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Colin Duggan
Colin Duggan

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Package Management in Go

Go Package Management

Go takes a semi-decentralized approach to package management, allowing any git repo to be used as a module package system. Modules can be downloaded directly from their source control servers or alternatively through a module proxy which performs a simple forwarding of requests to the appropriate source. The following diagram is a high-level take on what happens when go get attempts to resolve a package or module.

Image description

Module Proxy

The module proxy is an HTTP server which implements the module proxy protocol. It responds to GET requests matching a particular pattern, See examples below. The module proxy ensures changes or downtime to a module origin server will not bubble up and impact your build process. The module proxy can also act as a gatekeeper, restricting access to modules which have unsuitable licences or contain security vulnerabilities. Module proxies can also provide faster dependency resolution by caching previously fetched modules, which can then be used to serve future requests. A module's dependencies are defined in the following files:

Module Description
go.mod Defines the module name, the version of Go with which to build the project and the list of dependencies.
go.sum Contains cryptographic hashes of the module's direct and indirect dependencies.

Module Proxy Protocol

Since version 1.13, proxy.golang.org is the default module proxy for fetching modules.

It's possible to build your own module proxy, which consists of an HTTP server and REST API that satisfies the GOPROXY protocol.

Go Get Command

The go get command follows this protocol when attempting to resolve a package path:

  • Search modules in the current project build list (go.mod) including all build lists of transitive dependencies, looking for modules with paths which are prefixes of the requested package path.
  • If one module in the build list contains the required package, then that module will be used.
  • If none of the modules in the searched build lists provide the package or if two or more modules provide the requested package, then go get will report an error.

When go get is resolving a new module for a package path , it uses the GOPROXY environment variable to determine whether modules will be downloaded directly, through a module proxy or some other private mirror. GOPROXY accepts a list of proxy URLs separated by either commas or pipes. The separator used in this list determines how the go get command will fallback in different scenarios:

GOPROXY Separator Type Behaviour
List separated by commas go get will fallback to the next URL in the event of a 404 or 410 HTTP response. All other response codes are treated as terminal.
List separated by pipes go get will fallback to the next URL in the proxy list in the event of any HTTP or non-HTTP error.

The go get command visits each proxy in the list sequentially until it receives a successful response or error.

GoProxy Environment Variable Options

The default configuration for the GOPROXY environment variable is GOPROXY=proxy.golang.org,direct which effectively tells the go get command to first attempt module retrieval using the module mirror and if that approach is unsuccessful, fallback to the approach of connecting directly to the module origin repository. A number of other environment variables can also impact how go get will attempt to resolve dependencies. These variables are listed here.

GPROXY keywords Description
direct go get will download directly from version control repo
off Disallows downloading from any source

Go Module Proxy Request Patterns

A GET request for a particular module will follow the pattern: https://$base/$module/@v/$version.zip.

Where:

Request portion Description Sample value
$base The path portion of the proxy URL proxy.golang.org
$module Module path go.uber.org/zap
$version Version v1.21.0

The resulting GET request will download the module and dependencies from: https://proxy.golang.org/go.uber.org/zap/@v/v1.21.0.zip. The 'go get' and 'go mod tidy' commands abstract the details of this from the developer.

Go Module Proxy Response Codes

The type of separator used in the GOPROXY environment variable will determine if the go get command will fall back to the next option in the list or fail completely. See section on 'go get'.

Proxy Response Code Description
200 OK
300 Redirects are followed
404 Not found
403 Forbidden - Private proxies can choose to prevent access to modules based on unsuitable licenses or security vulnerabilities
410 Module not available on the proxy server but might be available elsewhere
4xx & 5xx Treated as errors

Go Environment Variables Which Impact Module Retrieval

If set with specific values, GOPRIVATE and GONOPROXY can impact the behaviour of GOPROXY, by preventing specific modules from being downloaded through the proxy.

Environment Variable Description
GOPRIVATE Comma separated list of module prefixes considered private. If set, this acts as a default value for GONOSUMDB
GONOPROXY Comma separated list of module prefixes, which should always be downloaded directly from source. If set, this acts as a default value for GOPRIVATE
GONOSUMDB Comma separated list of module prefixes which the go command should not verify checksums using the checksum database.

Verifying Modules

After downloading a .mod or .zip file, go get computes a cryptographic hash and checks that it matches a hash in the main modules go.sum. If the hash is not present in go.sum then the go command retrieves it from the checksum database. The checksum database, located at sum.golang.org, allows for global consistency and reliability for all publicly available modules. It also ensures bits associated with specific module versions do not change from one day to the next.

Module Caching

The 'go get' command will cache modules downloaded from both module proxies and directly from a module's source repository in $GOPATH/pkg/mod/cache/download. This local cache layout is identical to the proxy URL space.

Module Graph

Module graph pruning was introduced in version 1.17. See go.dev for more information. Module pruning uses Minimal version selection to select a set of module versions to use when building packages. It operates on a directed graph of modules, specified in go.mod. Each vertex in the graph represents a module version. Each edge represents a minimum required version of a dependency, specified using a require directive. The graph may be modified by exclude and replace directives in go.mod. MVS produces the build list as an output.

Command Description
go list -m all Prints a modules build list
go mod graph Prints the edges of the graph, one per line. Each line contains a module version and one of its dependencies
go mod graph -go <some-version> Identical to previous command however it prints based on how the selected go version would interrupt the graph
go mod verify Checks dependencies of the main module stored in module cache have not been modified since they were downloaded
go mod why Shows the shortest path in the import graph from the main module to each of the listed packages

Full details can be found at understanding-go-get

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