What is a Hash Table? What is a Key-Value pair?

0-2 years experience

1. 1Suppose you need to implement a simple cache for recent user lookups using a hash table. How would you store and retrieve a user object given its ID?
2. 2If you insert a new entry into a hash table and the computed bucket already contains another entry, what happens and how would you handle it in code?
3. 3What would be the result of trying to retrieve a key that was never added to the hash table?

2-5 years experience

1. 1We have a feature that stores session data in an in‑memory hash map, but we’re seeing occasional 'key not found' errors after a server restart. Walk me through how you would debug this.
2. 2When choosing a hash function for a hash table that will store URLs as keys, what trade‑offs would you consider?
3. 3If the load factor of our hash table grows beyond 0.75, what impact does that have and what would you do to mitigate it?

5-8 years experience

1. 1Design a distributed caching layer that uses consistent hashing to spread key‑value pairs across multiple nodes. What edge cases do you need to handle?
2. 2Our service is migrating from a relational DB to a key‑value store backed by a hash table. How would you ensure data consistency and handle collisions at scale?
3. 3Explain how you would monitor and tune the performance of a high‑throughput hash table used in a real‑time analytics pipeline.

8+ years experience

1. 1At a company‑wide level, we’re planning to replace several microservices’ internal hash‑table implementations with a shared, sharded key‑value store. What architectural considerations and migration steps would you propose?
2. 2How would you evaluate the long‑term maintainability and operational costs of using a custom hash table versus adopting an off‑the‑shelf distributed key‑value system?
3. 3If multiple teams need different collision‑resolution strategies, how would you design an extensible hash table library that accommodates that without fragmenting the codebase?