Chapter 4: COMMUNICATION PATTERNS & STATE - System Design Course: Complete Production Architecture | PlayBook Library

SLIDE 19: Sync vs Async: When to Use Each

Synchronous vs Asynchronous Communication

CODE

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Synchronous Communication:

Characteristic	Description
Blocking	Client waits for response
Immediate	Response within milliseconds
Simple	Request-response pattern
Coupled	Client and server must be available simultaneously

Asynchronous Communication:

Characteristic	Description
Non-blocking	Client continues immediately
Delayed	Response comes later
Complex	Requires queue, callbacks, or polling
Decoupled	Client and server can be unavailable at different times

SLIDE 19B: Sync vs Async: When to Use Each (continued)

Decision Framework:

Use Sync When	Use Async When
User is waiting for response	User does not need immediate result
Operation is fast (< 500ms)	Operation is slow (> 1 second)
Failure should be visible immediately	Failure can be retried later
Simple request-response pattern	Complex multi-step workflow
Strong consistency required	Eventual consistency acceptable

Examples:

Operation	Pattern	Why
Get user profile	Sync	User waiting, fast lookup
Login	Sync	User waiting, must know immediately
Upload photo	Async	Processing takes 5-10 seconds
Send notification	Async	User does not wait for delivery
Generate report	Async	Takes minutes, user can check later
Charge payment	Sync	User needs immediate confirmation

SLIDE 20: Sync vs Async: Synchronous Patterns

Synchronous Communication in Production

Request-Response Pattern:

CODE

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Request2. Forward3. Query4. Result5. Response6. Response

Timeout Configuration:

Hop	Timeout	Reasoning
Client to Gateway	30 seconds	Allow for slow networks
Gateway to Service	5 seconds	Service should be fast
Service to Database	2 seconds	Query should be optimized
Service to External API	3 seconds	External dependency budget

Rule: Inner timeouts must be shorter than outer timeouts.

If database timeout is 5 seconds but service timeout is 3 seconds, the service will timeout before the database, causing confusing errors.

SLIDE 20B: Sync vs Async: Synchronous Patterns (continued)

Synchronous Anti-Patterns:

Anti-Pattern 1: Chain of Synchronous Calls

CODE

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Problem:

Total latency = A + B + C + D
Any service failure breaks entire chain
Debugging spans multiple services

Anti-Pattern 2: Synchronous Fan-Out

CODE

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Problem:

Must wait for slowest service
One slow service delays entire response

Solution: Parallelize where possible, or make async.

SLIDE 21: Sync vs Async: Asynchronous Patterns

Asynchronous Communication in Production

Queue-Based Pattern:

CODE

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path{stroke:url(#mermaid-64a5ccf8-9fbe-43ac-9858-3a01f16f442f-gradient);filter:drop-shadow( 1px 2px 2px rgba(185,185,185,1));}#mermaid-64a5ccf8-9fbe-43ac-9858-3a01f16f442f :root{--mermaid-font-family:"trebuchet ms",verdana,arial,sans-serif;}ProducerQueueConsumerResult1. Enqueue2. Dequeue3. Process

When This Pattern Works:

Scenario	Example
Slow processing	Image resizing, video encoding
Unreliable consumer	External API that may be down
Spiky traffic	Absorb bursts, process at steady rate
Fan-out	One message to many consumers
Retry logic	Failed messages go back to queue

Queue Technologies:

Technology	Use Case	Throughput
AWS SQS	General purpose	10K+ msg/sec
RabbitMQ	Complex routing	20K+ msg/sec
Apache Kafka	High throughput streaming	1M+ msg/sec
Redis Streams	Simple queuing with Redis	100K+ msg/sec

SLIDE 21B: Sync vs Async: Asynchronous Patterns (continued)

Event-Driven Pattern:

CODE

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Publish-Subscribe Benefits:

Benefit	Explanation
Loose coupling	Publisher does not know subscribers
Easy extension	Add new subscriber without changing publisher
Independent scaling	Each subscriber scales independently
Failure isolation	One subscriber failure does not affect others

Event Schema Example:

CODE

{
  "event_type": "user.created",
  "event_id": "abc-123",
  "timestamp": "2025-01-15T10:30:00Z",
  "data": {
    "user_id": 12345,
    "email": "mukul@example.com"
  }
}

SLIDE 22: Sync vs Async: Hybrid Patterns

Combining Sync and Async

Pattern: Sync Response, Async Processing

CODE

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Upload Request2. Store in Queue3. Return 202 Accepted4. Process Later5. Notify

HTTP Response:

CODE

HTTP/1.1 202 Accepted

{
  "job_id": "abc-123",
  "status": "processing",
  "status_url": "/v1/jobs/abc-123"
}

