The spine
At minute twenty the board is still empty. Draw the boxes in a fixed order, write a verb on every arrow, and then walk one write through them hop by hop. The walk, not the drawing, is what the room grades, and it writes your deep-dive agenda for you.
What should the high-level diagram in a system design interview contain?
The boxes that carry the key operations and nothing else: the clients, the edge, the services, and then the cache, the store, the queue and the blob store only when a requirement needs each one. Every box gets its job written on it and every arrow gets a direction and a verb. Table columns, internal data structures and technology names wait for the data model and the deep dive. A small diagram with a reason behind every box beats a large one nobody walked.
In what order should I draw the boxes?
The order a request travels: clients, the edge (the load balancer), the services, then what the services call, which is the cache, the store, the queue and the blob store. Drawing in that order means the diagram is already in the shape of a walk when you finish, so the first walk costs nothing to set up. It also makes a missing box easy to see, because every arrow has to land somewhere.
What does it mean to walk an operation through the design?
You take one key operation, such as a write, and follow it from the client through every box it touches, saying at each hop what that box does and what it reads, writes, caches or queues. The walk proves the boxes connect, and it is where a missing piece shows itself: a version nobody stamps, a copy nobody makes. Walk the write first, because the durability requirement lives there, then the read, saying only where it differs.
How long should I spend drawing the high-level design?
The spine and the walks own minutes 20 to 30, and the data model takes 30 to 35. Drawing should be the short part of that; the walks are the long part. Stop drawing as soon as every key operation has a path through the boxes, and move to the walks even if the diagram looks sparse.
Should I name specific technologies on the diagram?
Name the kind, not the product, when you first draw it: a store, a cache, a queue. The product choice is a decision with a trade-off, and it belongs in the deep dive where you can defend it against an alternative. Writing a product name on a box at minute 22 invites a question you have not yet earned the time to answer.