go/doc/modules.md

Introduction

Modules, packages, and versions

A module is a collection of packages that are released, versioned, and distributed together. A module is identified by a module path, which is declared in a go.mod file, together with information about the module's dependencies. The module root directory is the directory that contains the go.mod file. The main module is the module containing the directory where the go command is invoked.

Each package within a module is a collection of source files in the same directory that are compiled together. A package path is the module path joined with the subdirectory containing the package (relative to the module root). For example, the module "golang.org/x/net" contains a package in the directory "html". That package's path is "golang.org/x/net/html".

Module paths

A module path is the canonical name for a module, declared with the module directive in the module's go.mod file. A module's path is the prefix for package paths within the module.

A module path should describe both what the module does and where to find it. Typically, a module path consists of a repository root path, a subdirectory within the repository (usually empty), and a major version suffix (for major version 2 or higher).

• The repository root path is part of a URL that corresponds to a version control repository. For example, golang.org/x/net. See Custom import paths for details on how paths are resolved to repositories.
• If the module is in a subdirectory of the version control repository, the subdirectory should be part of the module path (but not the repository root path). For example, the module golang.org/x/tools/gopls is in the /gopls subdirectory of the repository golang.org/x/tools.
• If the module is released at major version 2 or higher, the module path must end with a major version suffix like /v2. This may or may not be part of the subdirectory name. For example, the module with path golang.org/x/repo/sub/v2 could be in the /sub or /sub/v2 subdirectory of the repository golang.org/x/repo.

If a module might be depended on by other modules, these rules must be followed so that the go command can find and download the module. There are also several lexical restrictions on characters allowed in module paths.

Versions

A version identifies an immutable snapshot of a module, which may be either a release or a pre-release. Each version starts with the letter v, followed by a semantic version. See Semantic Versioning 2.0.0 for details on how versions are formatted, interpreted, and compared.

To summarize, a semantic version consists of three non-negative integers (the major, minor, and patch versions, from left to right) separated by dots. The patch version may be followed by an optional pre-release string starting with a hyphen. The pre-release string or patch version may be followed by a build metadata string starting with a plus. For example, v0.0.0, v1.12.134, v8.0.5-pre, and v2.0.9+meta are valid versions.

Each part of a version indicates whether the version is stable and whether it is compatible with previous versions.

• The major version must be incremented and the minor and patch versions must be set to zero after a backwards incompatible change is made to the module's public interface or documented functionality, for example, after a package is removed.
• The minor version must be incremented and the patch version set to zero after a backwards compatible change, for example, after a new function is added.
• The patch version must be incremented after a change that does not affect the module's public interface, such as a bug fix or optimization.
• The pre-release suffix indicates a version is a pre-release. Pre-release versions sort before the corresponding release versions. For example, v1.2.3-pre comes before v1.2.3.
• The build metadata suffix is ignored for the purpose of comparing versions. Tags with build metadata are ignored in version control repositories, but build metadata is preserved in versions specified in go.mod files. The suffix +incompatible denotes a version released before migrating to modules version major version 2 or later (see Compatibility with non-module repositories.

A version is considered unstable if its major version is 0 or it has a pre-release suffix. Unstable versions are not subject to compatibility requirements. For example, v0.2.0 may not be compatible with v0.1.0, and v1.5.0-beta may not be compatible with v1.5.0.

Go may access modules in version control systems using tags, branches, or revisions that don't follow these conventions. However, within the main module, the go command will automatically convert revision names that don't follow this standard into canonical versions. The go command will also remove build metadata suffixes (except for +incompatible) as part of this process. This may result in a pseudo-version, a pre-release version that encodes a revision identifier (such as a Git commit hash) and a timestamp from a version control system. For example, the command go get -d golang.org/x/net@daa7c041 will convert the commit hash daa7c041 into the pseudo-version v0.0.0-20191109021931-daa7c04131f5. Canonical versions are required outside the main module, and the go command will report an error if a non-canonical version like master appears in a go.mod file.

Major version suffixes

Starting with major version 2, module paths must have a major version suffix like /v2 that matches the major version. For example, if a module has the path example.com/mod at v1.0.0, it must have the path example.com/mod/v2 at version v2.0.0.

Major version suffixes implement the import compatibility rule:

If an old package and a new package have the same import path, the new package must be backwards compatible with the old package.

By definition, packages in a new major version of a module are not backwards compatible with the corresponding packages in the previous major version. Consequently, starting with v2, packages need new import paths. This is accomplished by adding a major version suffix to the module path. Since the module path is a prefix of the import path for each package within the module, adding the major version suffix to the module path provides a distinct import path for each incompatible version.

