Developing the User Authentication Service

2.1 Define Your Service's Functionalities

Before coding, you should clearly define what your User Authentication service needs to do. Some typical functionalities might include:

• User registration
• User login
• Password reset
• Session management
• User data retrieval and modification

2.2 Choose Your Technology Stack

Based on the defined functionalities, choose an appropriate technology stack. This usually includes a programming language, a web framework, and libraries for interacting with your database and other services.

2.3 Design the Database Schema

Design the schema for your user database. This should include tables for storing user information, hashed passwords, session tokens, and any other necessary data.

2.4 Implement the Service

Now it's time to start coding. Implement each of your service's functionalities one by one, starting with the most fundamental ones like user registration and login.

2.4.1 User Registration

Create a function to register new users. This should include validating the input data, hashing the user's password, and storing the new user's data in the database.

2.4.2 User Login

Create a function to authenticate users. This should include checking the provided password against the stored hashed password, and if the password is correct, generating a new session token for the user.

2.4.3 Password Reset

Create a function to handle password resets. This should include generating a password reset token, sending it to the user's email address, and allowing the user to change their password using the token.

2.4.4 Session Management

Create functions to handle user sessions. This should include generating session tokens upon login, verifying session tokens when the user accesses protected resources, and invalidating session tokens upon logout.

2.4.5 User Data Management

Create functions to handle user data retrieval and modification. This should include retrieving user data based on the session token and updating user data in the database.

2.5 Test Your Service

Finally, thoroughly test your User Authentication service. Write unit tests for your functions and perform integration tests to ensure your service works correctly as a whole.

Testing should also include load testing to see how your service performs under heavy load and security testing to ensure your user data is safe.

After this step, your User Authentication service should be ready for containerization and deployment. Keep in mind that developing a service is an iterative process, and you should always be ready to go back, make modifications, and retest as necessary.