To support server-to-server interactions, first create a service account for your project in the API Console. If you want to access user data for users in your G Suite domain, then delegate domain-wide access to the service account.
Then, your application prepares to make authorized API calls by using the service account's credentials to request an access token from the OAuth 2.0 auth server.
Finally, your application can use the access token to call Google APIs.
The Google OAuth 2.0 system supports server-to-server interactions such as those between a web application and a Google service. For this scenario you need a service account, which is an account that belongs to your application instead of to an individual end user. Your application calls Google APIs on behalf of the service account, so users aren't directly involved. This scenario is sometimes called "two-legged OAuth," or "2LO." (The related term "three-legged OAuth" refers to scenarios in which your application calls Google APIs on behalf of end users, and in which user consent is sometimes required.)
Typically, an application uses a service account when the application uses Google APIs to work with its own data rather than a user's data. For example, an application that uses Google Cloud Datastore for data persistence would use a service account to authenticate its calls to the Google Cloud Datastore API.
G Suite domain administrators can also grant service accounts domain-wide authority to access user data on behalf of users in the domain.
This document describes how an application can complete the server-to-server OAuth 2.0 flow by using either a Google APIs client library (recommended) or HTTP.
