hellodocker

README

hellodocker

This is a 4 container project. I'm using docker-compose to bring it all up (AKA orchestration).

• 1 data-only container. This is where Postgres' data will live.
• 1 Postgres container. Because Postgres is awesome!
• 1 Go container. This container will house the Go app.
• 1 Nginx container. This will be used as a reverse proxy between Go and the wild wild web.

Setup Docker

If you're on OS X, then you'll need to use Docker Machine to setup Docker first. Because Docker uses some Linux-specific kernel features, you'll need VirtualBox in order to run Docker.

docker-machine create --driver virtualbox default
eval $(docker-machine env default)

Download, Build, and Run

git clone git@github.com:variadico/hellodocker.git
cd hellodocker
docker-compose build
docker-compose up -d

Then fire up Chrome and point it to the Docker host IP. You can get this IP by running docker-machine ip default.

If you see Cannot start container 8675309: Cannot link to a non running container then just run docker-compose up -d again. I'm not sure how to prevent that.

Files

This is what each file is doing.

server.go

This run a server that listens on port :8080 inside its container. When you first start it up, it will attempt to connect to Postgres and create table if not exists. When you visit /, it calls the serveIndex handler and inserts data into the database. Finally, it queries all of the rows in the database and sends the row ID back to the web client. This can prove that the database is persisting across runs.

Dockerfile

This file will create a new container image by copying the Go source in your current directory, building it in the container, and running it in the container. If you want to see how this is happening check out the golang:onbuild Dockerfile here.

docker-compose.yml

This file describes the 4 containers that our app will use. You should read the source of this file. Don't worry, there are comments. To execute this file, you'll have to say docker-compose build to create the images. Then, docker-compose up will start the containers. You can pass the -d so that the containers run in the background, docker-compose up -d. Finally, to stop the running containers, you can use docker-compose stop.

nginx.conf

This is a basic configuration file that I grabbed from the stock nginx Docker image. I just added the server block so that web requests get forwarded to the Go container. The interesting bit is that I forward requests to http://app:8080, where app is an entry in /etc/hosts that points to the Go container's IP address and :8080 is the port I EXPOSE in my Dockerfile.

More About Linking

When you link two containers together, you get some information about where the other container is located.

go:
    links:
        - postgres:db

This is from docker-compose.yml with the comments removed and describes the Go container. It says send location information (ha that rhymes) about the Postgres container to the Go container.

[ Go ] <--loc-info-- [ Postgres ]

The Go container will be able to access this information in the form of environment variables. So you can use os.Getenv to access that information in your code.

connInfo := fmt.Sprintf(
	"user=%s dbname=%s password=%s host=%s port=%s sslmode=disable",
	"postgres",
	"postgres",
	os.Getenv("DB_ENV_POSTGRES_PASSWORD"),
	os.Getenv("HELLODOCKER_POSTGRES_1_PORT_5432_TCP_ADDR"),
	os.Getenv("HELLODOCKER_POSTGRES_1_PORT_5432_TCP_PORT"),
)

You can make sure this information is available if you run the env command in a running container.

docker exec hellodocker_go_1 env

Additionally, Docker will add entries in /etc/hosts that have the IP address of the linked container. You can verify this using this command.

docker exec hellodocker_go_1 cat /etc/hosts