Major version suffixes are not allowed at major versions v0 or v1. There is no need to change the module path between v0 and v1 because v0 versions are unstable and have no compatibility guarantee. Additionally, for most modules, v1 is backwards compatible with the last v0 version; a v1 version acts as a commitment to compatibility, rather than an indication of incompatible changes compared with v0.

As a special case, modules paths starting with gopkg.in/ must always have a major version suffix, even at v0 and v1. The suffix must start with a dot rather than a slash (for example, gopkg.in/yaml.v2).

Major version suffixes let multiple major versions of a module coexist in the same build. This may be necessary due to a diamond dependency problem. Ordinarily, if a module is required at two different versions by transitive dependencies, the higher version will be used. However, if the two versions are incompatible, neither version will satisfy all clients. Since incompatible versions must have different major version numbers, they must also have different module paths due to major version suffixes. This resolves the conflict: modules with distinct suffixes are treated as separate modules, and their packages—even packages in same subdirectory relative to their module roots—are distinct.

Many Go projects released versions at v2 or higher without using a major version suffix before migrating to modules (perhaps before modules were even introduced). These versions are annotated with a +incompatible build tag (for example, v2.0.0+incompatible). See Compatibility with non-module repositories for more information.

Resolving a package to a module

When the go command loads a package using a package path, it needs to determine which module provides the package.

The go command starts by searching the build list for modules with paths that are prefixes of the package path. For example, if the package example.com/a/b is imported, and the module example.com/a is in the build list, the go command will check whether example.com/a contains the package, in the directory b. At least one file with the .go extension must be present in a directory for it to be considered a package. Build constraints are not applied for this purpose. If exactly one module in the build list provides the package, that module is used. If two or more modules provide the package, an error is reported. If no modules provide the package, the go command will attempt to find a new module (unless the flags -mod=readonly or -mod=vendor are used, in which case, an error is reported).

When the go command looks up a new module for a package path, it checks the GOPROXY environment variable, which is a comma-separated list of proxy URLs or the keywords direct or off. A proxy URL indicates the go command should contact a module proxy using the GOPROXY protocol. direct indicates that the go command should communicate with a version control system. off indicates that no communication should be attempted. The GOPRIVATE and GONOPROXY environment variables can also be used to control this behavior.

For each entry in the GOPROXY list, the go command requests the latest version of each module path that might provide the package (that is, each prefix of the package path). For each successfully requested module path, the go command will download the module at the latest version and check whether the module contains the requested package. If one or more modules contain the requested package, the module with the longest path is used. If one or more modules are found but none contain the requested package, an error is reported. If no modules are found, the go command tries the next entry in the GOPROXY list. If no entries are left, an error is reported.

For example, suppose the go command is looking for a module that provides the package golang.org/x/net/html, and GOPROXY is set to https://corp.example.com,https://proxy.golang.org. The go command may make the following requests:

• To https://corp.example.com/ (in parallel):
  • Request for latest version of golang.org/x/net/html
  • Request for latest version of golang.org/x/net
  • Request for latest version of golang.org/x
  • Request for latest version of golang.org
• To https://proxy.golang.org/, if all requests to https://corp.example.com/ have failed with 404 or 410:
  • Request for latest version of golang.org/x/net/html
  • Request for latest version of golang.org/x/net
  • Request for latest version of golang.org/x
  • Request for latest version of golang.org

After a suitable module has been found, the go command will add a new requirement with the new module's path and version to the main module's go.mod file. This ensures that when the same package is loaded in the future, the same module will be used at the same version. If the resolved package is not imported by a package in the main module, the new requirement will have an // indirect comment.

go.mod files

A module is defined by a UTF-8 encoded text file named go.mod in its root directory. The go.mod file is line-oriented. Each line holds a single directive, made up of a keyword followed by arguments. For example:

module example.com/my/thing

go 1.12

require example.com/other/thing v1.0.2
require example.com/new/thing/v2 v2.3.4
exclude example.com/old/thing v1.2.3
replace example.com/bad/thing v1.4.5 => example.com/good/thing v1.4.5

The leading keyword can be factored out of adjacent lines to create a block, like in Go imports.

require (
    example.com/new/thing/v2 v2.3.4
    example.com/old/thing v1.2.3
)

The go.mod file is designed to be human readable and machine writable. The go command provides several subcommands that change go.mod files. For example, go get can upgrade or downgrade specific dependencies. Commands that load the module graph will automatically update go.mod when needed. go mod edit can perform low-level edits. The golang.org/x/mod/modfile package can be used by Go programs to make the same changes programmatically.

Lexical elements

When a go.mod file is parsed, its content is broken into a sequence of tokens. There are several kinds of tokens: whitespace, comments, punctuation, keywords, identifiers, and strings.

