Interview Q&A / System Design

1. How do you choose between a SQL and a NoSQL database?

   Choose SQL when data is relational, you need joins, transactions and flexible queries, and the schema is fairly stable. Choose NoSQL when you need a flexible schema, very high scale for simple access patterns known in advance, or a data model like documents, key-value or graphs. Many systems use both.

   BeginnerIn-depth4 min read
2. How would you design a URL shortener like bit.ly?

   Store a mapping from a short code to the long URL. Generate codes with a counter encoded in base62 (or random codes with a collision check), keep the mapping in a key-value store, put a cache in front for fast redirects, and return a 301 or 302 redirect. Add rate limits, expiry and analytics as extras.

   IntermediateIn-depth3 min read
3. How does leader-follower database replication work?

   All writes go to one leader, which streams its changes to follower replicas. Followers serve reads, which spreads read load and gives a standby copy if the leader fails, but they may lag behind.

   BeginnerQuick answer1 min read
4. What is the difference between sharding and partitioning?

   Partitioning splits a big table into smaller parts, often inside one database. Sharding is partitioning where the parts live on different servers, so both data and load are spread across machines.

   BeginnerQuick answer1 min read
5. Why is returning a total count with every page expensive, and what are the alternatives?

   COUNT(*) over a large filtered table scans many rows on every request. Alternatives are returning only has_next, using approximate counts, caching the count, or showing 'more than 1,000' instead of an exact total.

   IntermediateQuick answer1 min read