Specify dependencies

Apps that run in the Go 1.12+ standard runtime can use any linux/amd64-compatible package.

App Engine requires all import statements in your Go code to specify absolute paths. For example, use:

import "github.com/example/mypackage"

Use Go modules

We recommend that you use Go modules to manage dependencies in your Go app. You can continue to use the older GOPATH mode if you aren't ready to migrate to Go modules.

Starting in Go version 1.22 and later:

You can't use go get outside of a module in the legacy GOPATH mode (GO111MODULE=off). For more information, see Tools.
Go recommends that you use a go.mod file for managing dependencies. To install dependencies during deployment, include a go.mod file in the same folder as the app.yaml file. For more information about Go versions, and managing dependencies for vendor directories, see GOPATH and Modules.

When you deploy your app, App Engine uses the go build command to build your app and matches the behavior of Go itself. To ensure that your app uses module-aware mode, do the following in your development environment:

Create your module's go.mod file in the same directory as your app.yaml file. App Engine searches the current directory and the successive parent directories until it finds a go.mod file.

If App Engine doesn't find a go.mod file, it follows GOPATH mode.
If you set the GO111MODULE environment variable, make sure that the variable's value enables module-aware mode. When you deploy your app, App Engine checks your environment for GO111MODULE and matches the behavior of Go itself. App Engine only applies the GO111MODULE variable setting if you have included a go.mod file for your app.
For Go 1.12, don't locate your app directory within the $GOPATH/src folder. If your Go 1.12 app is anywhere in the $GOPATH/src directory tree, App Engine follows GOPATH mode even if you've defined a go.mod file for your app.

For Go 1.13 and later, App Engine uses module-aware mode by default, unless GO111MODULE overrides the default or a go.mod file doesn't exist in the app's directory.

Vendoring dependencies

Vendoring copies the packages your app uses into the application directory instead of downloading modules from their sources during the build process. Go provides the go build command to vendor the packages your app needs into a directory named vendor in your app's root directory.

In Go 1.14 and later, if your app's root directory contains a directory named vendor, the go build command and the App Engine deployment process use the packages in the vendor directory instead of downloading modules.

In Go 1.13 and earlier, vendoring is only available if you set up your environment to use GOPATH mode.

Use private dependencies

App Engine can't download your private dependencies during the build process, so you must include the dependencies with your application code upon deployment.

Use the replace directive in your go.mod file to declare private dependencies. The following example assumes your app is in the /myapp/ directory:

Change to your app directory:
```
cd /myapp
```
Create a directory containing your private dependencies:
```
mkdir private
```
Make sure your private dependency is in the private directory. One approach is by creating a symlink:
```
mkdir private/private.example.com
ln -s /path/to/private.example.com/foo private/private.example.com/foo
```

Update your go.mod file to use the replace directive to use the private directory for your dependency:

go mod edit -replace=private.example.com/foo=./private/private.example.com/foo

Your go.mod file should now look like:

Final `go.mod` file

module private.example.com/myapp

require private.example.com/foo v1.2.3

replace private.example.com/foo => ./private/private.example.com/foo

Original `go.mod` file

module private.example.com/myapp

require private.example.com/foo v1.2.3

Don't modify how you import and use your private package. Your import statement should look like this:
```
import "private.example.com/foo"
```
Include your private dependency in your deployment by deploying your app:
```
gcloud app deploy
```

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