White space consists of spaces (U+0020), tabs (U+0009), carriage returns (U+000D), and newlines (U+000A). White space characters other than newlines have no effect except to separate tokens that would otherwise be combined. Newlines are significant tokens.

Comments start with // and run to the end of a line. /* */ comments are not allowed.

Punctuation tokens include (, ), and =>.

Keywords distinguish different kinds of directives in a go.mod file. Allowed keywords are module, go, require, replace, and exclude.

Identifiers are sequences of non-whitespace characters, such as module paths or semantic versions.

Strings are quoted sequences of characters. There are two kinds of strings: interpreted strings beginning and ending with quotation marks (", U+0022) and raw strings beginning and ending with grave accents (<, U+0060). Interpreted strings may contain escape sequences consisting of a backslash (\, U+005C) followed by another character. An escaped quotation mark (\") does not terminate an interpreted string. The unquoted value of an interpreted string is the sequence of characters between quotation marks with each escape sequence replaced by the character following the backslash (for example, \" is replaced by ", \n is replaced by n). In contrast, the unquoted value of a raw string is simply the sequence of characters between grave accents; backslashes have no special meaning within raw strings.

Identifiers and strings are interchangeable in the go.mod grammar.

Module paths and versions

Most identifiers and strings in a go.mod file are either module paths or versions.

A module path must satisfy the following requirements:

• The path must consist of one or more path elements separated by slashes (/, U+002F). It must not begin or end with a slash.
• Each path element is a non-empty string made of up ASCII letters, ASCII digits, and limited ASCII punctuation (+, -, ., _, and ~).
• A path element may not begin or end with a dot (., U+002E).
• The element prefix up to the first dot must not be a reserved file name on Windows, regardless of case (CON, com1, NuL, and so on).

If the module path appears in a require directive and is not replaced, or if the module paths appears on the right side of a replace directive, the go command may need to download modules with that path, and some additional requirements must be satisfied.

• The leading path element (up to the first slash, if any), by convention a domain name, must contain only lower-case ASCII letters, ASCII digits, dots (., U+002E), and dashes (-, U+002D); it must contain at least one dot and cannot start with a dash.
• For a final path element of the form /vN where N looks numeric (ASCII digits and dots), N must not begin with a leading zero, must not be /v1, and must not contain any dots.
  • For paths beginning with gopkg.in/, this requirement is replaced by a requirement that the path follow the gopkg.in service's conventions.

Versions in go.mod files may be canonical or non-canonical.

A canonical version starts with the letter v, followed by a semantic version following the Semantic Versioning 2.0.0 specification. See Versions for more information.

Most other identifiers and strings may be used as non-canonical versions, though there are some restrictions to avoid problems with file systems, repositories, and module proxies. Non-canonical versions are only allowed in the main module's go.mod file. The go command will attempt to replace each non-canonical version with an equivalent canonical version when it automatically updates the go.mod file.

In places where a module path is associated with a verison (as in require, replace, and exclude directives), the final path element must be consistent with the version. See Major version suffixes.

Grammar

go.mod syntax is specified below using Extended Backus-Naur Form (EBNF). See the Notation section in the Go Language Specificiation for details on EBNF syntax.

GoMod = { Directive } .
Directive = ModuleDirective |
            GoDirective |
            RequireDirective |
            ExcludeDirective |
            ReplaceDirective .

Newlines, identifiers, and strings are denoted with newline, ident, and string, respectively.

Module paths and versions are denoted with ModulePath and Version.

ModulePath = ident | string . /* see restrictions above */
Version = ident | string .    /* see restrictions above */

module directive

A module directive defines the main module's path. A go.mod file must contain exactly one module directive.

ModuleDirective = "module" ( ModulePath | "(" newline ModulePath newline ")" newline .

Example:

module golang.org/x/net

go directive

A go directive sets the expected language version for the module. The version must be a valid Go release version: a positive integer followed by a dot and a non-negative integer (for example, 1.9, 1.14).

The language version determines which language features are available when compiling packages in the module. Language features present in that version will be available for use. Language features removed in earlier versions, or added in later versions, will not be available. The language version does not affect build tags, which are determined by the Go release being used.

The language version is also used to enable features in the go command. For example, automatic vendoring may be enabled with a go version of 1.14 or higher.

A go.mod file may contain at most one go directive. Most commands will add a go directive with the current Go version if one is not present.

GoDirective = "go" GoVersion newline .
GoVersion = string | ident .  /* valid release version; see above */

Example:

go 1.14

require directive

A require directive declares a minimum required version of a given module dependency. For each required module version, the go command loads the go.mod file for that version and incorporates the requirements from that file. Once all requirements have been loaded, the go command resolves them using minimal version selection (MVS) to produce the build list.