Use Cases:

Photo upload (return immediately, process thumbnails async)
Report generation (return job ID, poll for completion)
Batch operations (acknowledge receipt, process in background)

SLIDE 22B: Sync vs Async: Hybrid Patterns (continued)

Pattern: Async with Sync Fallback

CODE

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Example: Feed Generation

CODE

1. User requests feed
2. Check cache for pre-computed feed
3. If cache hit: Return immediately (sync, fast)
4. If cache miss:
   - Option A: Compute on demand (sync, slow)
   - Option B: Return stale + trigger refresh (hybrid)
   - Option C: Return empty + push when ready (async)

Trade-off:

Option	Latency	Freshness	Complexity
A: Sync compute	High (500ms)	Fresh	Low
B: Stale + refresh	Low (50ms)	Slightly stale	Medium
C: Push when ready	Very low	Eventually fresh	High

SLIDE 23: Sync vs Async: Trade-offs

Latency vs Consistency vs Complexity

The Trade-off Triangle:

Approach	Latency	Consistency	Complexity
Synchronous	Higher	Strong	Lower
Asynchronous	Lower	Eventual	Higher

Consistency Implications:

Synchronous:

Structure & Concept Flow

User creates post -> Sync write to DB -> Post visible immediately

Strong consistency: What you wrote is what you read.

Asynchronous:

Structure & Concept Flow

User creates post -> Enqueue -> Worker writes to DB -> Post visible later

Eventual consistency: What you wrote will eventually be readable.

Key Question:
Can the user tolerate seeing stale data for a few seconds?

Use Case	Tolerance	Pattern
Bank balance	None	Sync
Social feed	Seconds	Async OK
Inventory count	Low	Sync with cache invalidation
Analytics dashboard	Minutes	Async

SLIDE 23B: Sync vs Async: Trade-offs (continued)

Debugging Complexity:

Synchronous Debugging:

CODE

1. Request comes in
2. Follow the call stack
3. Find the error
4. Fix and deploy

Single trace, single log stream, straightforward.

Asynchronous Debugging:

CODE

1. Request comes in
2. Message goes to queue
3. Worker picks up message (maybe minutes later)
4. Worker processes (different machine, different logs)
5. Error occurs
6. Need to correlate across multiple systems

Production Practice:
For async systems, implement:

Correlation IDs (trace message across systems)
Dead letter queues (capture failed messages)
Message replay (re-process failed messages)
Distributed tracing (visualize message flow)

SLIDE 24: Stateful vs Stateless: The Basics

Why Stateless is Preferred

Stateless Service:
Does not store any client session data between requests.

Stateful Service:
Stores client session data that persists between requests.

CODE

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Stateless Benefits:

Benefit	Explanation
Horizontal scaling	Add servers without data migration
Simple load balancing	Any server can handle any request
Failure resilience	Server crash loses no client data
Simple deployment	Replace any server without draining sessions

SLIDE 24B: Stateful vs Stateless: The Basics (continued)

Stateless Architecture:

CODE

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State is Externalized:

State Type	External Storage
Session data	Redis, Memcached
User data	Database
File uploads	Object storage (S3)
Cache	Redis cluster

Request Contains All Context:

CODE

Authorization: Bearer {jwt_token}

JWT contains:
- user_id
- roles
- expiration

No server-side session lookup needed.

SLIDE 25: Stateful vs Stateless: When State is Unavoidable

Some State Cannot Be Externalized

WebSocket Connections:

CODE

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WebSocket connection is inherently stateful. The connection lives on a specific server.

In-Memory Caches:

CODE

Local cache on Server A: { user_123: {...} }
Local cache on Server B: { user_456: {...} }

For extreme low latency, local memory is faster than Redis.

Long-Running Computations:

CODE

Server A: Processing video encode job (30 minutes)

Cannot easily move this to another server mid-computation.

SLIDE 25B: Stateful vs Stateless: When State is Unavoidable (continued)

Strategies for Stateful Services:

Strategy 1: Sticky Sessions

CODE

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Load balancer routes same client to same server.

Problem: Server failure loses all sessions on that server.

Strategy 2: State Replication

CODE

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All servers have copy of all state.

Problem: Replication overhead, consistency complexity.

Strategy 3: Graceful Handoff

CODE

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Before shutdown, transfer active sessions to another server.

Problem: Complex implementation, transfer takes time.

