Most people study for backend interviews by reading system design books and trying to memorize architectures. They learn to draw boxes labeled Database and Cache. That approach gets you through the initial screening. It falls apart as soon as the interviewer asks a follow up question about what happens inside those boxes when traffic spikes. I compiled this list based on the questions that actually get asked in real rooms. I failed many interviews because I did not know the answers to these. I ask them now because they are excellent filters. They separate the people who only know the vocabulary from the people who know how the systems behave under pressure. You do not need to memorize textbook answers for these. You need to understand the underlying mechanics well enough to discuss them conversationally. Databases and query performance Databases are where most applications spend most of their time. You need to know how to get data in and out efficiently when the tables get large.…