Intro

If you’ve never heard about Docker, but that’s unlikely, the first thing you should know is that Docker allows you to run applications in isolation and with a great separation of concerns, yet allows them to communicate and interact with the external world.

And you should know that everyone uses it, and every major cloud provider has a solution dedicated to running containers, so you should learn it!

Install

Installation changes depending on your system, so check https://www.docker.com/get-docker.

I assume you already installed Docker and have the docker command available in your shell.

The Go official images

Docker maintains a list of official images for many different languages, and Go is no exception, being it part of the original official images launch back in 2014.

The official image repository can be found at https://hub.docker.com/_/golang/. There are many tags that identify both the Go version, and the operating system you want to fetch.

An example app

As an example, I’m going to deploy a little Go app in a Docker container. It listens on port 8000, gets a webpage as a q query parameter, fetches it and prints the links it finds:

findlinks.go

package main import ( "fmt" "log" "net/http" "golang.org/x/net/html" ) func main () { http . HandleFunc ( "/" , handler ) log . Fatal ( http . ListenAndServe ( "0.0.0.0:8000" , nil )) } func handler ( w http . ResponseWriter , r * http . Request ) { url := r . URL . Query (). Get ( "q" ) fmt . Fprintf ( w , "Page = %q

" , url ) if len( url ) == 0 { return } page , err := parse ( "https://" + url ) if err != nil { fmt . Printf ( "Error getting page %s %s

" , url , err ) return } links := pageLinks ( nil , page ) for _ , link := range links { fmt . Fprintf ( w , "Link = %q

" , link ) } } func parse ( url string ) ( * html . Node , error ) { fmt . Println ( url ) r , err := http . Get ( url ) if err != nil { return nil , fmt . Errorf ( "Cannot get page" ) } b , err := html . Parse ( r . Body ) if err != nil { return nil , fmt . Errorf ( "Cannot parse page" ) } return b , err } func pageLinks ( links [] string , n * html . Node ) [] string { if n . Type == html . ElementNode && n . Data == "a" { for _ , a := range n . Attr { if a . Key == "href" { links = append( links , a . Val ) } } } for c := n . FirstChild ; c != nil ; c = c . NextSibling { links = pageLinks ( links , c ) } return links }

Example usage:

Moving the app to Docker

I put the app on https://github.com/flaviocopes/findlinks.

Using go get I can easily download and install it, using go get github.com/flaviocopes/findlinks .

Running

docker run golang go get -v github.com/flaviocopes/findlinks

will first download the golang Docker image, if you don’t have it already, then it will fetch the repository and will scan for additional dependencies not included in the standard library. In this case, golang.org/x/net/html .

$ docker run golang go get -v github.com/flaviocopes/findlinks github.com/flaviocopes/findlinks ( download ) Fetching https://golang.org/x/net/html?go-get = 1 Parsing meta tags from https://golang.org/x/net/html?go-get = 1 ( status code 200 ) get "golang.org/x/net/html" : found meta tag main.metaImport { Prefix: "golang.org/x/net" , VCS: "git" , RepoRoot: "https://go.googlesource.com/net" } at https://golang.org/x/net/html?go-get = 1 get "golang.org/x/net/html" : verifying non-authoritative meta tag Fetching https://golang.org/x/net?go-get = 1 Parsing meta tags from https://golang.org/x/net?go-get = 1 ( status code 200 ) golang.org/x/net ( download ) golang.org/x/net/html/atom golang.org/x/net/html github.com/flaviocopes/findlinks

This command creates a container, and runs it. We can inspect it using docker ps -l (the -l option tells Docker to list the latest container ran):

CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES 343d96441f16 golang "go get -v github...." 3 minutes ago Exited ( 0 ) 2 minutes ago mystifying_swanson

The container exited when the go get command completed.

Docker just built an on demand image and ran it; to run it again, we would need to repeat the process, but images help us: let’s now create an image from this container, so we can run it later:

docker commit $( docker ps -lq ) findlinks

The above command gets the last container ID using docker ps -lq , and commits the image. The image has now been installed, as you can check using docker images findlinks :

$ docker images findlinks REPOSITORY TAG IMAGE ID CREATED SIZE findlinks latest 4e7ebb87d02e 11 seconds ago 720MB

We can run the findlinks command on our findlinks image with

docker run -p 8000:8000 findlinks findlinks

That’s it! Our app will now respond on http://192.168.99.100:8000/ , where 192.168.99.100 is the IP address of the Docker container.

