Docker and Go Modules

This is a text version of packagemain #14: Docker and Go modules video.

As you may know Go 1.11 includes opt-in feature for versioned modules. Before go modules Gophers used dependency managers like dep or glide, but with go modules you don't need a 3rd-party manager as they are included into standard go toolchain.

Also modules allow for the deprecation of the GOPATH, which was a blocker for some newcomers in Go.

In this video I am going to demonstrate how to enable go modules for your program and then package it with Docker. And you will see how easy it is.

Create a project

Let's create simple http server which will use logrus package for logging.

As I said before go modules is an opt-in feature, which can be enabled by setting environment variable GO111MODULE=on.

export GO111MODULE=on
mkdir httpserver && cd httpserver
go mod init
go get github.com/sirupsen/logrus

2 new files have been created in our folder: go.mod and go.sum.

package main

import (
    "fmt"
    "net/http"

    log "github.com/sirupsen/logrus"
)

func main() {
    http.Handle("/", loggingMiddleware(http.HandlerFunc(handler)))
    http.ListenAndServe(":8080", nil)
}

func handler(w http.ResponseWriter, req *http.Request) {
    fmt.Fprintf(w, "package main #14")
}

func loggingMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
        log.Infof("uri: %s", req.RequestURI)
        next.ServeHTTP(w, req)
    })
}

Now if we run go build it will download deoendencies and build a binary:

go build
./httpserver

Package with Docker

Let's create a simple Dockerfile for our server.

FROM golang

ENV GO111MODULE=on

WORKDIR /app

COPY . .

RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build

EXPOSE 8080
ENTRYPOINT ["/app/httpserver"]

docker build -t httpserver .
docker run -p 8080:8080 httpserver

Cache go modules

As you can see go build downloads our dependencies. But what is not good here is that it will do it every time we build an image. And imagine if your project have a lot of dependencies, it will slow down your build process. Let's change something in main.go file and run build again.

To fix this we can use go mod download which will download dependencies first. But we should re-run it if our go.mod / go.sum files have been changed.

We can do it by copying go.mod / go.sum files into docker first, then run go mod download, then copy all other files and run go build.

FROM golang

ENV GO111MODULE=on

WORKDIR /app

COPY go.mod .
COPY go.sum .

RUN go mod download

COPY . .

RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build

EXPOSE 8080
ENTRYPOINT ["/app/httpserver"]

Multi-stage build

One more thing I like to do with my Dockerfiles is to use multi-stage build to reduce the size of final image. To run our server we only need a binary file, we don't need the go installed, so inside one Dockerfile we can build program first using golang image, and then copy only a binary from it to scratch.

# build stage
FROM golang as builder

ENV GO111MODULE=on

WORKDIR /app

COPY go.mod .
COPY go.sum .

RUN go mod download

COPY . .

RUN CGO_ENABLED=0 GOOS=linux GOARCH=amd64 go build

# final stage
FROM scratch
COPY --from=builder /app/httpserver /app/
EXPOSE 8080
ENTRYPOINT ["/app/httpserver"]

Conclusion

So I think go modules is a nice feature, and you definitely should try it, I use it in all my services I write. Of course it needs some improvements, but it works well in practice.

Code on GitHub