genesis

Genesis is a utility for embedding static file assets into Go files to be included in static compilation. Genesis includes support for http.FileSystem as well versioning via SHA256 hashes.

Installation

To install genesis, first install Go and then run:

$ go get -u github.com/benbjohnson/genesis/...

You should now have a genesis binary in your $GOPATH/bin directory.

Usage

Command line usage

The genesis command includes a few basic options available from the CLI:

usage: genesis [options] path [paths]

Embeds listed assets in a Go file as hex-encoded strings.

The following flags are available:

	-pkg name
		Package name of the generated Go file. Required.

	-o output
		Output filename for generated code. Optional.
		(default stdout)

	-C dir
		Execute genesis from dir. Optional.

	-tags tags
		Comma-delimited list of build tags. Optional.

You can generate a new Go file with your embedded assets by specifying the package name and the list of files or directories you want to include. By default the output will go to STDOUT so you can redirect to the file of your choice.

$ genesis -pkg mypkg file1 file2 dir1 > assets.gen.go

Specifying the output file

You can also specify the output file using the -o flag. This is useful if you are running via //go:generate and file redirection is not available:

$ genesis -pkg mypkg -o assets.gen.go file1 file2 dir1

Working directory

One common approach to asset organization is to create a separate assets package that includes your asset files and your generated embedded file. You can use the -C flag to change the present working directory to this package and then execute relative to this directory:

$ genesis -C assets -pkg mypkg -o assets.gen.go file1 file2

Including build tags

Finally, you can optionally include build tags to optionally include assets in your final build. Tags should be comma separated for each individual build tag line:

$ genesis -pkg mypkg -o assets.386.gen.go -tags "linux darwin,386" file1 file2 dir1

This will output the following build tag comments at the top of the file:

// +build linux darwin
// +build 386

HTTP File System

Your generated embedded assets Go file will include an implementation of http.FileSystem that can be passed to http.FileServer to automatically serve your embedded assets through an http.Handler:

http.Handle("/", http.FileServer(assets.FileSystem()))

Cache control

Your embedded assets Go file also includes a method for returning the filename with an included SHA256 hash. This hash will change whenever the contents of the file changes so the http.FileSystem will tell the browser to cache the file indefinitely using the Cache-Control header.

To use this feature, simply use the AssetNameWithHash() function when writing your URLs that reference embedded assets:

fmt.Fprintf(w, `<script src="%s"></script>`, assets.AssetNameWithHash("bundle.js"))

This will output something like the following:

<script src="/bundle-c3ab8ff13720e8ad9047dd39466b3c8974e592c2fa383d4a3960714caef0c4f2.js"></script>

When a file is requested with a SHA256 hash it will be cached indefinitely which leads to improved web site latency on return visits.

Programmatic API

The generated embedded assets Go file includes several types and utility functions. You can view this information via the godoc for your package. A list of some of these types and functions are included below.

File

One File is generated for every asset you embed. This struct provides the name, content hash, size, last modification time, and raw data.

type File struct {}

func (f *File) Name() string
func (f *File) Hash() string
func (f *File) ModTime() time.Time
func (f *File) Data() []byte

Asset functions

Several utility functions are provided for accessing asset information. The Asset() function simply returns the underlying asset data while AssetFile() returns the entire File object. To retrieve a sorted list of all asset names, use the AssetNames() function.

func Asset(name string) []byte
func AssetFile(name string) *File
func AssetNames() []string

Content hash functions

The AssetNameWithHash() function will return a given asset name with its SHA256 content hash included. This is useful for embedding in HTML source names when combining with the included http.FileSystem implementation.

func AssetNameWithHash(name string) string

There are also utility functions for working with asset names and hashes. The JoinNameHash() function will build an asset name hash with the given name and hash. The SplitNameHash() function will return the base asset name and an asset name hash. The HasNameHash() returns true if a given name includes an asset hash.

func JoinNameHash(name, hash string) string
func SplitNameHash(name string) string 
func HasNameHash(name string) bool