Hello there! 👋
I would like to present a complete guide for Salesforce to Google REST API Integration.
Below you can find the step-by-step configuration.
No more talk. Let's begin!

Architecture

Custom App

Google

Goal: Enabling custom client applications access to online resources needed by those applications.

create a project, enable API, configure OAuth consent screen and set scopes, create credentials.

1. Go to Manage resources,
   1. Sign in with Google Account.
   2. Create a new project. - CREATE PROJECT button.
2. Go to API Library.
   1. Make sure that previously created project is selected (picklist next to the Google Cloud logo).
   2. Click the needed API card.
   3. IMPORTANT! Click TRY THIS API. It redirects you to complete documentation about the selected endpoint. Here you can find HTTP Method, HTTP URL and required OAuth 2.0 Scopes. Keep this page for the future use.
   4. When you are fine with all permissions enable API by hitting the ENABLE button on the previous page.
3. Go to OAuth consent screen.
   1. Execute all steps.
   2. Add all needed scopes for the selected Endpoint. Get scopes from step 2.3. Note! Be careful, add only necessary access! Scope defines what type of access your app can guarantee.
4. If you added sensitive scopes, you need to go through the verification. You can do it with the PREPARE FOR VERIFICATION button. It's not necessary to accomplish our tutorial, let's skip it for now.
5. Go to Credentials.
   1. Hit + CREATE CREDENTIALS. Choose Create OAuth client ID.
   2. Set Application type as Web application.
   3. Skip URIs configuration just for now.
6. You should see Client ID and Client Secret. Note! Copy and save it for future use.

Auth. Provider

Salesforce

Goal: Allow your Salesforce org to access protected third-party data on behalf of users.

configure Auth. Provider, get callback URL.

Configure a Salesforce Authentication Provider

1. Setup > Auth. Providers > New

2. Hit Save.
3. Copy the Callback URL from the Salesforce Configuration section.
4. Go to Credentials and edit the previously added OAuth 2.0 Client ID (Step Custom App#5).
5. Add copied Callback URL to the Authorised redirect URLs section.

Named Credentials

Salesforce

Goal: Specifies the URL of a callout endpoint and its required authentication parameters in one definition.

configure Named Credentials with Auth. Provider

Legacy Named Credentials

Creaate a Legacy Named Credentials

1. Setup > Named Credentials > Arrow next to New > New Legacy
2. Complete the configuration form

3. Save

New Named Credentials

External Credenitials

Create and Edit an External Credential

1. Setup > Named Credentials
2. Choose the External Credentials tab and click New.

3. Save

Named Credentials

Create and Edit a Named Credential

1. Setup > Named Credentials.
2. Choose the Named Credentials tab and click New.

Permission Set

1. Setup > Permission Sets > New

2. Setup > Named Credentials > External Credentials
3. Click New next to Permission Sets Mappings.

4. Assign Permission Set to your integration user.
5. Go back External Credentials page. Click the arrow next to the Permission Set Mappings and hit Authenticate.

Apex Code

Salesforce

1. Create an APEX class. e.g GoogleWebService.
2. Go to API Library. Choose your API and open documentation.
3. Choose a concrete endpoint. Check REST API method and HTTP request params.

Here you can find really simple code to make a callout.

public with sharing class GoogleWebService {
    public static void makeCallout() {
        HttpRequest request = new HttpRequest();
        request.setMethod('REST API METHOD');
        //request.setMethod('GET');
        request.setEndpoint('callout:NAMED_CREDENTIALS_NAME/' + path);
        //request.setEndpoint('callout:NAMED_CREDENTIALS_NAME' + '/users/me/calendarList');

        Http http = new Http();
        HTTPResponse response = http.send(request);

        if (response.getStatusCode() == 200) {
            System.debug(response.getBody());
        }
    }
}

If you have some questions feel free to ask in the comment section below. 🙂

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Resources

• Create and Edit an External Credential
• Create and Edit a Named Credentials
• Configure a Salesforce Authentication Provider
• API Library