SYS ARCHITECTLearning Platform

Phase 1

  • Interview Framework
  • Back-of-the-Envelope Estimation
  • ACID
  • CAP
  • Consistency vs Integrity
  • Idempotency
  • NFR
  • SPOF
  • Stateless vs Stateful
  • SOLID
  • SRP
  • OCP
  • LSP
  • DIP vs LSP
  • Clear Architecture Principles
  • 12-Factor App

    • Phase 2

  • SQL vs NoSQL
  • Normalization vs Denormalization
  • Partition vs Sharding
  • Sharding
  • Partition Tolerance
  • Serializable Transactions
  • MVCC
  • Optimistic vs Pessimistic Locking
  • Row Level Locking
  • Distributed Locks
  • Redis Lock
  • Replication
  • Primary Replica Setup
  • Eventual Consistency
  • Data Consistency
  • Quorum
  • Write Ahead Logs
  • Redo Logs
  • DB Durability
  • Data Recovery
  • Two Phase Commit
  • Saga Pattern
  • Cassandra
  • S3
  • HDFS
  • Timeseries DB

    • Phase 3

  • Caching
  • Cache Eviction Policies
  • Hot Keys
  • Hot Reads and Writes
  • Latency
  • Latency vs Throughput
  • Tail Latency
  • Low Latency Patterns
  • Backpressure
  • Sudden Traffic Spike
  • Buffer Burst

    • Phase 4

  • Replication
  • Consistent Hashing
  • Leader Election
  • Distributed Locking
  • Fault Tolerance
  • Resiliency
  • Raft Consensus
  • Paxos
  • Event Driven Architecture
  • Event Sourcing
  • Outbox Pattern
  • CQRS

    • Phase 5

  • How HTTP Works
  • HTTP vs HTTPS
  • TCP vs UDP
  • TLS
  • Long Polling vs WebSockets
  • Server Sent Events
  • Load Balancer
  • L4 vs L7
  • Load Balancer vs Reverse Proxy vs API Gateway
  • CDN
  • Geo DNS

    • Phase 6

  • JWT
  • OAuth2
  • Opaque vs JWT Token
  • TLS
  • OWASP
  • Secrets Management

    • Phase 7

  • Creating Docker Image
  • Kubernetes ConfigMap
  • Kubernetes Ingress
  • Persistent Volume
  • Prometheus and Grafana

    • Phase 8

  • Monolith vs Microservices
  • API Gateway
  • Unique ID Generator
  • Retry Mechanism
  • Deployment Strategy
  • Rate Limiting

    • Phase 9

  • URL Shortener
  • Chat System
  • News Feed
  • Web Crawler
  • Notifications Design
  • System Design Problems
    • Settings
    Theory

    TCP vs UDP: Transport Layer Decision Matrix

    📋 Overview

    At the Transport Layer (Layer 4), the choice between TCP (Transmission Control Protocol) and UDP (User Datagram Protocol) is the ultimate trade-off between reliability and speed. Understanding these protocols is critical for designing low-latency real-time systems versus data-integrity-critical transactional systems.

    🏗️ Core Principles & Characteristics

    • TCP (Connection-Oriented):
      • Reliability: Uses sequence numbers, ACKs (Acknowledgements), and retransmissions.
      • Flow Control: Sliding windows prevent the sender from overwhelming the receiver.
      • Congestion Control: Algorithms (like BBR or Cubic) detect network congestion and slow down.
    • UDP (Connectionless):
      • "Fire and Forget": No handshake, no ACKs, no retransmissions.
      • Minimal Header: 8 bytes (UDP) vs. 20+ bytes (TCP), reducing overhead for tiny packets.
      • Broadcasting: Supports one-to-many communication natively.

    ⚖️ Trade-offs: Pros & Cons

    TCP

    • Pros: Guaranteed delivery, in-order packets, handles network congestion gracefully.
    • Cons: High overhead (Handshake + ACKs), "Head-of-Line Blocking" (one lost packet stalls all subsequent packets).

    UDP

    • Pros: Ultra-low latency, no handshake delay, full control over timing (good for real-time).
    • Cons: Packets can be lost, duplicated, or arrive out of order; no built-in congestion protection (can cause "Network Collapse").

    🌍 Real-World Implementation

    • HTTP/1.1 & HTTP/2 (TCP): Reliability is required for loading web resources (HTML/JS) correctly.
    • Online Gaming (UDP): A lost packet (movement glitch) is better than a 2-second stall to wait for retransmission.
    • Live Video (UDP/RTP): Dropping a frame is acceptable to keep the stream "Live."
    • DNS (UDP): Requests are usually a single packet; if it fails, the client just retries.
    • HTTP/3 (QUIC on UDP): Uses UDP to avoid TCP's Head-of-Line blocking while adding its own reliability layer in user-space.

    💡 Interview "Gotchas" & Tips

    • The "Three-Way Handshake": Be ready to explain SYN -> SYN-ACK -> ACK and the latency cost (1.5 RTTs) before data even moves.
    • Head-of-Line (HoL) Blocking: The single biggest reason for moving from TCP to QUIC/UDP for the modern web.
    • TLS Handshake: Remember that on top of TCP, TLS adds another 1-2 RTTs of delay.
    • MTU (Maximum Transmission Unit): If a UDP packet is too large, it gets fragmented by routers, often leading to it being dropped.

    📐 Suggested Architecture Primitives

    • Socket.io / WebSockets: Built on TCP for persistent, reliable full-duplex communication.
    • WebRTC: Uses UDP (via DTLS/SRTP) for ultra-low latency peer-to-peer video/audio.
    • HAProxy / Nginx: L4 Load Balancers can be configured to handle both TCP and UDP traffic.
    • QUIC/HTTP3: The emerging standard for high-performance web traffic.
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