The go command automatically adds // indirect comments for some requirements. An // indirect comment indicates that no package from the required module is directly imported by any package in the main module. The go command adds an indirect requirement when the selected version of a module is higher than what is already implied (transitively) by the main module's other dependencies. That may occur because of an explicit upgrade (go get -u), removal of some other dependency that previously imposed the requirement (go mod tidy), or a dependency that imports a package without a corresponding requirement in its own go.mod file (such as a dependency that lacks a go.mod file altogether).

RequireDirective = "require" ( RequireSpec | "(" newline { RequireSpec } ")" newline ) .
RequireSpec = ModulePath Version newline .

Example:

require golang.org/x/net v1.2.3

require (
    golang.org/x/crypto v1.4.5 // indirect
    golang.org/x/text v1.6.7
)

exclude directive

An exclude directive prevents a module version from being loaded by the go command. If an excluded version is referenced by a require directive in a go.mod file, the go command will list available versions for the module (as shown with go list -m -versions) and will load the next higher non-excluded version instead. Both release and pre-release versions are considered for this purpose, but pseudo-versions are not. If there are no higher versions, the go command will report an error. Note that this may change in Go 1.15.

exclude directives only apply in the main module's go.mod file and are ignored in other modules. See Minimal version selection for details.

ExcludeDirective = "exclude" ( ExcludeSpec | "(" newline { ExcludeSpec } ")" ) .
ExcludeSpec = ModulePath Version newline .

Example:

exclude golang.org/x/net v1.2.3

exclude (
    golang.org/x/crypto v1.4.5
    golang.org/x/text v1.6.7
)

replace directive

A replace directive replaces the contents of a specific version of a module, or all versions of a module, with contents found elsewhere. The replacement may be specified with either another module path and version, or a platform-specific file path.

If a version is present on the left side of the arrow (=>), only that specific version of the module is replaced; other versions will be accessed normally. If the left version is omitted, all versions of the module are replaced.

If the path on the right side of the arrow is an absolute or relative path (beginning with ./ or ../), it is interpreted as the local file path to the replacement module root directory, which must contain a go.mod file. The replacement version must be omitted in this case.

If the path on the right side is not a local path, it must be a valid module path. In this case, a version is required. The same module version must not also appear in the build list.

Regardless of whether a replacement is specified with a local path or module path, if the replacement module has a go.mod file, its module directive must match the module path it replaces.

replace directives only apply in the main module's go.mod file and are ignored in other modules. See Minimal version selection for details.

ReplaceDirective = "replace" ( ReplaceSpec | "(" newline { ReplaceSpec } ")" newline ")" ) .
ReplaceSpec = ModulePath [ Version ] "=>" FilePath newline
            | ModulePath [ Version ] "=>" ModulePath Version newline .
FilePath = /* platform-specific relative or absolute file path */

Example:

replace golang.org/x/net v1.2.3 => example.com/fork/net v1.4.5

replace (
    golang.org/x/net v1.2.3 => example.com/fork/net v1.4.5
    golang.org/x/net => example.com/fork/net v1.4.5
    golang.org/x/net v1.2.3 => ./fork/net
    golang.org/x/net => ./fork/net
)

Automatic updates

The go command automatically updates go.mod when it uses the module graph if some information is missing or go.mod doesn't accurately reflect reality. For example, consider this go.mod file:

module example.com/M

require (
    example.com/A v1
    example.com/B v1.0.0
    example.com/C v1.0.0
    example.com/D v1.2.3
    example.com/E dev
)

exclude example.com/D v1.2.3

The update rewrites non-canonical version identifiers to canonical semver form, so example.com/A's v1 becomes v1.0.0, and example.com/E's dev becomes the pseudo-version for the latest commit on the dev branch, perhaps v0.0.0-20180523231146-b3f5c0f6e5f1.

The update modifies requirements to respect exclusions, so the requirement on the excluded example.com/D v1.2.3 is updated to use the next available version of example.com/D, perhaps v1.2.4 or v1.3.0.

The update removes redundant or misleading requirements. For example, if example.com/A v1.0.0 itself requires example.com/B v1.2.0 and example.com/C v1.0.0, then go.mod's requirement of example.com/B v1.0.0 is misleading (superseded by example.com/A's need for v1.2.0), and its requirement of example.com/C v1.0.0 is redundant (implied by example.com/A's need for the same version), so both will be removed. If the main module contains packages that directly import packages from example.com/B or example.com/C, then the requirements will be kept but updated to the actual versions being used.

Finally, the update reformats the go.mod in a canonical formatting, so that future mechanical changes will result in minimal diffs. The go command will not update go.mod if only formatting changes are needed.

