go modules – Manually fetch dependencies from go.mod?-ThrowExceptions

Exception or error:

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?

How to solve:

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:

FROM golang:1.12-alpine as builder
RUN apk --no-cache add ca-certificates git
WORKDIR /build/myapp

# Fetch dependencies
COPY go.mod ./
RUN go mod download

# Build
COPY . ./
RUN CGO_ENABLED=0 go build

# Create final image
FROM alpine
WORKDIR /root
COPY --from=builder /build/myapp/myapp .
EXPOSE 8080
CMD ["./myapp"]

Answer:

You may use the go mod vendor command which will create a vendor folder in the main module’s root folder, and copy all dependencies into it. After this you may pass the -mod=vendor param to the go tool, and then dependencies from the vendor folder will be used to build / compile / test your app.

So what you may do to speed up your builds:

  1. Run the go mod vendor command to have an actual version of your dependencies.
  2. Save / cache this vendor folder.
  3. During builds, restore this vendor folder, and build / install your app by passing the -mod=vendor argument to the go tool, so no dependencies will be downloaded, but the content of the vendor folder will be used.

Quoting from go help mod:

Modules and vendoring

When using modules, the go command completely ignores vendor directories.

By default, the go command satisfies dependencies by downloading modules
from their sources and using those downloaded copies (after verification,
as described in the previous section). To allow interoperation with older
versions of Go, or to ensure that all files used for a build are stored
together in a single file tree, ‘go mod vendor’ creates a directory named
vendor in the root directory of the main module and stores there all the
packages from dependency modules that are needed to support builds and
tests of packages in the main module.

To build using the main module’s top-level vendor directory to satisfy
dependencies (disabling use of the usual network sources and local
caches), use ‘go build -mod=vendor’. Note that only the main module’s
top-level vendor directory is used; vendor directories in other locations
are still ignored.

Answer:

You can use a package manager, There are many of them like dep, glide, and govendor. dep is newer and is going to be integrated into go toolchain as official dependency management tool.

We also make docker images for go applications and we use dind to make those images and we prepared a CI/CD image with all dependencies preinstalled to make the builds faster. Though, it took a little bit of scripting to glue everything together.

Moreover, layering up the dependencies could result in big size of docker images. I suggest try multi-stage builds which could help making images super lite.

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