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+# Dependency Management in Go
+
+Go takes an unusual approach to dependency management, in that it is
+source-based instead of artifact-based. In an artifact-based dependency
+management system, packages consist of artifacts generated from source code and
+are stored in a separate repository system from source code. For example, many
+NodeJS packages use `npmjs.org` as a package repository and `github.com` as a
+source repository. On the other hand, packages in Go *are* source code and
+releasing a package does not involve artifact generation or a separate
+repository. Go packages must be stored in a version control repository on a VCS
+server. Dependencies are fetched directly from their VCS server or via an
+intermediary proxy which itself fetches them from their VCS server.
+
+## Versioning
+
+Go 1.11 introduced modules and first-class package versioning to the Go ecosystem.
+Prior to this, Go did not have any well-defined mechanism for version management.
+While 3rd party version management tools existed, the default Go experience had
+no support for versioning.
+
+Go modules use [semantic versioning](https://semver.org). The versions of a
+module are defined as VCS (version control system) tags that are valid semantic
+versions prefixed with `v`. For example, to release version `1.0.0` of
+`gitlab.com/my/project`, the developer must create the Git tag `v1.0.0`.
+
+For major versions other than 0 and 1, the module name must be suffixed with
+`/vX` where X is the major version. For example, version `v2.0.0` of
+`gitlab.com/my/project` must be named and imported as
+`gitlab.com/my/project/v2`.
+
+Go uses 'pseudo-versions', which are special semantic versions that reference a
+specific VCS commit. The prerelease component of the semantic version must be or
+end with a timestamp and the first 12 characters of the commit identifier:
+
+- `vX.0.0-yyyymmddhhmmss-abcdefabcdef`, when no earlier tagged commit exists for X.
+- `vX.Y.Z-pre.0.yyyymmddhhmmss-abcdefabcdef`, when most recent prior tag is vX.Y.Z-pre.
+- `vX.Y.(Z+1)-0.yyyymmddhhmmss-abcdefabcdef`, when most recent prior tag is vX.Y.Z.
+
+If a VCS tag matches one of these patterns, it is ignored.
+
+For a complete understanding of Go modules and versioning, see [this series of
+blog posts](https://blog.golang.org/using-go-modules) on the official Go
+website.
+
+## 'Module' vs 'Package'
+
+- A package is a folder containing `*.go` files.
+- A module is a folder containing a `go.mod` file.
+- A module is *usually* also a package, that is a folder containing a `go.mod`
+  file and `*.go` files.
+- A module may have subdirectories, which may be packages.
+- Modules usually come in the form of a VCS repository (Git, SVN, Hg, and so on).
+- Any subdirectories of a module that themselves are modules are distinct,
+  separate modules and are excluded from the containing module.
+  - Given a module `repo`, if `repo/sub` contains a `go.mod` file then
+    `repo/sub` and any files contained therein are a separate module and not a
+    part of `repo`.
+
+## Naming
+
+The name of a module or package, excluding the standard library, must be of the
+form `(sub.)*domain.tld(/path)*`. This is similar to a URL, but is not a URL.
+The package name does not have a scheme (such as `https://`) and cannot have a
+port number. `example.com:8443/my/package` is not a valid name.
+
+## Fetching Packages
+
+Prior to Go 1.12, the process for fetching a package was as follows:
+
+1. Query `https://{package name}?go-get=1`.
+1. Scan the response for the `go-import` meta tag.
+1. Fetch the repository indicated by the meta tag using the indicated VCS.
+
+The meta tag should have the form `<meta name="go-import" content="{prefix}
+{vcs} {url}">`. For example, `gitlab.com/my/project git
+https://gitlab.com/my/project.git` indicates that packages beginning with
+`gitlab.com/my/project` should be fetched from
+`https://gitlab.com/my/project.git` using Git.
+
+## Fetching Modules
+
+Go 1.12 introduced checksum databases and module proxies.
+
+### Checksums
+
+In addition to `go.mod`, a module will have a `go.sum` file. This file records a
+SHA-256 checksum of the code and the `go.mod` file of every version of every
+dependency that is referenced by the module or one of the module's dependencies.
+Go continually updates `go.sum` as new dependencies are referenced.
+
+When Go fetches the dependencies of a module, if those dependencies already have
+an entry in `go.sum`, Go will verify the checksum of these dependencies. If the
+checksum does not match what is in `go.sum`, the build will fail. This ensures
+that a given version of a module cannot be changed by its developers or by a
+malicious party without causing build failures.