Because the module graph defines the meaning of import statements, any commands that load packages also use and therefore update go.mod, including go build, go get, go install, go list, go test, go mod graph, go mod tidy, and go mod why.

The -mod=readonly flag prevents commands from automatically updating go.mod. However, if a command needs to perform an action that would update to go.mod, it will report an error. For example, if go build is asked to build a package not provided by any module in the build list, go build will report an error instead of looking up the module and updating requirements in go.mod.

Minimal version selection (MVS)

Compatibility with non-module repositories

Module-aware commands

Most go commands may run in Module-aware mode or GOPATH mode. In module-aware mode, the go command uses go.mod files to find versioned dependencies, and it typically loads packages out of the module cache, downloading modules if they are missing. In GOPATH mode, the go command ignores modules; it looks in vendor directories and in GOPATH to find dependencies.

Module-aware mode is active by default whenever a go.mod file is found in the current directory or in any parent directory. For more fine-grained control, the GO111MODULE environment variable may be set to one of three values: on, off, or auto.

• If GO111MODULE=off, the go command ignores go.mod files and runs in GOPATH mode.
• If GO111MODULE=on, the go command runs in module-aware mode, even when no go.mod file is present. Not all commands work without a go.mod file: see Module commands outside a module.
• If GO111MODULE=auto or is unset, the go command runs in module-aware mode if a go.mod file is present in the current directory or any parent directory (the default behavior).

In module-aware mode, GOPATH no longer defines the meaning of imports during a build, but it still stores downloaded dependencies (in GOPATH/pkg/mod; see Module cache) and installed commands (in GOPATH/bin, unless GOBIN is set).

Build commands

Vendoring

go get

go list -m

Usage:

go list -m [-u] [-versions] [list flags] [modules]

Example:

$ go list -m all
$ go list -m -versions example.com/m
$ go list -m -json example.com/m@latest

The -m flag causes go list to list modules instead of packages. In this mode, the arguments to go list may be modules, module patterns (containing the ... wildcard), module queries, or the special pattern all, which matches all modules in the build list. If no arguments are specified, the main module is listed.

When listing modules, the -f flag still specifies a format template applied to a Go struct, but now a Module struct:

type Module struct {
    Path      string       // module path
    Version   string       // module version
    Versions  []string     // available module versions (with -versions)
    Replace   *Module      // replaced by this module
    Time      *time.Time   // time version was created
    Update    *Module      // available update, if any (with -u)
    Main      bool         // is this the main module?
    Indirect  bool         // is this module only an indirect dependency of main module?
    Dir       string       // directory holding files for this module, if any
    GoMod     string       // path to go.mod file for this module, if any
    GoVersion string       // go version used in module
    Error     *ModuleError // error loading module
}

type ModuleError struct {
    Err string // the error itself
}

The default output is to print the module path and then information about the version and replacement if any. For example, go list -m all might print:

example.com/main/module
golang.org/x/text v0.3.0 => /tmp/text
rsc.io/pdf v0.1.1

The Module struct has a String method that formats this line of output, so that the default format is equivalent to -f '{{.String}}'.

Note that when a module has been replaced, its Replace field describes the replacement module module, and its Dir field is set to the replacement module's source code, if present. (That is, if Replace is non-nil, then Dir is set to Replace.Dir, with no access to the replaced source code.)

The -u flag adds information about available upgrades. When the latest version of a given module is newer than the current one, list -u sets the module's Update field to information about the newer module. The module's String method indicates an available upgrade by formatting the newer version in brackets after the current version. For example, go list -m -u all might print:

example.com/main/module
golang.org/x/text v0.3.0 [v0.4.0] => /tmp/text
rsc.io/pdf v0.1.1 [v0.1.2]

(For tools, go list -m -u -json all may be more convenient to parse.)

The -versions flag causes list to set the module's Versions field to a list of all known versions of that module, ordered according to semantic versioning, lowest to highest. The flag also changes the default output format to display the module path followed by the space-separated version list.

The template function module takes a single string argument that must be a module path or query and returns the specified module as a Module struct. If an error occurs, the result will be a Module struct with a non-nil Error field.

go mod download

Usage:

go mod download [-json] [-x] [modules]

Example:

$ go mod download
$ go mod download golang.org/x/mod@v0.2.0

The go mod download command downloads the named modules into the module cache. Arguments can be module paths or module patterns selecting dependencies of the main module or module queries of the form path@version. With no arguments, download applies to all dependencies of the main module.

The go command will automatically download modules as needed during ordinary execution. The go mod download command is useful mainly for pre-filling the module cache or for loading data to be served by a module proxy.

By default, download writes nothing to standard output. It prints progress messages and errors to standard error.

