StealthCoder

Last Minute System Design Prep with AI (What to Review in 1 Day)

2025-09-18

System design interviews are intimidating because they test how you think at scale. Normally, you’d want weeks to practice, but sometimes the interview lands on your calendar tomorrow and you don’t have that luxury. If you have just one day left, your goal should be to focus on the high-leverage areas that deliver the biggest return on preparation. Here’s how to make the most of 24 hours.

1. Review the Core Building Blocks

Don’t try to memorize every possible system. Instead, refresh the foundational pieces you’ll almost certainly need to mention:

  • Databases: When to use SQL vs NoSQL, partitioning, replication, indexing.
  • Caching: What it solves, common tools like Redis or Memcached, and cache invalidation basics.
  • Load Balancers: How they distribute requests and why they’re essential for scaling.
  • Message Queues: Decoupling services, handling spikes, ensuring reliability.

These four topics appear in nearly every design discussion. If you can talk through them clearly, you’ll sound confident.

2. Rehearse Popular Systems at a High Level

Spend 2–3 hours skimming common design prompts:

  • Design a URL shortener
  • Design Twitter/Instagram feed
  • Design a chat app
  • Design a payment system

You don’t need deep diagrams here. Instead, practice articulating the main components: client, API gateway, database choice, scaling strategy, and trade-offs. The interviewer cares more about your reasoning than a perfect diagram.

3. Focus on Trade-Offs

System design is less about “the right answer” and more about trade-offs. Be ready to answer:

  • Why would you shard a database vs replicate it?
  • Why choose eventual consistency over strong consistency?
  • Why use a queue instead of synchronous calls?

Spending a couple of hours on these “why” questions is often more impactful than memorizing architectures.

4. Practice Out Loud

Take a prompt and talk through it as if you’re in the interview. Time yourself. Most system design interviews last 30–45 minutes, so practice explaining your thought process clearly within that window. Even a few dry runs will make you sound sharper.

5. Use AI to Simulate the Interview

One of the fastest ways to cover ground in a single day is to simulate interview scenarios with AI. Instead of passively reading notes, you can:

  • Generate mock prompts and practice live responses.
  • Get feedback on whether your answer covers scalability, fault tolerance, and trade-offs.
  • Review diagrams and receive instant critiques.

If you don’t want to spin up your own setup, you can try tools like StealthCoder. It includes an AI system design mode that gives you realistic prompts, feedback, and diagrams so you can maximize your prep in the limited time you have.