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Sneak Peek: Linkerd 2.14

This is the documentation - and executable code! - for the Service Mesh Academy workshop about what's coming in Linkerd 2.14. The easiest way to use this file is to execute it with demosh.

Things in Markdown comments are safe to ignore when reading this later. When executing this with demosh, things after the horizontal rule below (which is just before a commented @SHOW directive) will get displayed.

BAT_STYLE="grid,numbers"

Sneak Peek: Linkerd 2.14

Welcome to the Service Mesh Academy Sneak Peek for Linkerd 2.14! We're going to show off a multicluster version of the Faces demo using k3d.

NOTE: k3d is great because it's easy for anyone to lay hands on. However, setting up k3d for a flat-network multicluster demo is extremely tricky, enough so that we're not going to do it during the demo. Instead, all the gory details are in create-clusters.sh and setup-demo.sh, and this demo assumes that you've run both of those.

So! To set everything up, first run

`bash create-clusters.sh`

to create the clusters with all the necessary network setup, and then

`bash setup-demo.sh`

to install Linkerd, the Linkerd multicluster extension, Emissary, and the multicluster Faces demo, and link the clusters together in the mesh. You are strongly encouraged to read these scripts, but recognize that some of create-clusters.sh is serious k3d wizardry. 🙂

That's all done? Good. At this point, our Faces demo should be happily running. Let's take a look in the browser.

This looks like a pretty typical Faces demo, but under the hood it's quite different: we have three clusters running, all on a flat network so that pods in each cluster can talk to the pods in the others, with Linkerd installed and all three clusters linked. Let's take a look at what that looks like.

The clusters

Our three clusters are called north, east, and west just to make them easier to tell apart than using numbers or some such:

  • The north cluster has the Faces GUI and the face workload, and is cluster that users will first talk to (so it's where the north-south communication) happens. An instance of the color workload runs here, too.
  • The east cluster is running only a smiley workload.
  • The west cluster runs another color workload.

Our KUBECONFIG has a context for each cluster, with the same name as the cluster, so that we can easily talk to any of them from the command line:

kubectl --context north cluster-info
kubectl --context east cluster-info
kubectl --context west cluster-info

Multicluster Links

We've linked our clusters together so that pods in the north cluster can communicate with pods in the east and west clusters. We actually did the linking in setup-demo.sh, with the linkerd multicluster link command, for example:

linkerd --context=east multicluster link --gateway=false --cluster-name east \
    | kubectl --context=north apply -f -

Pay careful attention to the contexts here.

  • We run linkerd multicluster link in the east context; that tells east's Linkerd installation to generate a Link resource that will tell some other cluster's Linkerd how to talk to it.

  • We also supply --gateway=false to tell Linkerd to use Pod-to-Pod networking.

  • We apply that Link resource in the north context, which tells north's Linkerd how to talk to east's Linkerd.

If we wanted to, we could do basically the same thing to set up a link from east back to north, but we don't need it here so we didn't set it up.

If we want, we can get a lower-level look at the links by examing the Link resources:

kubectl --context north get link -n linkerd-multicluster

We see one for each linked cluster, and if we look at the resource itself we'll see a bunch of information about the linked cluster:

kubectl --context north get link -n linkerd-multicluster east -o yaml | bat -l yaml

We can use the same trick to verify that there aren't any Links back from the other clusters:

kubectl --context east get link -n linkerd-multicluster
kubectl --context west get link -n linkerd-multicluster

As we said before, we made links from north to east and west, but not the other direction. We could have - it would work fine - but we don't need them for this demo.

No Gateways

As a quick aside for those familiar with Linkerd 2.13 multicluster operations: when we use Linkerd 2.14's Pod-to-Pod networking, the multicluster gateways you'd see in Linkerd 2.14 don't exist:

linkerd --context north multicluster gateways

The way this works is that the Linkerd control plane in the north cluster talks directly to the east and west control planes to do service discovery, and the gateways aren't necessary to rely traffic any more, so they don't appear.

The north cluster

The north cluster is the one users connect to. It has a lot of stuff running in it. First, let's take a look at what's running in the faces namespace.

kubectl --context north get pods -n faces

We see the Faces GUI, the face workload, and the color workload... but... uh... no smiley workload? So... where are the smileys coming from?

What do the services here look like?

kubectl --context north get svc -n faces

We see faces-gui for the GUI, face and color, and... smiley and smiley-east?

Mirrored services

What's going on here is that smiley-east is a mirror of the smiley Service from the linked east cluster. If we look at the faces namespace in the east cluster, we can see a little bit more of the picture:

kubectl --context east get svc,pods -n faces

That's Faces' typical smiley and smiley2 setup, so the smiley workloads must actually be running here. The magic is actually in the labels on the smiley Service:

kubectl --context east get svc smiley -n faces -o yaml | bat -l yaml

The mirror.linkerd.io/exported: remote-discovery label tells the Linkerd control plane in other clusters linked to this one to mirror the Service for direct pod-to-pod communication. (If you're familiar with the way Linkerd did multicluster in 2.13, you might remember this label with a value of enabled. That still works if you configure multicluster with gateways, but it's not what we want here.)

When remote discovery is enabled, the Linkerd control plane mirrors the Service across the Link. In this case, the north cluster has a Link connecting it to the east cluster, so when the Linkerd control plane sees the mirror.linkerd.io/exported label, it will pull a copy of the smiley Service from east into north.

