The download mode file

Athens accepts an HCL formatted file that has instructions for how it should behave when a module@version isn’t found in its storage. This functionality gives Athens the flexibility configure Athens to fit your organization’s needs. The most popular uses of this download file are:

  • Configure Athens to never download or serve a module or group of modules
  • Redirect to a different module proxy for a module or group of modules

This document will outline how to use this file - called the download mode file - to accomplish these tasks and more.

Please see the “Use cases” section below for more details on how to enable these behaviors and more.

Configuration

First, once you’ve created your download mode file, you tell Athens to use it by setting the DownloadMode configuration parameter in the config.toml file, or setting the ATHENS_DOWNLOAD_MODE environment variable. You can set this configuration value to one of two values to tell Athens to use your file:

  1. Set its value to file:$FILE_PATH, where $FILE_PATH is the path to the HCL file
  2. Set its value to custom$BASE_64 where $BASE_64 is the base64 encoded HCL file

Instead of one of the above two values, you can set this configuration to sync, async, none, redirect, or async_redirect. If you do, the download mode will be set globally rather than for specific sub-groups of modules. See below for what each of these values mean.

Download mode keywords

If Athens receives a request for the module github.com/pkg/errors at version v0.8.1, and it doesn’t have that module and version in its storage, it will consult the download mode file for specific instructions on what action to take:

  1. sync: Synchronously download the module from VCS via go mod download, persist it to the Athens storage, and serve it back to the user immediately. Note that this is the default behavior.
  2. async: Return a 404 to the client, and asynchronously download and persist the module@version to storage.
  3. none: Return a 404 and do nothing.
  4. redirect: Redirect to an upstream proxy (such as proxy.golang.org) and do nothing after.
  5. async_redirect: Redirect to an upstream proxy (such as proxy.golang.org) and asynchronously download and persist the module@version to storage.

Athens expects these keywords to be used in conjunction with module patterns (github.com/pkg/*, for example). You combine the keyword and the pattern to specify behavior for a specific group of modules.

Athens uses the Go path.Match function to parse module patterns.

Below is an example download mode file.

downloadURL = "https://proxy.golang.org"

mode = "async_redirect"

download "github.com/gomods/*" {
    mode = "sync"
}

download "golang.org/x/*" {
    mode = "none"
}

download "github.com/pkg/*" {
    mode = "redirect"
    downloadURL = "https://gocenter.io"
}

The first two lines describe the default behavior for all modules. This behavior is overridden for select module groups below. In this case, the default behavior is:

  • Immediatley redirect all requests to https://proxy.golang.org
  • In the background, download the module from the version control system (VCS) and store it

The rest of the file contains download blocks. These override the default behavior for specific groups of modules.

The first block specifies that any module matching github.com/gomods/* (such as github.com/gomods/athens) will be downloaded from GitHub, stored, and then returned to the user.

The second block specifies that any module matching golang.org/x/* (such as golang.org/x/text) will always return a HTTP 404 response code. This behavior ensures that Athens will never store or serve any module names starting with golang.org/x.

If a user has their GOPROXY environment variable set with a comma separated list, their go command line tool will always try the option next in the list. For example, if a user has their GOPROXY environment variable set to https://athens.azurefd.net,direct, and then runs go get golang.org/x/text, they will still download golang.org/x/text to their machine. The module just won’t come from Athens.

The last block specifies that any module matching github.com/pkg/* (such as github.com/pkg/errors) will always redirect the go tool to https://gocenter.io. In this case, Athens will never persist the given module to its storage.

Use cases

The download mode file is versatile and allows you to configure Athens in a large variety of different ways. Below are some of the mode common.

Blocking certain modules

If you’re running Athens to serve a team of Go developers, it might be useful to ensure that the team doesn’t use a specific group or groups of modules (for example, because of licensing or security issues).

In this case, you would write this in your file:

download "bad/module/repo/*" {
    mode = "none"
}

Preventing storage overflow

If you are running Athens using a storage backend that has limited space, you may want to prevent Athens from storing certain groups of modules that take up a lot of space. To avoid exhausting Athens storage, while still ensuring that the users of your Athens server still get access to the modules you can’t store, you would use a redirect directive, as shown below:

download "very/large/*" {
    mode = "redirect"
    url = "https://reliable.proxy.com"
}

If you use the redirect mode, make sure that you specify a url value that points to a reliable proxy.

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