go-githubapp

A library for building GitHub Apps and other services that handle GitHub webhooks.

The library provides an http.Handler implementation that dispatches webhook events to the correct place, removing boilerplate and letting you focus on the logic of your application.

Usage
- Examples
- Dependencies
Structured Logging
GitHub Clients
- Metrics
Background Jobs and Multi-Organization Operations
OAuth2
Stability and Versioning Guarantees
Contributing

Usage

Most users will implement githubapp.EventHandler for each webhook event that needs to be handled. A single implementation can also respond to multiple event types if they require the same actions:

type CommentHandler struct {
    githubapp.ClientCreator
}

func (h *CommentHandler) Handles() []string {
    return []string{"issue_comment"}
}

func (h *CommentHandler) Handle(ctx context.Context, eventType, deliveryID string, payload []byte) error {
    // from github.com/google/go-github/github
    var event github.IssueCommentEvent
    if err := json.Unmarshal(payload, &event); err != nil {
        return err
    }

    // do something with the content of the event
}

We recommend embedding githubapp.ClientCreator in handler implementations as an easy way to access GitHub clients.

Once you define handlers, register them with an event dispatcher and associate it with a route in any net/http-compatible HTTP router:

func registerRoutes(c githubapp.Config) {
    cc := githubapp.NewDefaultCachingClientCreator(c)

    http.Handle("/api/github/hook", githubapp.NewDefaultEventDispatcher(c,
        &CommentHandler{cc},
        // ...
    ))
}

Examples

The example package contains a fully functional server using go-githubapp. The example app responds to comments on pull requests by commenting with a copy of the comment body.

To run the app, update example/config.yml with appropriate secrets and then run:

./godelw dep
./godelw run example

Dependencies

go-githubapp has minimal dependencies, but does make some decisions:

rs/zerolog for logging
rcrowley/go-metrics for metrics
google/go-github for v3 (REST) API client
shurcooL/githubv4 for v4 (GraphQL) API client

Logging and metrics are only active when they are configured (see below). This means you can add your own logging or metrics libraries without conflict, but will miss out on the free built-in support.

Structured Logging

go-githubapp uses rs/zerolog for structured logging. A logger must be stored in the context.Context associated with each http.Request:

func (d *EventDispatcher) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    logger := zerolog.Ctx(r.Context())
    ...
}

If there is no logger in the context, log output is disabled. It's expected that HTTP middleware, like that provided by the hlog package, configures the http.Request context automatically.

Below are the standard keys used when logging events. They are also exported as constants.

exported constant	key	definition
`LogKeyEventType`	`github_event_type`	the github event type header
`LogKeyDeliveryID`	`github_delivery_id`	the github event delivery id header
`LogKeyInstallationID`	`github_installation_id`	the installation id the app is authenticating with
`LogKeyRepositoryName`	`github_repository_name`	the repository name of the pull request being acted on
`LogKeyRepositoryOwner`	`github_repository_owner`	the repository owner of the pull request being acted on
`LogKeyPRNum`	`github_pr_num`	the number of the pull request being acted on

Where appropriate, the library creates derived loggers with the above keys set to the correct values.

GitHub Clients

Authenticated and configured GitHub clients can be retrieved from githubapp.ClientCreator implementations. The library provides a basic implementation and a caching version.

There are three types of clients and two API versions for a total of six distinct clients:

An application client authenticates as the application and can only call limited APIs that mostly return application metadata.
An installation client authenticates as an installation and can call any APIs where the has been installed and granted permissions.
A token client authenticates with a static OAuth2 token associated with a user account.

go-githubapp also exposes various configuration options for GitHub clients. These are provided when calling githubapp.NewClientCreator:

githubapp.WithClientUserAgent sets a User-Agent string for all clients
githubapp.WithClientCaching enables response caching for all v3 (REST) clients. The cache can be configured to always validate responses or to respect the cache headers returned by GitHub. Re-validation is useful if data often changes faster than the requested cache duration.
githubapp.WithClientMiddleware allows customization of the http.RoundTripper used by all clients and is useful if you want to log requests or emit metrics about GitHub requests and responses.

