Manually fetch dependencies from go.mod?

Question

I'm using go 1.11 with module support. I understand that the go tool now installs dependencies automatically on build/install. I also understand the reasoning.

I'm using docker to build my binaries. In many other ecosystems its common to copy over your dependency manifest (package.json, requirements.txt, etc) and install dependencies as a separate stage from build. This takes advantage of docker's layer caching, and makes rebuilds much faster since generally code changes vastly outnumber dependency changes.

I was wondering if vgo has any way to do this?

Answer 1

It was an issue #26610, which is fixed now.

So now you can just use:

go mod download 

For this to work you need just the go.mod / go.sum files.

For example, here's how to have a cached multistage Docker build: (source)

FROM golang:1.17-alpine as builder RUN apk --no-cache add ca-certificates git WORKDIR /build  # Fetch dependencies COPY go.mod go.sum ./ RUN go mod download  # Build COPY . ./ RUN CGO_ENABLED=0 go build  # Create final image FROM alpine WORKDIR / COPY --from=builder /build/myapp . EXPOSE 8080 CMD ["./myapp"] 

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Also see the article Containerize Your Go Developer Environment – Part 2, which describes how to leverage the Go compiler cache to speed up builds even further.