Ensure that a user (or a database was read) is used only by one application

Question

Imagine that I have 100 users list in the database and I have 100 or more console applications that are using those users to log in somewhere. I need to use a unique user per application, I must not use the same user more than one application. How can I solve this issue, what is the best practice or pattern for that? That means a record has to read only one application/service I have also some requirements/restrictions as I mentioned below.

1. Each application/service an instance of a microservice. So, it can be scale-down or scale-up. It means I cannot set a user to a specific application/service in a config file.
2. Users in the database can be disabled or removed in the future or added new users. So user count is not fixed.

Answer 1

You didn't say what database you use or what database access technology stack. In general, you need the following:

• a field in the database that signals whether a row is in use
• each application:
  • locks the table
  • grabs a row that is not in use
  • updates the in-use flag to "currently used"
  • releases the lock on the table
  • uses this user in the program
  • makes sure that it releases the in-use flag when it's done

Any decent database and any decent driver should be able to do that.

Answer 2

If I don't misunderstand you, you need a class that handles 1:1 relationships between two classes. Maybe you can find an answer here?