The library provides the following middleware:

githubapp.ClientMetrics emits the standard metrics described below
githubapp.ClientLogging logs metadata about all requests and responses

baseHandler, err := githubapp.NewDefaultCachingClientCreator(
    config.Github,
    githubapp.WithClientUserAgent("example-app/1.0.0"),
    githubapp.WithClientCaching(false, func() httpcache.Cache { return httpcache.NewMemoryCache() }),
    githubapp.WithClientMiddleware(
        githubapp.ClientMetrics(registry),
        githubapp.ClientLogging(zerolog.DebugLevel),
    ),
    ...
)

Metrics

go-githubapp uses rcrowley/go-metrics to provide metrics. GitHub clients emit the metrics below if configured with the githubapp.ClientMetrics middleware.

metric name	type	definition
`github.requests`	`counter`	the count of successfully completed requests made to GitHub
`github.requests.2xx`	`counter`	like `github.requests`, but only counting 2XX status codes
`github.requests.3xx`	`counter`	like `github.requests`, but only counting 3XX status codes
`github.requests.4xx`	`counter`	like `github.requests`, but only counting 4XX status codes
`github.requests.5xx`	`counter`	like `github.requests`, but only counting 5XX status codes
`github.requests.cached`	`counter`	the count of successfully cached requests
`github.rate.limit[installation:<id>]`	`gauge`	the maximum number of requests permitted to make per hour, tagged with the installation id
`github.rate.remaining[installation:<id>]`	`gauge`	the number of requests remaining in the current rate limit window, tagged with the installation id

Note that metrics need to be published in order to be useful. Several publishing options are available or you can implement your own.

Background Jobs and Multi-Organization Operations

While applications will mostly operate on the installation IDs provided in webhook payloads, sometimes there is a need to run background jobs or make API calls against multiple organizations. In these cases, use an application client to look up specific installations of the application and then construct an installation client to make API calls:

func getOrganizationClient(cc githubapp.ClientCreator, org name) (*github.Client, error) {
    // create a client to perform actions as the application
    appClient, err := cc.NewAppClient()
    if err != nil {
        return nil, err
    }

    // look up the installation ID for a particular organization
    installations := githubapp.NewInstallationsService(app)
    install := installations.GetByOwner(context.Background(), org)

    // create a client to perform actions on that specific organization
    return cc.NewInstallationClient(install.ID)
}

OAuth2

The oauth2 package provides an http.Handler implementation that simplifies OAuth2 authentication with GitHub. When a user visits the endpoint, they are redirected to GitHub to authorize the application. GitHub redirects back to the same endpoint, which performs the code exchange and obtains a token for the user. The token is passed to a callback for further processing.

func registerOAuth2Handler(c githubapp.Config) {
    http.Handle("/api/auth/github", oauth2.NewHandler(
        oauth2.GetConfig(c, []string{"user:email"}),
        // force generated URLs to use HTTPS; useful if the app is behind a reverse proxy
        oauth2.ForceTLS(true),
        // set the callback for successful logins
        oauth2.OnLogin(func(w http.ResponseWriter, r *http.Request, login *oauth2.Login) {
            // look up the current user with the authenticated client
            client := github.NewClient(login.Client)
            user, _, err := client.Users.Get(r.Context(), "")
            // handle error, save the user, ...

            // redirect the user back to another page
            http.Redirect(w, r, "/dashboard", http.StatusFound)
        }),
    ))
}

Production applications should also use the oauth2.WithStore option to set a secure StateStore implementation. oauth2.SessionStateStore is a good choice that uses alexedwards/scs to store the state in a session.

Customizing Webhook Responses

For most applications, the default responses should be sufficient: they use correct status codes and include enough information to match up GitHub delivery records with request logs. If your application has additional requirements for responses, two methods are provided for customization:

Error responses can be modified with a custom error callback. Use the WithErrorCallback option when creating an event dispatcher.
Non-error responses can be modified with a custom response callback. Use the WithResponseCallback option when creating an event dispatcher.
Individual hook responses can be modified by calling the SetResponder function before the handler returns. Note that if you register a custom response handler as described above, you must make it aware of handler-level responders if you want to keep using SetResponder. See the default response callback for an example of how to implement this.

Stability and Versioning Guarantees

While we've used this library to build multiple applications internally, there's still room for API tweaks and improvements as we find better ways to solve problems. These will be backwards compatible when possible and should require only minor changes when not.

Releases will be tagged periodically and will follow semantic versioning, with new major versions tagged after any backwards-incompatible changes. Still, we recommend vendoring this library to avoid surprises.

In general, fixes will only be applied to trunk and future releases, not backported to older versions.

Contributing

Contributions and issues are welcome. For new features or large contributions, we prefer discussing the proposed change on a GitHub issue prior to a PR.

New functionality should avoid adding new dependencies if possible and should be broadly useful. Feature requests that are specific to certain uses will likely be declined unless they can be redesigned to be generic or optional.

Before submitting a pull request, please run tests and style checks:

./godelw verify

License

This library is made available under the Apache 2.0 License.

suthsc/go-githubapp