The mirrored service currently gets a name of $ServiceName-$ClusterName, so smiley-east is the mirrored smiley Service from the east cluster.

It's also instructive to check out the endpoints of both our smiley Service (in the east cluster) and our smiley-east Service (in the north cluster):

kubectl --context east get endpoints smiley -n faces

Two endpoints, since we have two replicas. This makes sense.

kubectl --context north get endpoints smiley-east -n faces

No endpoints. On the face of it, this seems wrong - how can anything work with no endpoints - but what's going on here is that the Linkerd control plane is tracking endpoints for us. The linkerd diagnostics endpoints command will show them to us (note that we have to give it the fully-qualified name of the Service):

linkerd --context north diagnostics endpoints smiley-east.faces.svc.cluster.local:80

Two endpoints, like we'd expect because there are two replicas. Also note that the IP addresses are the same, which is a great sign since this is supposed to be a flat network.

Bringing in the Gateway API

OK, so we have a mirrored smiley-east Service, the Linkerd control plane is managing its endpoints, all is well. However, the Faces app doesn't talk to smiley-east: it just talks to smiley. So how is that working?

Answer: we're using an HTTPRoute under the hood.

First, the astute reader will have noticed that the north cluster has a smiley Service in addition to smiley-east. Let's take a quick, but careful, look at that.

kubectl --context north get svc -n faces smiley -o yaml | bat -l yaml

If you look closely, you'll realize that something is missing: this Service has no selector, so it can match no Pods, so it will never have any endpoints.

The smiley-router HTTPRoute

In fact, the only purpose of the smiley Service is to have this HTTPRoute attached to it:

#@immed
yq 'select(document_index==5)' k8s/01-face/faces-mc.yaml | bat -l yaml

This HTTPRoute says "take all traffic sent to the smiley service and divert it to smiley-east instead, with a five-second timeout". That's what's permitting the Faces demo to work.

Some inconvenient truths

Note that we use a policy.linkerd.io/v1beta3 HTTPRoute, not a gateway.networking.k8s.io HTTPRoute. That's because we're using a timeout.

We're using Gateway API v0.8.0-rc1 at the moment -- and HTTPRoute timeouts are defined by GEP-1742, which didn't quite make the cut for v0.8.0. They'll be in Gateway API v1.0.0, but to use HTTPRoute timeouts with Linkerd, you need to stick with policy.linkerd.io/v1beta3.

Also, you can't use an HTTPRoute in the north cluster to direct traffic to a mirrored Service from east, then have an HTTPRoute in the east cluster further split the traffic. That's because the north cluster finds the endpoints and goes directly to one of them, rather than allowing anything in the east cluster to affect the chosen endpoint. (This is also true with HTTPRoutes in the same cluster: you can't stack them.)

HTTPRoute timeouts

Let's see if we can put timeouts to work. Setting a 300ms timeout has worked out well in the past to improve the Faces user experience, so let's edit the HTTPRoute and change its timeout to 300ms.

kubectl --context north edit httproute smiley-router -n faces

If we flip to the browser, sure enough, we'll see fewer faded cells and more counts incrementing in the corners.

(The reason why the fading cells don't all disappear is left as an exercise for the reader. 😇)

Splitting across clusters

We aren't limited, of course, to just redirecting all our traffic to another cluster. We can also use an HTTPRoute to split traffic across multiple clusters. Let's take 50% of our color traffic and send it across to the west cluster's color2 Service as a demo.

We need to start by mirroring the color2 Service into our north cluster (which is already linked to the west cluster). All we need to make that happen is to put the correct label on the Service in the west cluster:

kubectl --context west -n faces label svc/color2 mirror.linkerd.io/exported=remote-discovery

As soon as we do that, we'll see the mirrored Service color2-west appear in the north cluster:

kubectl --context north -n faces get svc

And, if we check out the endpoints, we should be good to go:

kubectl --context west get endpoints color2 -n faces
linkerd --context north diagnostics endpoints color2-west.faces.svc.cluster.local:80

So Linkerd knows the right endpoints for the color2-west Service.

At that point, we can add an HTTPRoute to split traffic.

#@immed
bat k8s/02-canary/color-canary.yaml

Let's apply that and see how it goes:

kubectl --context north apply -f k8s/02-canary/color-canary.yaml

Back to the browser to see what we see...

Now that our split is working, we can do something kind of cool. We can migrate color completely off the north cluster just by editing the HTTPRoute. We'll use kubectl edit to delete the backendRef for the local color Service.

kubectl --context north edit httproute color-canary -n faces

At this point, we'll see all blue cells in the browser.

Just to round things out, let's delete the color Deployment in the face cluster...

kubectl --context north delete -n faces deployment color

...and prove that it's gone.

kubectl --context north get pods -n faces

This is an amazing thing about having multicluster running smoothly: you can migrate between clusters simply by deleting things. 🙂

Sneak Peek: Linkerd 2.14

So there's a quick sampling of Linkerd 2.14 pod-to-pod multicluster, with a touch of timeouts thrown in. You can find the source for this demo at

https://github.com/BuoyantIO/service-mesh-academy

in the sneak-peek-2-14 directory -- the black magic of setup, again, is in create-clusters.sh and setup-demo.sh.

As always, we welcome feedback! Join us at https://slack.linkerd.io/ for more.