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Cats and Dogs with Service Discovery

This example demonstrates communication (HTTP and UDP) between a frontend and multiple backend applications using service discovery over the container network.

To see a visual representation of the steps taken in this demo, see here.

We're assuming that you've deployed to BOSH lite. If you've deployed to AWS or another environment, substitute bosh-lite.com below with the domain name of your installation.

Prerequisites

You have service discovery enabled in your deployment.

You have downloaded and installed the CF CLI in order to configure policies and create internal routes.

Preparing to push your apps

To prepare to push your apps, you will want to target your CF org and space.

cf api api.bosh-lite.com --skip-ssl-validation
cf auth admin admin
cf create-org cats-and-dogs
cf target -o cats-and-dogs
cf create-space cats
cf target -o cats-and-dogs -s cats

Then clone this cats-and-dogs-example-apps repo into a directory. Set this directory as the $DIR env variable.

Frontend

The frontend serves a form at http://frontend.bosh-lite.com/.

The frontend allows you to test out container network communication via two methods:

  • connect to the backend via HTTP
  • connect to the backend via UDP

In either case, the response from the backend to the frontend will be rendered as a web page.

Deploying

cd $DIR/frontend
cf push frontend

Use Case 1: Frontend Connects to Single Backend

Backend

The backend will be pushed with no external route and therefore should not be accessible via the public internet.

Backend-A serves a picture of a typing cat on the TCP ports specified in the environment variable CATS_PORTS, and responds to simple text messages on the UDP ports specified in the environment variable UDP_PORTS.

Backend-B serves a picture of a party cat on the TCP ports specified in the environment variable CATS_PORTS, and responds to simple text messages on the UDP ports specified in the environment variable UDP_PORTS.

We will give both backends internal hostnames that will map to the app's container ips and can be used to connect via container-to-container networking. An internal hostname is configured via the CF CLI map-route command, with the domain provided set to the reserved internal domain of apps.internal.

Deploying (Diagram 1)

Backend-A (example with internal route defined in app manifest)

cd $DIR/backend-a
cf push backend-a --no-start
cf set-env backend-a CATS_PORTS "7007,7008"
cf set-env backend-a UDP_PORTS "9003,9004"
cf start backend-a

Backend-B (example with internal route defined using CLI)

cd $DIR/backend-b
cf push backend-b --no-start -d apps.internal
cf set-env backend-b CATS_PORTS "7007,7008"
cf set-env backend-b UDP_PORTS "9003,9004"
cf start backend-b

Usage

After both frontend and backend apps have been deployed, you can visit http://frontend.bosh-lite.com/ in a browser. You should see something like:

Frontend Sample App

HTTP Test
Backend HTTP URL: [....] [ Submit ]

UDP Test
Backend UDP Server Address: [....]
Message: [....] [ Submit ]

Usage with HTTP (Diagrams 3-6)

In the frontend app, in the Backend HTTP URL field enter backend-a's internal hostname and a cats port (backend-a.apps.internal:7007). Hit submit.

You will see an error message saying the connection is refused. This is because the two apps have not been configured to allow connections from the frontend to the backend.

If you see an error message saying no such host, service discovery is incorrectly configured.

Now allow access:

cf add-network-policy frontend --destination-app backend-a --port 7007 --protocol tcp

Now if you try again from the frontend:

[GIF OF CAT]
Hello from the backend, port: 7007

Doing the same thing with backend-b should result in a different cat (specifically, a party cat) being shown.

Usage with UDP

In the frontend app, in the Backend UDP Server Address field, enter the backend's internal hostname and UDP port (backend-a.apps.internal:9003) and a message. Hit submit.

You will see an error message. This is because the two apps have not been configured to allow connections from the frontend to the backend.

Now allow access:

cf add-network-policy frontend --destination-app backend-a --port 9003 --protocol udp

Now if you try again from the frontend:

You sent the message: hello world

Backend UDP server replied: HELLO WORLD

Use Case 2: Frontend Connects to Multiple Backends

Backend

We will use the two apps pushed in Use Case 1 and also create a third internal route that maps to both apps. When queried, the route should return both apps as possible destinations.

Creating Route (Diagram 2)

Creating an internal route shared across both backends

cf create-route cats apps.internal --hostname backend
cf map-route backend-a apps.internal --hostname backend
cf map-route backend-b apps.internal --hostname backend

Usage

After both frontend and backend apps have been deployed, you can visit http://frontend.bosh-lite.com/ in a browser. You should see something like:

Frontend Sample App

HTTP Test
Backend HTTP URL: [....] [ Submit ]

UDP Test
Backend UDP Server Address: [....]
Message: [....] [ Submit ]

Usage with HTTP (Diagram 7)

When policy is configured for the frontend to reach both backend-a and backend-b on the same port, entering the shared internal hostname and port (backend.apps.internal:7007) in Backend HTTP URL field will show

[GIF OF CAT]
Hello from the backend, port: 7007

Trying this multiple times should result in seeing both cat gifs returned (as both apps are routed via that hostname).