SLIDE 26: Stateful vs Stateless: Session Management

How to Handle User Sessions

Option 1: Server-Side Sessions (Stateful)

Structure & Concept Flow

Client sends: session_id=abc123

Server looks up: sessions[abc123] -> {user_id: 1, cart: [...]}

Problems:

Session lives on one server
Requires sticky sessions or shared session store
Server memory grows with active users

Option 2: JWT Tokens (Stateless)

CODE

Client sends: Authorization: Bearer eyJhbGc...
Server decodes: {user_id: 1, exp: 1234567890, roles: ["user"]}

Benefits:

No server-side lookup
Any server can validate
Scales horizontally

Problems:

Cannot revoke immediately (must wait for expiration)
Token size grows with claims

SLIDE 26B: Stateful vs Stateless: Session Management (continued)

Production Pattern: JWT with Refresh Tokens

CODE

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Token Configuration:

Token Type	Lifetime	Storage	Purpose
Access Token	15 minutes	Memory	API authentication
Refresh Token	7 days	Secure cookie	Get new access token

Revocation Strategy:

CODE

Access tokens cannot be revoked (short-lived, acceptable).
Refresh tokens can be revoked (stored in database with revoked flag).

For immediate logout:
1. Revoke refresh token in database
2. Access token expires within 15 minutes
3. User cannot get new access token

SLIDE 27: Stateful vs Stateless: Sticky Sessions

When and How to Use Sticky Sessions

What is Sticky Session:
Load balancer routes requests from same client to same server.

Implementation:

Structure & Concept Flow

Load Balancer Cookie:

Set-Cookie: SERVERID=server-a; Path=/

Subsequent requests:

Cookie: SERVERID=server-a

-> Routed to server-a

When to Use:

Use Case	Why Sticky
WebSocket connections	Connection is inherently to one server
In-memory session cache	Avoid distributed cache latency
Long polling	Client waiting for specific server response
Gradual rollout	Keep user on same version during session

SLIDE 27B: Stateful vs Stateless: Sticky Sessions (continued)

Sticky Session Problems:

Problem 1: Uneven Load Distribution

CODE

Server A: 1000 sessions (overloaded)
Server B: 100 sessions (idle)
Server C: 50 sessions (idle)

Some users happen to be more active than others.

Problem 2: Server Failure Loses Sessions

Structure & Concept Flow

Server A crashes

-> 1000 users lose their sessions

-> 1000 users must re-authenticate

Problem 3: Deployment Complexity

Structure & Concept Flow

Deploy new version to Server A

-> Must drain 1000 sessions first

-> Or lose 1000 active sessions

Mitigation:

Problem	Solution
Uneven load	Periodic session rebalancing
Server failure	Session replication to standby
Deployment	Graceful drain with session handoff

SLIDE 28: Architecture Boundaries: Summary

Key Takeaways

Production Requirements:

Uptime SLAs compound across dependencies
Latency budgets force optimization decisions
Cost constraints shape architecture choices
Team structure determines service boundaries
Compliance requirements restrict data flow

Service Boundaries:

Start with monolith, split when you feel pain
Every service needs a clear owner
API contracts are the law between services
Backward compatibility protects consumers

Sync vs Async:

Use sync when user waits and operation is fast
Use async when operation is slow or failure is tolerable
Hybrid patterns combine the best of both
Async increases debugging complexity

Stateful vs Stateless:

Prefer stateless for horizontal scaling
Externalize state to databases and caches
Use JWT for session management
Sticky sessions are a last resort with known trade-offs

SLIDE 29: Architecture Boundaries: Decision Framework

How to Make These Decisions in Practice

Decision 1: Monolith vs Microservices

CODE

IF team_size < 10 AND system_age < 2_years:
    START with monolith
ELIF different_scaling_needs OR different_deployment_cadence:
    SPLIT into services
ELSE:
    STAY with monolith

Decision 2: Sync vs Async

CODE

IF user_waiting AND operation < 500ms:
    USE sync
ELIF operation > 1_second OR can_fail_silently:
    USE async
ELSE:
    USE hybrid (sync response, async processing)

Decision 3: Stateful vs Stateless

CODE

IF can_externalize_state:
    USE stateless with external store
ELIF connection_inherently_stateful (WebSocket):
    USE sticky sessions with replication
ELSE:
    EVALUATE trade-offs case by case

SLIDE 30: Section 1-2 Complete

What We Covered

Section 1: Introduction

The gap between interview design and production reality
What production constraints actually look like
The production mindset of senior engineers
Course structure and what is coming next

Section 2: Production Requirements and Architecture Boundaries

Uptime, latency, cost, and compliance constraints
Team ownership and Conway's Law
Service boundaries: when to split, API contracts, versioning
Sync vs async: patterns, trade-offs, and when to use each
Stateful vs stateless: session management, sticky sessions

Coming Up Next: Section 3 - Data in Production

Schema evolution and migrations
Replication lag and consistency
Hot partitions and data distribution
Backups and disaster recovery