The -json flag causes download to print a sequence of JSON objects to standard output, describing each downloaded module (or failure), corresponding to this Go struct:

type Module struct {
    Path     string // module path
    Version  string // module version
    Error    string // error loading module
    Info     string // absolute path to cached .info file
    GoMod    string // absolute path to cached .mod file
    Zip      string // absolute path to cached .zip file
    Dir      string // absolute path to cached source root directory
    Sum      string // checksum for path, version (as in go.sum)
    GoModSum string // checksum for go.mod (as in go.sum)
}

The -x flag causes download to print the commands download executes to standard error.

go mod edit

go mod init

Usage:

go mod init [module-path]

Example:

go mod init
go mod init example.com/m

The go mod init command initializes and writes a new go.mod file in the current directory, in effect creating a new module rooted at the current directory. The go.mod file must not already exist.

init accepts one optional argument, the module path for the new module. See Module paths for instructions on choosing a module path. If the module path argument is omitted, init will attempt to infer the module path using import comments in .go files, vendoring tool configuration files, and the current directory (if in GOPATH).

If a configuration file for a vendoring tool is present, init will attempt to import module requirements from it. init supports the following configuration files.

• GLOCKFILE (Glock)
• Godeps/Godeps.json (Godeps)
• Gopkg.lock (dep)
• dependencies.tsv (godeps)
• glide.lock (glide)
• vendor.conf (trash)
• vendor.yml (govend)
• vendor/manifest (gvt)
• vendor/vendor.json (govendor)

Vendoring tool configuration files can't always be translated with perfect fidelity. For example, if multiple packages within the same repository are imported at different versions, and the repository only contains one module, the imported go.mod can only require the module at one version. You may wish to run go list -m all to check all versions in the build list, and go mod tidy to add missing requirements and to drop unused requirements.

go mod tidy

go mod verify

go clean -modcache

Module queries

Module commands outside a module

Retrieving modules

GOPROXY protocol

A module proxy is an HTTP server that can respond to GET requests for paths specified below. The requests have no query parameters, and no specific headers are required, so even a site serving from a fixed file system (including a file:// URL) can be a module proxy.

Successful HTTP responses must have the status code 200 (OK). Redirects (3xx) are followed. Responses with status codes 4xx and 5xx are treated as errors. The error codes 404 (Not Found) and 410 (Gone) indicate that the requested module or version is not available on the proxy, but it may be found elsewhere. Error responses should have content type text/plain with charset either utf-8 or us-ascii.

The go command may be configured to contact proxies or source control servers using the GOPROXY environment variable, which is a comma-separated list of URLs or the keywords direct or off (see Environment variables for details). When the go command receives a 404 or 410 response from a proxy, it falls back to later proxies in the list. The go command does not fall back to later proxies in response to other 4xx and 5xx errors. This allows a proxy to act as a gatekeeper, for example, by responding with error 403 (Forbidden) for modules not on an approved list.

The table below specifies queries that a module proxy must respond to. For each path, $base is the path portion of a proxy URL,$module is a module path, and $version is a version. For example, if the proxy URL is https://example.com/mod, and the client is requesting the go.mod file for the module golang.org/x/text at version v0.3.2, the client would send a GET request for https://example.com/mod/golang.org/x/text/@v/v0.3.2.mod.

To avoid ambiguity when serving from case-insensitive file systems, the $module and $version elements are case-encoded by replacing every uppercase letter with an exclamation mark followed by the corresponding lower-case letter. This allows modules example.com/M and example.com/m to both be stored on disk, since the former is encoded as example.com/!m.

Path	Description
$base/$module/@v/list	Returns a list of known versions of the given module in plain text, one per line. This list should not include pseudo-versions.
$base/$module/@v/$version.info	Returns JSON-formatted metadata about a specific version of a module. The response must be a JSON object that corresponds to the Go data structure below: type Info struct { Version string // version string Time time.Time // commit time } The Version field is required and must contain a valid, canonical version (see Versions). The $version in the request path does not need to be the same version or even a valid version; this endpoint may be used to find versions for branch names or revision identifiers. However, if $version is a canonical version with a major version compatible with $module, the Version field in a successful response must be the same. The Time field is optional. If present, it must be a string in RFC 3339 format. It indicates the time when the version was created. More fields may be added in the future, so other names are reserved.
$base/$module/@v/$version.mod	Returns the go.mod file for a specific version of a module. If the module does not have a go.mod file at the requested version, a file containing only a module statement with the requested module path must be returned. Otherwise, the original, unmodified go.mod file must be returned.
$base/$module/@v/$version.zip	Returns a zip file containing the contents of a specific version of a module. See Module zip format for details on how this zip file must be formatted.
$base/$module/@latest	Returns JSON-formatted metadata about the latest known version of a module in the same format as $base/$module/@v/$version.info. The latest version should be the version of the module that the go command should use if $base/$module/@v/list is empty or no listed version is suitable. This endpoint is optional, and module proxies are not required to implement it.

