Day42
60-day SSE plan
Rate Limiter (Token Bucket) • WhatsApp-like Chat • Binary Search on Answer • Tech: Kafka
Algorithm
DSA
Binary Search on Answer — Capacity to Ship Packages (LC 1011)
Minimize ship capacity so all packages ship within D days using feasibility check.
Example
Input: weights=[1,2,3,1,1], D=4 → Output: 3
Where it shows up
Capacity planning, throughput sizing, SLO budgeting.
References
Low-level design
LLD
Rate Limiter (Token Bucket)
Requirements
- Per-user limits with rate & capacity.
- Thread-safe tryAcquire(); refill over time.
Expectations
- UML for TokenBucket & RateLimiterService.
- Concurrency-safe Java implementation + tests.
Where it shows up
API gateways, throttling abusive clients.
References
High-level design
HLD
WhatsApp-like Chat
Requirements
- 1:1 + group messaging, delivery/read receipts, offline sync.
- Real-time delivery via WebSockets; scalable fan-out.
Expectations
- APIs, WebSocket fan-out, message broker, storage model.
- Multi-region strategy, exactly-once UX via idempotency keys.
Where it shows up
Consumer chat apps, support chat in products.
References
Today's deep-dive
Tech
Kafka
Write a producer & consumer; demonstrate consumer groups and rebalancing; handle offsets safely.
Where it shows up
—
References