How to use gargle for auth in a client package

gargle provides common infrastructure for use with Google APIs. This vignette describes one possible design for using gargle to deal with auth, in a client package that provides a high-level wrapper for a specific API.

There are frequent references to (the development version of) googledrive, which is a functioning test bed. The released version of googledrive already presents a very similar interface to users, but using internal functions. The release of gargle means these functions now exist in gargle, where they can be shared across packages, and the redundant versions in googledrive can be removed. The auth approach described here is already used in bigrquery (v1.2.0 and higher) and the (GitHub-only) googlesheets4 package (the successor to googlesheets). gmailr is also slated for retrofitting.

Key choices

Getting a token requires several pieces of information and there are stark differences in how much users (need to) know or control about this process. Let’s review them, with an eye towards identifying the responsibilities of the package author versus the user.

User-facing auth

In googledrive, the main user-facing auth function is googledrive::drive_auth(). Here is its definition (at least approximately, remember this is static code):

drive_auth() is called automatically upon the first need of a token and that can lead to user interaction, but does not necessarily do so. token_fetch() is described in the vignette How gargle gets tokens. The internal .auth object maintains googledrive’s auth state and is explained next.

Auth state

A client package can use an internal object of class gargle::AuthClass to hold the auth state. Here’s how it is initialized in googledrive:

The Oauth app and api_key are configurable by the user and, when NULL, downstream functions can fall back to internal credentials. The cred field is populated by the first call to drive_auth() (direct or indirectly via drive_token()).

OAuth app

Most users should present OAuth user credentials to Google APIs. However, most users can also be spared the fiddly details surrounding this. The OAuth app is one example. The app is a component that most users do not even know about and they are content to use the same app for all work through a client package: possibly, the app built into the package.

There is a field in the .auth auth state to hold the OAuth app. Exported auth helpers, drive_oauth_app() and drive_auth_configure(), retrieve and modify the current app to support users ready to take that level of control.

Do not “borrow” an OAuth app (OAuth client ID and secret) from gargle or any other package; always use credentials associated with your package or provided by your user. Per the Google User Data Policy https://developers.google.com/terms/api-services-user-data-policy, your application must accurately represent itself when authenticating to Google API services.

API key

Some Google APIs can be used in an unauthenticated state, if and only if requests include an API key. For example, this is a great way to read a Google Sheet that is world-readable or readable by “anyone with a link” from a Shiny app, thereby designing away the need to manage user credentials on the server.

The user can provide their own API key via drive_auth_configure() and retrieve that value with drive_api_key(), just like the OAuth app. The API key is stored in the api_key field of the .auth auth state.

Many users aren’t motivated to take this level of control and appreciate when a package provides a built-in default API key. As with the app, packages should obtain their own API key and not borrow the gargle or tidyverse key.

Some APIs are not usable without a token, in which case a wrapper package may not even expose functionality for managing an API key. Among the packages mentioned as examples, this is true of bigrquery.

Email or Google identity

In contrast to the OAuth app and API key, every user must express which identity they wish to present to the API. This is a familiar concept and users expect to specify this. Since users may have more than one Google account, it’s quite likely that they will want to switch between accounts, even within a single R session, or that they might want to explicitly declare the identity to be used in a specific script or app.

That explains why drive_auth() has the optional email argument that lets users proactively specify their identity. drive_auth() is usually called indirectly upon first need, but a user can also call it proactively in order to specify their target email:

If email is not given, gargle also checks for an option named “gargle_oauth_email”. The email is used to look up tokens in the cache and, if no suitable token is found, it is used to pre-configure the OAuth chooser in the browser. Read more in the help for gargle::gargle_oauth_email().

Scopes

Most users have no concept of scopes. They just know they want to work with, e.g., Google Drive or Google Sheets. A client package can usually pick sensible default scopes, that will support what most users want to do.

Here’s a reminder of the signature of googledrive::drive_auth():

googledrive ships with a default scope, but a motivated user could call drive_auth() pre-emptively at the start of the session and request different scopes. For example, if they intend to only read data and want to guard against inadvertent file modification, they might opt for the drive.readonly scope.

OAuth cache and Out-of-bound auth

The location of the token cache and whether to prefer out-of-bound auth are two aspects of OAuth where most users are content to go along with sensible default behaviour. For those who want to exert control, that can be done in direct calls to drive_auth() or by configuring an option. Read the help for gargle::gargle_oauth_cache() and gargle::gargle_oob_default() for more about these options.

Overview of mechanics

Here’s a concrete outline of how one could set up a client package to get its auth functionality from gargle.

  1. Add gargle to your package’s Imports.
  2. Create a file R/YOURPKG_auth.R.
  3. Create an internal gargle::AuthClass object to hold auth state. R/YOURPKG_auth.R is a good place to do this.
  4. Define standard functions for the auth interface between gargle and your package; do this in R/YOURPKG_auth.R. Examples: tidyverse/googledrive/R/drive_auth.R and r-dbi/bigrquery/R/bq_auth.R.
  5. Use gargle’s roxygen helpers to create the docs for your auth functions. This relieves you from writing docs and you inherit standard wording. See previously cited examples for inspiration.
  6. Use the functions YOURPKG_token() and YOURPKG_api_key() (defined in the standard auth interface) to insert a token or API key in your package’s requests.