You can test calling http://192.168.99.100:8000/?q=flaviocopes.com , which will print the same output we had when we run the app locally:

Page = "flaviocopes.com" Link = "https://flaviocopes.com/index.xml" Link = "https://twitter.com/flaviocopes" Link = "https://github.com/flaviocopes" Link = "https://stackoverflow.com/users/205039/flaviocopes" Link = "https://linkedin.com/in/flaviocopes/" Link = "mailto:copesc@gmail.com" Link = "/" Link = "/page/contact/" Link = "/page/about/" Link = "https://flaviocopes.com/golang-tutorial-rest-api/" Link = "https://flaviocopes.com/golang-environment-variables/" Link = "https://flaviocopes.com/golang-sql-database/" Link = "https://flaviocopes.com/golang-is-go-object-oriented/" Link = "https://flaviocopes.com/golang-comparing-values/" Link = "https://flaviocopes.com/golang-data-structures/" Link = "https://flaviocopes.com/golang-data-structure-binary-search-tree/" Link = "https://flaviocopes.com/golang-data-structure-graph/" Link = "https://flaviocopes.com/golang-data-structure-linked-list/" Link = "https://flaviocopes.com/golang-data-structure-queue/" Link = "https://flaviocopes.com/golang-data-structure-stack/" Link = "https://flaviocopes.com/golang-event-listeners/"

Trim the Docker image

This strategy is now deprecated in favor of multi-stage builds

The problem with the above result is, the image is huge: 720MB for this simple program is not really acceptable, keep in mind this is a very simple scenario. We might want to deploy thousands of instances of the application, and this size is not going to work.

Why is the image this big? Because what happens is that the Go app is compiled inside the container. So the image needs to have a Go compiler installed. And everything needed by the compiler of course, GCC, and a whole Linux distribution (Debian Jessie). It downloads Go and installs it, compiles the app and runs it.

It’s all so fast we don’t even realize. But we can do better. How? I apply what I learned on Building Minimal Docker Containers for Go Applications by Nick Gauthier

We tell Docker to run the golang:1.8.3 image and statically compile our application, disabling CGO, which means the image doesn’t even need the C libraries it normally needs to access when dynamically linked, using:

docker run --rm -it -v " $GOPATH " :/gopath -v " $( pwd ) " :/app -e "GOPATH=/gopath" -w /app golang:1.8.3 sh -c 'CGO_ENABLED=0 go build -a --installsuffix cgo --ldflags="-s" -o findlinks'

We now have a findlinks binary in the folder:

$ ll .rw-r--r-- 77 flavio 17 Aug 18:57 Dockerfile .rwxr-xr-x 4.2M flavio 17 Aug 19:13 findlinks .rw-r--r-- 1.1k flavio 12 Aug 18:10 findlinks.go $ file findlinks findlinks: ELF 64-bit LSB executable, x86-64, version 1 ( SYSV ) , statically linked, stripped

Notice that this of course is not the same file that I get if I go build on OSX, it’s a linux-ready binary:

$ file findlinks findlinks: Mach-O 64-bit executable x86_64

We then create a Dockerfile, telling Docker to use iron/base, a very light image:

FROM iron/base WORKDIR /app COPY findlinks /app/ ENTRYPOINT [ "./findlinks" ]

We can now build the image, tagging it flaviocopes/golang-docker-example-findlinks :

docker build -t flaviocopes/golang-docker-example-findlinks .

and run it:

docker run --rm -it -p 8000:8000 flaviocopes/golang-docker-example-findlinks

The output is the same as before, but this time the image is not 720MB, but just 11.1MB

REPOSITORY TAG IMAGE ID CREATED SIZE flaviocopes/golang-docker-example-findlinks latest f32d2fd74638 14 minutes ago 11.1MB findlinks latest c60f6792b9f3 20 minutes ago 720MB

Multi-stage builds

This section was added thanks to Reddit comments pointing me to multi-stage builds, a recent addition to Docker 17.05

Multi-stage builds allow us to have a lightweight image very easily, without the need to compile the binary and then run it separately. This is the Dockerfile to put in the app:

FROM golang:1.8.3 as builder WORKDIR /go/src/github.com/flaviocopes/findlinks RUN go get -d -v golang.org/x/net/html COPY findlinks.go . RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o findlinks . FROM alpine:latest RUN apk --no-cache add ca-certificates WORKDIR /root/ COPY --from=builder /go/src/github.com/flaviocopes/findlinks/findlinks . CMD ["./findlinks"]

Run

$ docker build -t flaviocopes/golang-docker-example-findlinks .

which builds a lightweight (10.8MB) image:

$ docker images REPOSITORY TAG IMAGE ID CREATED SIZE flaviocopes/golang-docker-example-findlinks latest aa2081ca7016 12 seconds ago 10.8MB

Run the image with

$ docker run --rm -it -p 8000:8000 flaviocopes/golang-docker-example-findlinks