What I am going to say next is probably the most important thing to know before your System Design interview: in a System Design interview, your interviewer is not interested in knowing WHAT your design is for the system that you are building. What your interviewer is interested in knowing is how many design choices you are presenting, and what trade-offs you are making in each of these design choices, pros and cons of each design choice, what design decision you are taking and WHY you are leaning towards one design choice vs the other. So make sure every design choice you make, you discuss trade-offs and bottlenecks, and THE REASON behind the design choice. For example, if you are deciding what kind of storage you are going to use and are making a design decision between whether you are going to use SQL DB or NoSQL DB, you need to talk in details whether you are gonna use SQL DB or NoSQl DB or a hybrid model, and why and what are the trade-offs of each option. You interviewer is really trying to find out what are your reasonings are behind each of the design choices you are making and whether you are exploring multiple design options and understand the pros and cons of each of the design options. Once you elaborate that, your interviewer would often encourage you to just choose of your described design choices and move forward with the rest of design discussion. It is because, it is not important what design you go with, what is important is if you understand the trade-offs and bottelenecks of each design option.
A productive System Design interview starts with interviewee asking a lot of clarifying questions to the interviewer. Remember, your mindset should be as if you are a consultant and your interviewer is your customer. This is your first meeting with your customer and your goal is to know everything about the system that your customers wants you to design. So you will try to know what your customer has in mind for the system and then you will quickly try to give your customer a consultation on what you think about how the system can be designed keeping the customer requirements in mind and you will also want to give your customer a walk-through of different design choices you might have for the system and what the trade-offs are for different design choices.So basically you are advising your interviewer on what you think the design should be for the system so that is scales, what different design options are there, what the trade-offs are for each of the design options. Discuss in details WHY you are taking certain design decision and WHY you think one decision option is better (or worse) than the other and what you are gaining or compromising in each of the design options. Each design choice is good for some use case and not-so-good for other scenarios. Your are responsible to discuss these in details and advice your customer keeping his/her scale requirements in mind. You are responsible to show your client the right path to architect his/her product in a scalable manner: this should be your mindset throughout the interview. You should propose various solutions and compare your options.