When resolving the latest version of a module, the go command will request $base/$module/@v/list, then, if no suitable versions are found, $base/$module/@latest. The go command prefers, in order: the semantically highest release version, the semantically highest pre-release version, and the chronologically most recent pseudo-version. In Go 1.12 and earlier, the go command considered pseudo-versions in $base/$module/@v/list to be pre-release versions, but this is no longer true since Go 1.13.

A module proxy must always serve the same content for successful responses for $base/$module/$version.mod and $base/$module/$version.zip queries. This content is cryptographically authenticated using go.sum files and, by default, the checksum database.

The go command caches most content it downloads from module proxies in its module cache in $GOPATH/pkg/mod/cache/download. Even when downloading directly from version control systems, the go command synthesizes explicit info, mod, and zip files and stores them in this directory, the same as if it had downloaded them directly from a proxy. The cache layout is the same as the proxy URL space, so serving $GOPATH/pkg/mod/cache/download at (or copying it to) https://example.com/proxy would let users access cached module versions by setting GOPROXY to https://example.com/proxy.

Communicating with proxies

Communicating with version control systems

Custom import paths

File name and path constraints

Module zip format

Private modules

Module cache

Authenticating modules

When deciding whether to trust the source code for a module version just fetched from a proxy or origin server, the go command first consults the go.sum lines in the go.sum file of the current module. If the go.sum file does not contain an entry for that module version, then it may consult the checksum database.

go.sum file format

Checksum database

The checksum database is a global source of go.sum lines. The go command can use this in many situations to detect misbehavior by proxies or origin servers.

The checksum database allows for global consistency and reliability for all publicly available module versions. It makes untrusted proxies possible since they can't serve the wrong code without it going unnoticed. It also ensures that the bits associated with a specific version do not change from one day to the next, even if the module's author subsequently alters the tags in their repository.

The checksum database is served by sum.golang.org, which is run by Google. It is a Transparent Log (or “Merkle Tree”) of go.sum line hashes, which is backed by Trillian. The main advantage of a Merkle tree is that independent auditors can verify that it hasn't been tampered with, so it is more trustworthy than a simple database.

The go command interacts with the checksum database using the protocol originally outlined in Proposal: Secure the Public Go Module Ecosystem.

The table below specifies queries that the checksum database must respond to. For each path, $base is the path portion of the checksum database URL, $module is a module path, and $version is a version. For example, if the checksum database URL is https://sum.golang.org, and the client is requesting the record for the module golang.org/x/text at version v0.3.2, the client would send a GET request for https://sum.golang.org/lookup/golang.org/x/text@v0.3.2.

To avoid ambiguity when serving from case-insensitive file systems, the $module and $version elements are case-encoded by replacing every uppercase letter with an exclamation mark followed by the corresponding lower-case letter. This allows modules example.com/M and example.com/m to both be stored on disk, since the former is encoded as example.com/!m.

Parts of the path surrounded by square brakets, like [.p/$W] denote optional values.

Path	Description
$base/latest	Returns a signed, encoded tree description for the latest log. This signed description is in the form of a note, which is text that has been signed by one or more server keys and can be verified using the server's public key. The tree description provides the size of the tree and the hash of the tree head at that size. This encoding is described in golang.org/x/mod/sumdb/tlog#FormatTree.
$base/lookup/$module@$version	Returns the log record number for the entry about $module at $version, followed by the data for the record (that is, the go.sum lines for $module at $version) and a signed, encoded tree description that contains the record.
$base/tile/$H/$L/$K[.p/$W]	Returns a [log tile](https://research.swtch.com/tlog#serving_tiles), which is a set of hashes that make up a section of the log. Each tile is defined in a two-dimensional coordinate at tile level $L, $Kth from the left, with a tile height of $H. The optional .p/$W suffix indicates a partial log tile with only $W hashes. Clients must fall back to fetching the full tile if a partial tile is not found.
$base/tile/$H/data/$K[.p/$W]	Returns the record data for the leaf hashes in /tile/$H/0/$K[.p/$W] (with a literal data path element).

If the go command consults the checksum database, then the first step is to retrieve the record data through the /lookup endpoint. If the module version is not yet recorded in the log, the checksum database will try to fetch it from the origin server before replying. This /lookup data provides the sum for this module version as well as its position in the log, which informs the client of which tiles should be fetched to perform proofs. The go command performs “inclusion” proofs (that a specific record exists in the log) and “consistency” proofs (that the tree hasn’t been tampered with) before adding new go.sum lines to the main module’s go.sum file. It's important that the data from /lookup should never be used without first authenticating it against the signed tree hash and authenticating the signed tree hash against the client's timeline of signed tree hashes.

