Absolutely! Implementing the Google Calendar API system across domain and subdomain websites (e.g., www.japansailing.com and kaikoma.japansailing.com) introduces a few key considerations and best practices, both on the Google API side and in your website development. Let's go over them step by step:

✅ 1. Google Cloud OAuth Authorized Domains

➤ What Google Requires:

Google enforces strict authorized domain policies when using OAuth 2.0. You need to:

• List all domains and subdomains in your OAuth consent screen.
• Only domains/subdomains you own and verify in Google Search Console are allowed.

➤ Example Setup for Japan Sailing:

1. Add and verify:
   • www.japansailing.com
   • kaikoma.japansailing.com
2. In Google Cloud Console ➜ OAuth consent screen ➜ Authorized domains, add:
   • japansailing.com

➡️ Subdomains are automatically covered if you add the root domain, but Google requires them to be verified in Search Console first.

✅ 2. Authorized Redirect URIs for Each Website

When you set up OAuth 2.0 client credentials, you specify redirect URIs for each site or subdomain.

➤ If you have multiple sites/subdomains:

• Each OAuth flow needs a redirect URI back to its own site.

Example:

➡️ You must list every redirect URI in your OAuth client settings.

✅ 3. Separate or Shared Google Cloud Projects

You have two options:

➤ Option 1: Shared Google Cloud Project

• Use one Google Cloud project for all sites (simpler management).
• List all authorized domains and redirect URIs under one OAuth credential.

➡️ Best for same company/brand (like your example: japansailing.com and its subdomains).

➤ Option 2: Separate Projects

• Separate Google Cloud projects for each site/subdomain.
• More isolated but harder to manage.

➡️ Useful if sites are independent, run by different teams, or have different privacy policies.

✅ 4. Cookies, Sessions, and OAuth Tokens Across Domains

If you're running two different sites, each:

• Manages its own sessions/cookies.
• Has its own OAuth token storage.

➤ Cross-Domain Considerations:

• Cookies from www.japansailing.com do not apply to kaikoma.japansailing.com (different scopes).
• If you want to share login/authentication state between domains, you'll need:
  • A Single Sign-On (SSO) implementation.
  • Or a shared database/session server.

➡️ For Google Calendar API, tokens are tied to a specific OAuth flow and site. You don’t share tokens across domains without explicit user authorization on each domain.

✅ 5. CORS (Cross-Origin Resource Sharing)

If you are:

• Using APIs on one domain (say www.japansailing.com)
• And want to interact with those APIs from another (say kaikoma.japansailing.com)

➡️ You’ll need to configure CORS policies on your API endpoints to allow cross-origin requests.

✅ 6. Security: HTTPS Everywhere

Every domain/subdomain involved:

• Must use HTTPS (required by Google OAuth 2.0).
• Ensure SSL certificates are installed on both www.japansailing.com and kaikoma.japansailing.com.

✅ 7. Branding and User Trust

Google displays your authorized domains on the OAuth consent screen:

• Users see which domains are authorized to request permissions.
• Make sure the brand and domain match the user's expectations to avoid confusion.

➡️ If kaikoma.japansailing.com asks for permissions, but the user is only familiar with www.japansailing.com, explain the relationship on your site.

✅ 8. Google API Quotas and Rate Limits

• If both sites/subdomains share a single Google Cloud project, they share the same API quota.
• If you need higher quotas for different sites, separate projects can help, but may complicate management.

✅ Example Architecture

✅ TL;DR Key Considerations

✅ Ready to Move On?

If you:

1. Confirm the domains/subdomains you’ll be using
2. Have SSL and verification set up
3. Know whether you’re going single project or multiple projects

➡️ We can move on to the OAuth 2.0 flow setup and coding (Node.js, Python, or PHP—your pick).

Let me know what tech stack you’re using and if you want sample code for one or both domains!