Software developers are in high demand today, and technology is advancing rapidly. 

One of the most sought-after roles in the tech industry is that of a back-end developer. Back-end developers are responsible for creating and maintaining the server side of web applications, which is crucial to the functioning of any web-based system. 

To secure a back-end developer position, one must undergo a rigorous interview process. 

This article will discuss four common back-end developer interview questions one can expect to face during the interview process.

Can you explain the difference between a PUT and a POST request?

HTTP requests are the backbone of any web application. A back-end developer should be well-versed in the various HTTP requests, including PUT and POST requests. The main difference between PUT and POST requests is how they handle data.

PUT requests are used to update or replace an existing resource on the server. For example, if a user wants to update their profile picture, they can send a PUT request to the server, replacing the existing image with the new one.

On the other hand, POST requests are used to create a new resource on the server. For example, if a user wants to make a new blog post, they can send a POST request to the server, creating a new post with the data provided.

It is important to note that PUT requests are idempotent, which means that making multiple identical PUT requests will produce the same result. However, POST requests are not idempotent, meaning making various similar POST requests may result in numerous resources being created on the server.

How would you handle a database migration?

Database migrations are an essential part of any web application's lifecycle. Database migration can be complex and must be done correctly to avoid data loss. A back-end developer should be familiar with migrating a database, which involves updating the database structure without losing data.

One common approach to handling a database migration is to use a tool like Flyway or Liquibase. These tools allow developers to write migration scripts in a version control system like Git, which can be applied to the database in a controlled manner. The migration script contains instructions for updating the database schema and associated data.

Another approach to database migration is using an ORM (Object-Relational Mapping) tool like Hibernate or Entity Framework. These tools abstract away the details of the database and allow developers to work with the database using object-oriented code. When a migration is required, the ORM tool can generate the necessary SQL statements to update the database schema.

Regardless of the approach, testing the migration thoroughly before applying it to a production database is essential. This can be done by setting up a staging environment that mirrors the production environment and using the migration to the staging database first.

How would you implement authentication and authorisation in a web application?

Authentication and authorisation are critical components of any web application. Authentication refers to the process of verifying a user's identity, while authorisation refers to the process of determining what actions a user is allowed to perform.

There are several ways to implement authentication and authorisation in a web application. One common approach is to use a framework like Spring Security or Django Authentication. These frameworks provide tools and libraries for implementing authentication and authorisation, including user management, password hashing, and role-based access control.

Another approach is to use a third-party authentication service like Google Sign-In or OAuth. These services allow users to authenticate with their existing social media or email accounts, making the authentication process more accessible and secure.

When implementing authentication and authorisation, it is essential to follow best practices to ensure the application's security. This includes using strong password hashing algorithms, enforcing password policies, and implementing measures like two-factor authentication to add protection.

Ensuring users have appropriate access levels to different application parts is also essential. This can be done through role-based access control, where users are assigned roles based on their job function or level of authority, and those roles dictate what actions they are allowed to perform within the application.

How do you optimise a database query?

Database queries are a common source of performance issues in web applications. A back-end developer should understand how to optimise database queries to ensure they execute quickly and efficiently.

One common approach to optimising database queries is to use indexes. An index is a data structure that allows the database to quickly look up data based on a specific set of columns. The database can retrieve data faster by creating indexes on columns frequently used in queries, improving performance.

Another approach to optimising database queries is to use query optimisation techniques like query caching or query rewriting. Query caching involves storing the results of frequently used questions in memory to retrieve them more quickly. Query rewriting involves rewriting a query to use more efficient SQL syntax or to break the query up into smaller, more manageable parts.

It is also essential to ensure that queries are written efficiently, with only the necessary data retrieved from the database. This can be done by selecting only the required columns, using LIMIT and OFFSET clauses to retrieve data in smaller chunks, and avoiding using wildcard characters like * in queries.

Back-end developer interviews can be challenging, but preparing for common questions like these can increase your chances of success. 

By having a good understanding of HTTP requests, database migrations, authentication and authorisation, and query optimisation, you will be well-equipped to tackle any technical question that comes your way. 

Remember also to showcase your problem-solving skills, communication skills, and ability to work in a team, as these soft skills are just as essential as technical knowledge in software development.