Protocol & Design Interview Questions - Hard

calendarDec 13, 2025 Β· 4 min read Β· protocol interview hard advanced-systems  Β·
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Hard-level protocol and design interview questions covering advanced distributed systems and protocol design.

Q1: Design a custom protocol for real-time multiplayer gaming.

Answer:

Protocol Design

Message Format

Header Fields:

  • Magic Number: Protocol identifier
  • Version: Protocol version
  • Flags: Reliable, ordered, encrypted
  • Sequence: For ordering and deduplication
  • Timestamp: For latency calculation
  • Message Type: Action, state, event
  • Player ID: Source player
  • Payload: Message-specific data

Client-Server Architecture

Key Techniques:

  • Client Prediction: Immediate feedback
  • Server Reconciliation: Correct mispredictions
  • Lag Compensation: Rewind time for hit detection
  • Delta Compression: Send only changes
  • Interest Management: Only send relevant updates

Q2: Explain Byzantine Fault Tolerance and PBFT.

Answer:

Byzantine Generals Problem

PBFT (Practical Byzantine Fault Tolerance)

PBFT Protocol Flow

Requirements:

  • N β‰₯ 3f + 1: To tolerate f Byzantine faults
  • Quorum: 2f + 1 nodes must agree
  • View Change: Replace faulty primary

Use Cases:

  • Blockchain consensus (Hyperledger Fabric)
  • Distributed databases
  • Critical infrastructure

Q3: Design a distributed lock service (like Chubby/ZooKeeper).

Answer:

Architecture

Lock Acquisition

Session Management

Lock Types

Features:

  • Advisory Locks: Clients cooperate
  • Fencing Tokens: Prevent stale lock holders
  • Watch Mechanism: Notify on lock release
  • Lock Queuing: Fair ordering

Fencing Token:

Q4: Explain distributed transactions and 2PC/3PC.

Answer:

Two-Phase Commit (2PC)

2PC Protocol

2PC Failure Scenario

2PC Problems:

  • Blocking: If coordinator fails after prepare
  • Single point of failure: Coordinator
  • Not partition tolerant

Three-Phase Commit (3PC)

3PC Advantages:

  • Non-blocking (with timeout)
  • Can make progress during coordinator failure

3PC Disadvantages:

  • More complex
  • More latency
  • Still not partition tolerant

Saga Pattern (Alternative)

Saga: Sequence of local transactions with compensating actions

Q5: Design a content delivery network (CDN) protocol.

Answer:

CDN Architecture

Request Flow

Cache Invalidation

Purge Protocol:

Cache Hierarchy

Advanced Features:

  • Anycast: Same IP, routed to nearest POP
  • Dynamic Content: Edge compute (Cloudflare Workers)
  • Image Optimization: On-the-fly resizing
  • DDoS Protection: Absorb attacks at edge

Q6: Explain QUIC protocol and HTTP/3.

Answer:

TCP vs QUIC Handshake

Head-of-Line Blocking

Connection Migration

QUIC Benefits:

  • Faster connection establishment
  • Better performance on lossy networks
  • Improved mobile experience
  • Easier to deploy (UDP not blocked)

Q7: Design a gossip protocol for distributed systems.

Answer:

Gossip Algorithm

Information Spread

Gossip Variants

Anti-Entropy

Use Cases:

  • Cluster membership (Consul, Cassandra)
  • Failure detection
  • Database replication
  • Configuration propagation

Trade-offs:

  • Pros: Scalable, fault-tolerant, simple
  • Cons: Eventually consistent, message overhead, convergence time

Q8: Explain vector clocks and conflict resolution.

Answer:

Vector Clock Structure

Format: [A:3, B:1, C:2] = A has seen 3 events from A, 1 from B, 2 from C

Vector Clock Evolution

Conflict Detection

Comparison Rules:

  • V1 < V2: All counters in V1 ≀ V2, at least one <
  • V1 > V2: All counters in V1 β‰₯ V2, at least one >
  • V1 || V2: Neither < nor >

Conflict Resolution

Example - Shopping Cart:

Use Cases:

  • Dynamo-style databases (Riak, Cassandra)
  • Collaborative editing
  • Distributed caches
  • Mobile offline-first apps

Q9: Design a distributed rate limiter.

Answer:

Architecture Options

Centralized Approach

Pros: Accurate, simple Cons: Single point of failure, latency

Distributed with Gossip

Pros: No single point, scalable Cons: Eventually consistent, may exceed limit temporarily

Token Bucket with Redis

Lua Script (atomic):

 1local key = KEYS[1]
 2local capacity = tonumber(ARGV[1])
 3local rate = tonumber(ARGV[2])
 4local now = tonumber(ARGV[3])
 5
 6local last_refill = redis.call('HGET', key, 'last_refill') or now
 7local tokens = redis.call('HGET', key, 'tokens') or capacity
 8
 9local elapsed = now - last_refill
10local new_tokens = math.min(capacity, tokens + elapsed * rate)
11
12if new_tokens >= 1 then
13  redis.call('HSET', key, 'tokens', new_tokens - 1)
14  redis.call('HSET', key, 'last_refill', now)
15  return {new_tokens - 1, 1}
16else
17  return {new_tokens, 0}
18end

Sliding Window with Redis

Trade-offs:

  • Accuracy: Centralized > Hybrid > Distributed
  • Latency: Distributed < Hybrid < Centralized
  • Scalability: Distributed > Hybrid > Centralized

Q10: Explain consensus in blockchain (Proof of Work, Proof of Stake).

Answer:

Proof of Work (Bitcoin)

Mining Process: $$\text{SHA256}(\text{SHA256}(\text{block header})) < \text{target}$$

PoW Characteristics:

  • Security: 51% attack expensive
  • Decentralized: Anyone can mine
  • Energy: High consumption
  • Finality: Probabilistic (6 confirmations)

Proof of Stake (Ethereum 2.0)

Validator Selection

PoS Characteristics:

  • Energy: Low consumption
  • Security: Slashing for misbehavior
  • Finality: Faster (2 epochs β‰ˆ 13 minutes)
  • Barrier: Need stake to participate

Slashing

Comparison

Use Cases:

  • PoW: Bitcoin, Litecoin, Monero
  • PoS: Ethereum 2.0, Cardano, Polkadot
  • DPoS: EOS, Tron
  • PoA: Private blockchains

Summary

Hard protocol and design topics:

  • Game Protocol: Low latency, client prediction, lag compensation
  • Byzantine Fault Tolerance: PBFT, 3f+1 nodes
  • Distributed Locks: Chubby/ZooKeeper, fencing tokens
  • Distributed Transactions: 2PC/3PC, Saga pattern
  • CDN Protocol: Edge caching, invalidation, anycast
  • QUIC/HTTP3: UDP-based, 0-RTT, connection migration
  • Gossip Protocol: Epidemic spread, anti-entropy
  • Vector Clocks: Causality tracking, conflict detection
  • Distributed Rate Limiting: Token bucket, sliding window
  • Blockchain Consensus: PoW, PoS, trade-offs

These advanced concepts enable designing complex distributed systems and protocols.

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