Practice
1. A SaaS platform must ensure strict data isolation between tenants while allowing shared infrastructure. Which component best enforces tenant data isolation at the storage layer?
easy
Solution
Step 1: Understand tenant isolation requirements
Strict isolation requires preventing any data leakage between tenants, which network-level controls and separate DB instances provide.Step 2: Evaluate options for isolation guarantees
Use a single shared database schema with tenant_id column filtering in application code relies on application filtering, prone to bugs; Use a multi-tenant database with row-level security policies enforced by the DBMS depends on DBMS features that may be complex and less secure; Use a caching layer to segregate tenant data in memory for faster isolation only caches data and doesn't guarantee persistent isolation.Final Answer:
Option B -> Option BQuick Check:
Separate DB instances + network controls -> strongest tenant isolation [check]
Hint: Separate DB instances isolate tenants physically [check]
Common Mistakes:
2. In the order matching flow, at step 5 the Matching Engine receives a new buy order for stock XYZ. What state does the Matching Engine hold immediately after processing this order if no matching sell orders exist?
easy
Solution
Step 1: Trace Matching Engine behavior at step 5
When no matching sell orders exist, the buy order is added to the order book's buy side and marked as open, waiting for future matches.Step 2: Eliminate other options
The order is marked as matched and removed from the order book is wrong because no match occurred; The order is rejected due to lack of matching sell orders is wrong because orders are not rejected for no matches; The order is forwarded to the Market Data Feed before being stored is wrong because Market Data Feed only publishes after matching or order book update.Final Answer:
Option B -> Option BQuick Check:
Unmatched order -> added to order book as open [check]
Hint: Unmatched orders stay open in order book [check]
Common Mistakes:
3. If the Layer 7 load balancer's HTTP header inspection module fails, what is the most likely failure mode for incoming requests?
medium
Solution
Step 1: Understand failure impact on Layer 7 inspection
If HTTP header inspection fails, the load balancer may fallback to Layer 4 forwarding to maintain availability.Step 2: Eliminate options inconsistent with graceful degradation
Dropping all connections (B) is unlikely; forwarding blindly to default backend (A) loses load distribution; encryption errors (D) unrelated.Final Answer:
Option C -> Option CQuick Check:
Layer 7 failure -> fallback to Layer 4 forwarding without HTTP routing [check]
Hint: Layer 7 fail -> fallback to Layer 4 forwarding [check]
Common Mistakes:
4. At 10x scale, which component in the multi-tenant SaaS platform is most likely to fail first, and why?
hard
Solution
Step 1: Analyze scaling bottlenecks
Billing DB (A) can be scaled with sharding; Tenant Metadata (C) can be cached; API Gateway (D) handles stateless HTTP mostly.Step 2: Evaluate Feature Flag Service cache at scale
Cache invalidation and stale data grow exponentially, causing inconsistent feature exposure and client errors.Final Answer:
Option A -> Option AQuick Check:
Feature flag cache stale -> inconsistent behavior at scale [check]
Hint: Feature flag cache stale -> first scale failure [check]
Common Mistakes:
5. At 10x scale, which component in your design explanation is most likely to fail first due to stateful connection bottlenecks, and why?
hard
Solution
Step 1: Identify stateful components at scale
WebSocket servers maintain persistent connections bound to server memory, limiting horizontal scaling.Step 2: Compare with stateless components
Stateless API servers and load balancers scale horizontally more easily; databases fail later due to storage.Final Answer:
Option A -> Option AQuick Check:
Stateful WebSocket servers bottleneck first due to connection affinity [check]
Hint: Stateful WebSocket servers bottleneck first at scale [check]
Common Mistakes: