Low-level design

Class and object design. Start with the TTL cache, then add the next problem as a markdown file.

Low-level design interviews ask you to model types, relationships, and a few operations in code-level detail. The first worked problem on this track is an in-memory cache with TTL and LRU eviction. It is a common follow-up in service-style coding rounds: O(1) get and put, expiry, and a rule that failures are not cached for the success TTL. Read it once, then close it and redraw the two structures and the lock from memory.

The system-design track is still the bulk of the site, because that is the Mercor loop. This track is where a problem becomes types and one hard method.

When you add the next problem, use the same shape: what is in scope, the types, the tricky method, the concurrency story, and what you would not build. Put the quiz in frontmatter or in src/content/quizzes/lld/<slug>.yaml. A parking lot or a multi-threaded job runner is a reasonable second page. Do not paste a problem from a paid course. Write the version you can redraw from memory.

You can also mine the system-design notes. The claim query on the labeling page is already halfway to an object: TaskQueue.claim(skill) -> Task with a lease. Writing that as a class, with tests, is a good next page.

Further reading

Check yourself