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".
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.
v1.2.3-pre comes before v1.2.3.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.
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.
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:
https://corp.example.com/ (in parallel):golang.org/x/net/htmlgolang.org/x/netgolang.org/xgolang.orghttps://proxy.golang.org/, if all requests to https://corp.example.com/ have failed with 404 or 410:golang.org/x/net/htmlgolang.org/x/netgolang.org/xgolang.orgAfter 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 filesgo.mod file formatgo mod downloadgo mod verifygo mod editgo clean -modcacheGOPROXY protocolA 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.
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.
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.
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.
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.
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 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.