+
+Go 1.12+ can be configured to use a checksum database. If configured to do so,
+when Go fetches a dependency and there is no corresponding entry in `go.sum`, Go
+will query the configured checksum database(s) for the checksum of the
+dependency instead of calculating it from the downloaded dependency. If the
+dependency cannot be found in the checksum database, the build will fail. If the
+downloaded dependency's checksum does not match the result from the checksum
+database, the build will fail. The following environment variables control this:
+
+- `GOSUMDB` identifies the name, and optionally the public key and server URL,
+  of the checksum database to query.
+  - A value of `off` will entirely disable checksum database queries.
+  - Go 1.13+ uses `sum.golang.org` if `GOSUMDB` is not defined.
+- `GONOSUMDB` is a comma-separated list of module suffixes that checksum
+  database queries should be disabled for. Wildcards are supported.
+- `GOPRIVATE` is a comma-separated list of module names that has the same
+  function as `GONOSUMDB` in addition to disabling other features.
+
+### Proxies
+
+Go 1.12+ can be configured to fetch modules from a Go proxy instead of directly
+from the module's VCS. If configured to do so, when Go fetches a dependency, it
+attempts to fetch the dependency from the configured proxies, in order. The
+following environment variables control this:
+
+- `GOPROXY` is a comma-separated list of module proxies to query.
+  - A value of `direct` will entirely disable module proxy queries.
+  - If the last entry in the list is `direct`, Go will fall back to the process
+    described [above](#fetching-packages) if none of the proxies can provide the
+    dependency.
+  - Go 1.13+ uses `proxy.golang.org,direct` if `GOPROXY` is not defined.
+- `GONOPROXY` is a comma-separated list of module suffixes that should be
+  fetched directly and not from a proxy. Wildcards are supported.
+- `GOPRIVATE` is a comma-separated list of module names that has the same
+  function as `GONOPROXY` in addition to disabling other features.
+
+### Fetching
+
+From Go 1.12 onward, the process for fetching a module or package is as follows:
+
+1. If `GOPROXY` is a list of proxies and the module is not excluded by
+   `GONOPROXY` or `GOPRIVATE`, query them in order, and stop at the first valid
+   response.
+1. If `GOPROXY` is `direct`, or the module is excluded, or `GOPROXY` ends with
+   `,direct` and no proxy provided the module, fall back.
+   1. Query `https://{module or package name}?go-get=1`.
+   1. Scan the response for the `go-import` meta tag.
+   1. Fetch the repository indicated by the meta tag using the indicated VCS.
+   1. If the `{vcs}` field is `mod`, the URL should be treated as a module proxy instead of a VCS.
+1. If the module is being fetched directly and not as a dependency, stop.
+1. If `go.sum` contains an entry corresponding to the module, validate the checksum and stop.
+1. If `GOSUMDB` identifies a checksum database and the module is not excluded by
+   `GONOSUMDB` or `GOPRIVATE`, retrieve the module's checksum, add it to
+   `go.sum`, and validate the downloaded source against it.
+1. If `GOSUMDB` is `off` or the module is excluded, calculate a checksum from
+   the downloaded source and add it to `go.sum`.
+
+The downloaded source must contain a `go.mod` file. The `go.mod` file must
+contain a `module` directive that specifies the name of the module. If the
+module name as specified by `go.mod` does not match the name that was used to
+fetch the module, the module will fail to compile.
+
+If the module is being fetched directly and no version was specified, or if the
+module is being added as a dependency and no version was specified, Go uses the
+most recent version of the module. If the module is fetched from a proxy, Go
+queries the proxy for a list of versions and chooses the latest. If the module is
+fetched directly, Go queries the repository for a list of tags and chooses the
+latest that is also a valid semantic version.
+
+## Authenticating
+
+In versions prior to Go 1.13, support for authenticating requests made by Go was
+somewhat inconsistent. Go 1.13 improved support for `.netrc` authentication. If
+a request is made over HTTPS and a matching `.netrc` entry can be found, Go will
+add HTTP Basic authentication credentials to the request. Go will not
+authenticate requests made over HTTP. Go will reject HTTP-only entries in
+`GOPROXY` that have embedded credentials.
+
+In a future version, Go may add support for arbitrary authentication headers.
+Follow [golang/go#26232](https://github.com/golang/go/issues/26232) for details.