Signed tree hashes and new tiles served by the checksum database are stored in the module cache, so the go command only needs to fetch tiles that are missing.

The go command doesn't need to directly connect to the checksum database. It can request module sums via a module proxy that mirrors the checksum database and supports the protocol above. This can be particularly helpful for private, corporate proxies which block requests outside the organization.

The GOSUMDB environment variable identifies the name of checksum database to use and optionally its public key and URL, as in:

GOSUMDB="sum.golang.org"
GOSUMDB="sum.golang.org+<publickey>"
GOSUMDB="sum.golang.org+<publickey> https://sum.golang.org"

The go command knows the public key of sum.golang.org, and also that the name sum.golang.google.cn (available inside mainland China) connects to the sum.golang.org checksum database; use of any other database requires giving the public key explicitly. The URL defaults to https:// followed by the database name.

GOSUMDB defaults to sum.golang.org, the Go checksum database run by Google. See https://sum.golang.org/privacy for the service's privacy policy.

If GOSUMDB is set to off, or if go get is invoked with the -insecure flag, the checksum database is not consulted, and all unrecognized modules are accepted, at the cost of giving up the security guarantee of verified repeatable downloads for all modules. A better way to bypass the checksum database for specific modules is to use the GOPRIVATE or GONOSUMDB environment variables. See Private Modules for details.

The go env -w command can be used to set these variables for future go command invocations.

Privacy

Environment variables

Glossary

build list: The list of module versions that will be used for a build command such as go build, go list, or go test. The build list is determined from the main module's go.mod file and go.mod files in transitively required modules using minimal version selection. The build list contains versions for all modules in the module graph, not just those relevant to a specific command.

canonical version: A correctly formatted version without a build metadata suffix other than +incompatible. For example, v1.2.3 is a canonical version, but v1.2.3+meta is not.

go.mod file: The file that defines a module's path, requirements, and other metadata. Appears in the module's root directory. See the section on go.mod files.

import path: A string used to import a package in a Go source file. Synonymous with package path.

main module: The module in which the go command is invoked.

major version: The first number in a semantic version (1 in v1.2.3). In a release with incompatible changes, the major version must be incremented, and the minor and patch versions must be set to 0. Semantic versions with major version 0 are considered unstable.

major version suffix: A module path suffix that matches the major version number. For example, /v2 in example.com/mod/v2. Major version suffixes are required at v2.0.0 and later and are not allowed at earlier versions. See the section on Major version suffixes.

minimal version selection (MVS): The algorithm used to determine the versions of all modules that will be used in a build. See the section on Minimal version selection for details.

minor version: The second number in a semantic version (2 in v1.2.3). In a release with new, backwards compatible functionality, the minor version must be incremented, and the patch version must be set to 0.

module: A collection of packages that are released, versioned, and distributed together.

module cache: A local directory storing downloaded modules, located in GOPATH/pkg/mod. See Module cache.

module graph: The directed graph of module requirements, rooted at the main module. Each vertex in the graph is a module; each edge is a version from a require statement in a go.mod file (subject to replace and exclude statements in the main module's go.mod file.

module path: A path that identifies a module and acts as a prefix for package import paths within the module. For example, "golang.org/x/net".

module proxy: A web server that implements the GOPROXY protocol. The go command downloads version information, go.mod files, and module zip files from module proxies.

module root directory: The directory that contains the go.mod file that defines a module.

package: A collection of source files in the same directory that are compiled together. See the Packages section in the Go Language Specification.

package path: The path that uniquely identifies a package. A package path is a module path joined with a subdirectory within the module. For example "golang.org/x/net/html" is the package path for the package in the module "golang.org/x/net" in the "html" subdirectory. Synonym of import path.

patch version: The third number in a semantic version (3 in v1.2.3). In a release with no changes to the module's public interface, the patch version must be incremented.

pre-release version: A version with a dash followed by a series of dot-separated identifiers immediately following the patch version, for example, v1.2.3-beta4. Pre-release versions are considered unstable and are not assumed to be compatible with other versions. A pre-release version sorts before the corresponding release version: v1.2.3-pre comes before v1.2.3. See also release version.

pseudo-version: A version that encodes a revision identifier (such as a Git commit hash) and a timestamp from a version control system. For example, v0.0.0-20191109021931-daa7c04131f5. Used for compatibility with non-module repositories and in other situations when a tagged version is not available.

release version: A version without a pre-release suffix. For example, v1.2.3, not v1.2.3-pre. See also pre-release version.

version: An identifier for an immutable snapshot of a module, written as the letter v followed by a semantic version. See the section on Versions.