Getting that first token

I focus on early use, by the naive user, with the OAuth flow. When the user first calls a high-level googledrive function such as drive_find(), a Drive request is ultimately generated with a call to googledrive::request_generate(). Here is its definition, at least approximately:

# googledrive::
request_generate <- function(endpoint = character(),
                             params = list(),
                             key = NULL,
                             token = drive_token()) {
  ept <- .endpoints[[endpoint]]
  if (is.null(ept)) {
    stop_glue("\nEndpoint not recognized:\n  * {endpoint}")
  }

  ## modifications specific to googledrive package
  params$key <- key %||% params$key %||%
    drive_api_key() %||% <BUILT_IN_DEFAULT_API_KEY>
  if (!is.null(ept$parameters$supportsTeamDrives)) {
    params$supportsTeamDrives <- TRUE
  }

  req <- gargle::request_develop(endpoint = ept, params = params)
  gargle::request_build(
    path = req$path,
    method = req$method,
    params = req$params,
    body = req$body,
    token = token
  )
}

googledrive::request_generate() is a thin wrapper around gargle::request_develop() and gargle::request_build() that only implements details specific to googledrive, before delegating to more general functions in gargle. The vignette Request Helper Functions documents these gargle functions.

googledrive::request_generate() gets a token with drive_token(), which is defined like so:

# googledrive::
drive_token <- function() {
  if (isFALSE(.auth$auth_active)) {
    return(NULL)
  }
  if (!drive_has_token()) {
    drive_auth()
  }
  httr::config(token = .auth$cred)
}

where drive_has_token() in a helper defined as:

# googledrive::
drive_has_token <- function() {
  inherits(.auth$cred, "Token2.0")
}

By default, auth is active, and, for a fresh start, we won’t have a token stashed in .auth yet. So this will result in a call to drive_auth() to obtain a credential, which is then cached in .auth$cred for the remainder of the session. All subsequent calls to drive_token() will just spit back this token.

Above, we discussed scenarios where an advanced user might call drive_auth() proactively, with non-default arguments, possibly even loading a service token or using alternative flows, like Application Default Credentials or a Google Cloud Engine flow. Any token loaded in that way is stashed in .auth$cred and will be returned by subsequent calls to drive_token().

Multiple gargle-using packages can use a shared token by obtaining a suitably scoped token with one package, then registering that token with the other packages. For example, the default scope requested by googledrive is also sufficient for operations available in googlesheets4. You could use a shared token like so:

library(googledrive)
library(googlesheets4)

drive_auth(email = "jane_doe@example.com") # gets a suitably scoped token
                                           # and stashes for googledrive use

sheets_auth(token = drive_token())         # registers token with googlesheets4

# now work with both packages freely ...

It is important to make sure that the token-requesting package (googledrive, above) is using an OAuth app (client ID and secret) for which all the necessary APIs and scopes are enabled.

Auth interface

The exported functions like drive_auth(), drive_token(), etc. constitute the auth interface between googledrive and gargle and are centralized in tidyverse/googledrive/R/drive_auth.R. That is a good template for how to use gargle to manage auth in a client package. In addition, the docs for these gargle-backed functions are generated automatically from standard information maintained in the gargle package.

De-auth

APIs split into two classes: those that can be used, at least partially, without a token and those that cannot. If an API is usable without a token – which is true for the Drive API – then requests must include an API key. Therefore, the auth design for a client package is different for these two types of APIs.

For an API that can be used without a token: drive_deauth() can be used at any time to enter a de-authorized state. It sets auth_active to FALSE and .auth$cred to NULL. In this state, requests are sent out with an API key and no token. This is a great way to eliminate any friction re: auth if there’s no need for it, i.e. if all requests are for resources that are world readable or available to anyone who knows how to ask for it, such as files shared via “Anyone with the link”. The de-authorized state is especially useful in non-interactive settings or where user interaction is indirect, such as via Shiny.

For an API that cannot be used without a token: BigQuery is an example of such an API. bq_deauth() just clears the current token, so that the auth flow starts over the next time a token is needed.

BYOAK = Bring Your Own App and Key

Advanced users can use their own OAuth app and API key. drive_auth_configure() lives in R/drive_auth() and it provides the ability to modify the current app and api_key. Recall that drive_oauth_app() and drive_api_key() also exist for targeted, read-only access.

The vignette How to get your own API credentials" describes how to an API key and OAuth app.

Packages that always send token will omit the API key functionality here.

Changing identities (and more)

One reason for a user to call drive_auth() directly and proactively is to switch from one Google identity to another or to make sure they are presenting themselves with a specific identity. drive_auth() accepts an email argument, which is honored when gargle determines if there is already a suitable token on hand. Here is a sketch of how a user could switch identities during a session, possibly non-interactive:

library(googledrive)

drive_auth(email = "janedoe_work@gmail.com")
# do stuff with Google Drive here, with Jane Doe's "work" account

drive_auth(email = "janedoe_personal@gmail.com")
# do other stuff with Google Drive here, with Jane Doe's "personal" account

drive_auth(path = "/path/to/a/service-account.json")
# do other stuff with Google Drive here, using a service account