Design users table for single sign in to use across sub domains

Question

We have a site which has sub domains like:

1. example.com - Main Site
2. food.example.com
3. fashion.example.com

And each domain

Answer 1

The UserID you mentioned is only identical in a specific Users table in a single DB. It's actually not an identifier of any user in your domain. To identify users across multiple databases, you will need a domain-level ID for every user. A simple way is to add one more property (column) named UID or something like that to User class (table), of Global Unique Id (GUID, see https://msdn.microsoft.com/en-us/library/system.guid(v=vs.110).aspx) and use it as domain-level ID to identify users instead of UserId. This way you can freely store user information in multiple databases in your domain. So:

• You can hold only domain-level ID in every sub-domain's database and use that ID to query full user profiles from the main domain DB. Pros: costs less memory. Trade-off: in a distributed system, it takes longer to query user information from sub-domain because the information residing in main DB.
• You can hold full user information in all DB, including main's and subs'. Pro: faster queries. Cons: costs more memory and when a user information changes, you will have to synchronize it across all databases.