, so it may take up to 1 minute before the stats look like this):

> linkerd viz -n traffic-shift-demo stat --from deploy/slow-cooker deploy
NAME   MESHED   SUCCESS      RPS   LATENCY_P50   LATENCY_P95   LATENCY_P99   TCP_CONN
v1        1/1   100.00%   9.0rps           1ms           1ms           1ms          1
v2        1/1   100.00%   1.0rps           1ms           1ms           1ms          1

From here, we can continue to tweak the weights in the HTTPRoute to gradually shift traffic over to the bb-v2 Service or shift things back if it’s looking dicey. To conclude this demo, let’s shift 100% of traffic over to bb-v2:

cat <<EOF | kubectl apply -f -
---
apiVersion: policy.linkerd.io/v1beta2
kind: HTTPRoute
metadata:
  name: bb-route
  namespace: traffic-shift-demo
spec:
  parentRefs:
    - name: bb
      kind: Service
      group: core
      port: 8080
  rules:
    - backendRefs:
      - name: bb-v2
        port: 8080
        weight: 100
EOF

> linkerd viz -n traffic-shift-demo stat --from deploy/slow-cooker deploy
NAME   MESHED   SUCCESS       RPS   LATENCY_P50   LATENCY_P95   LATENCY_P99   TCP_CONN
v1        1/1         -         -             -             -             -          -
v2        1/1   100.00%   10.0rps           1ms           1ms           2ms          1