Envoy Step By Step

This is a simple example of how one can use Envoy to create scalable Flask apps.

The Tools

Tools we'll be using:

You'll need to get Minikube installed before starting. You do not need to install envoy though! It will magically Just Happen as we build containers out.

The Process

We're going to work with a very, very simple application: a simple Flask app that allows creating users, then reading them back.

Step 0: Minikube

First we need to start Minikube. On a Mac for your first startup, you need to decide if you're going to use the VirtualBox driver for Minikube, or the xhyve driver. I use xhyve:

minikube start --vm-driver xhyve

Once minikube is started, run

eval $(minikube docker-env)

to get hooked up to the Minikube Docker daemon (which we'll be using when we build Docker images later).

To prep everything for Minikube, run

bash prep.sh -

Should you want to clean everything up when done, use

bash clean.sh

Step 1: Basic Flask App

Start the Postgres and usersvc containers:

bash up.sh postgres
bash up.sh usersvc

and then you should be able to check things out:

curl $(minikube service --url usersvc)/user/health

should show you something like

{ 
  "hostname": "usersvc-1941676296-zlrt2",
  "msg": "user health check OK",
  "ok": true,
  "resolvedname": "172.17.0.10" 
}

Next up we can try saving and retrieving a user:

curl -X PUT \
     -H "Content-Type: application/json" \
     -d '{ "fullname": "Alice", "password": "alicerules" }' \
     $(minikube service --url usersvc)/user/alice

This should give us a user record for Alice, including her UUID but not her password:

{
  "fullname": "Alice",
  "hostname": "usersvc-1941676296-zlrt2",
  "ok": true,
  "resolvedname": "172.17.0.10",
  "uuid": "44FD5687B15B4AF78753E33E6A2B033B" 
}

and we should be able to read the user back (sans password again) with

curl $(minikube service --url usersvc)/user/alice

Step 2: Enter Envoy

Start the edge-envoy container:

bash up.sh edge-envoy

then drop the usersvc container and replace it with the usersvc2 container:

bash down.sh usersvc
bash up.sh usersvc2

and now going through Envoy should work:

curl $(minikube service --url edge-envoy)/user/health
curl $(minikube service --url edge-envoy)/user/alice

Step 3: Scaling the App

We can scale the app pretty simply:

kubectl scale --replicas=3 deployment/usersvc

but that will reveal that the DNS discovery we've been using so far won't work. We need to bring Envoy's Service Discovery Service into play:

bash up.sh usersvc-sds
bash down.sh edge-envoy
bash up.sh edge-envoy2

and once that's done, you'll be able to see requests cycling through all the usersvc endpoints.