Module 11 - Interview Prep

Trade-off Analysis

Every design decision has a cost-understanding trade-offs is what separates good from great.

1No Free Lunch

Simple Analogy

You can have a car that's fast, cheap, and fuel-efficient-pick two. Want fast and cheap? It'll guzzle gas. Want cheap and efficient? It won't be fast. System design works the same way: consistency vs availability, latency vs throughput, simplicity vs flexibility.

Trade-off analysis is the art of making and justifying engineering decisions when you can't have everything. Interviewers care less about what you choose and more about why you chose it.

2Classic Trade-offs

ConsistencyvsAvailability

CAP theorem. Strong consistency requires coordination, reducing availability. Eventually consistent systems stay up longer.

Example: Banking: consistency. Social feeds: availability.

LatencyvsThroughput

Batching improves throughput but adds latency. Streaming is low-latency but lower throughput.

Example: Real-time chat: latency. ETL pipelines: throughput.

StoragevsCompute

Precompute and store results (use storage) or compute on demand (use compute).

Example: Materialized views vs live queries.

SimplicityvsFlexibility

Simple systems are easier to maintain. Flexible systems handle more use cases but are complex.

Example: Monolith vs microservices.

CostvsPerformance

More servers, faster databases, premium services cost money.

Example: Reserved instances vs on-demand.

3How to Present Trade-offs

State the Options

'We could use SQL for strong consistency or NoSQL for better scalability.'

Explain the Trade-off

'SQL gives us ACID but limits horizontal scaling. NoSQL scales but requires handling consistency in application.'

Connect to Requirements

'Given our requirements for 100K writes/sec and eventual consistency being acceptable...'

Make a Decision

'...I'd choose NoSQL with application-level consistency checks.'

Acknowledge Downsides

'The downside is more application complexity, but it's worth it for our scale.'

4Database Trade-offs

Choice	You Get	You Give Up
SQL	ACID, JOINs, mature tooling	Horizontal scaling, schema flexibility
NoSQL	Scalability, flexibility	JOINs, consistency guarantees
Indexing	Fast reads	Slower writes, more storage
Denormalization	Fast reads, no JOINs	Data duplication, update complexity
Sharding	Horizontal scale	Cross-shard queries, complexity

5Architecture Trade-offs

Choice	You Get	You Give Up
Microservices	Independent scaling, deployment	Network complexity, debugging difficulty
Monolith	Simplicity, easy debugging	Scaling flexibility, deployment coupling
Sync Processing	Simple, immediate response	Latency, coupling
Async Processing	Decoupling, resilience	Complexity, debugging, eventual consistency
Caching	Speed, reduced DB load	Stale data, cache invalidation complexity

6Interview Phrases

Use These Phrases

"The trade-off here is between X and Y..."

"Given our requirements for [specific requirement], I'd choose..."

"The downside of this approach is... but it's acceptable because..."

"An alternative would be... but that would sacrifice..."

"If the requirements change to [X], we might reconsider..."

7Key Takeaways

1Every decision has a cost. There's no perfect solution.

2Explain your reasoning. Why you chose matters more than what you chose.

3Connect to requirements. Trade-offs depend on context.

4Acknowledge downsides. Shows maturity and awareness.

5Classic trade-offs: consistency/availability, latency/throughput, cost/performance

?Quiz

1. System needs strong consistency and high availability. What do you say?

2. You chose NoSQL. Interviewer asks about downsides. Best response?