Global server load balancing (GSLB) in HLD - Architecture Diagram

System Overview - Global server load balancing (GSLB)

Global Server Load Balancing (GSLB) distributes user requests across multiple geographically dispersed data centers. It improves availability, reduces latency by directing users to the closest or healthiest server, and balances traffic globally.

Architecture Diagram

User
  |
  v
+-----------------+
|   DNS Resolver  |
+-----------------+
          |
          v
+-----------------+       +-----------------+       +-----------------+
| GSLB DNS Server |<----->| Data Center A   |<----->| Local Load      |
+-----------------+       | (Web Servers)   |       | Balancer        |
          |               +-----------------+       +-----------------+
          |                       |                         |
          |                       v                         v
          |               +-----------------+       +-----------------+
          |               | Cache Layer     |       | Application     |
          |               +-----------------+       | Servers         |
          |                       |                         |
          |                       v                         v
          |               +-----------------+       +-----------------+
          |               | Database        |       | Monitoring      |
          |               +-----------------+       +-----------------+
          |
          |               +-----------------+       +-----------------+
          |               | Data Center B   |<----->| Local Load      |
          |               | (Web Servers)   |       | Balancer        |
          |               +-----------------+       +-----------------+
          |                       |                         |
          |                       v                         v
          |               +-----------------+       +-----------------+
          |               | Cache Layer     |       | Application     |
          |               +-----------------+       | Servers         |
          |                       |                         |
          |                       v                         v
          |               +-----------------+       +-----------------+
          |               | Database        |       | Monitoring      |
          |               +-----------------+       +-----------------+

Components

User

client

End user making requests to the service

DNS Resolver

dns_resolver

Resolves domain names for users

GSLB DNS Server

gslb_dns

Directs user requests to the best data center based on health and proximity

Data Center A

data_center

Hosts web servers and services for serving user requests

Local Load Balancer

load_balancer

Distributes traffic within a data center to application servers

Cache Layer

cache

Speeds up data access by caching frequently requested data

Application Servers

service

Processes business logic and user requests

Database

database

Stores persistent data for the application

Monitoring

monitoring_service

Checks health and performance of servers and services

Data Center B

data_center

Secondary data center for redundancy and load distribution

Request Flow - 14 Hops

User → DNS Resolver

DNS Resolver → GSLB DNS Server

GSLB DNS Server → Monitoring

Monitoring → GSLB DNS Server

GSLB DNS Server → DNS Resolver

DNS Resolver → User

User → Local Load Balancer

Local Load Balancer → Cache Layer

Cache Layer → Local Load Balancer

Local Load Balancer → Application Servers

Application Servers → Database

Database → Application Servers

Application Servers → Local Load Balancer

Local Load Balancer → User

Failure Scenario

Component Fails:Data Center A

Impact:Users directed to Data Center A experience service disruption; requests fail or timeout

Mitigation:GSLB DNS detects health failure via monitoring and stops directing traffic to Data Center A, routing users to Data Center B instead

Architecture Quiz - 3 Questions

Which component decides which data center a user should connect to?

A GSLB DNS Server

B Local Load Balancer

C Cache Layer

D Database

Design Principle

This design uses DNS-based global load balancing to direct users to the best data center, improving latency and availability. Local load balancers distribute traffic inside data centers. Health monitoring enables automatic failover. Caches reduce database load and speed up responses.

Practice

1. What is the primary purpose of Global Server Load Balancing (GSLB)?

easy

A. To balance load only within a single data center

B. To direct user requests to the best global server based on location and health

C. To encrypt data between servers globally

D. To store user data in a centralized database

Solution

1. Step 1: Understand GSLB's role in system design

   GSLB directs traffic globally to optimize speed and availability by choosing the best server.

2. Step 2: Differentiate GSLB from local load balancing

   Local load balancing manages traffic inside a data center, while GSLB manages traffic across data centers worldwide.

3. Final Answer:

   To direct user requests to the best global server based on location and health -> Option B

4. Quick Check:

   GSLB = directs users globally [OK]

Hint: GSLB chooses best global server, not just local load [OK]

Common Mistakes:

• Confusing GSLB with local load balancing
• Thinking GSLB handles data encryption
• Assuming GSLB stores user data centrally

2. Which of the following is a correct component used in GSLB architecture?

easy

A. Global DNS server with health checks

B. Single local load balancer without DNS

C. Centralized database for user sessions

D. Static IP routing without failover

Solution

1. Step 1: Identify key GSLB components

   GSLB uses global DNS servers that perform health checks to route users to healthy servers.

2. Step 2: Eliminate incorrect options

   Local load balancers alone don't handle global routing; centralized databases and static IP routing don't provide dynamic failover.

3. Final Answer:

   Global DNS server with health checks -> Option A

4. Quick Check:

   GSLB = global DNS + health checks [OK]

Hint: GSLB needs DNS with health checks, not just local balancers [OK]

Common Mistakes:

• Ignoring the role of DNS in GSLB
• Confusing local load balancer as GSLB
• Assuming static routing works for failover

3. Consider a GSLB setup with three data centers: US, Europe, and Asia. If the US data center is down, where will the DNS direct a user from Europe?

medium

A. To the closest healthy data center, Europe or Asia

B. To a random data center without health checks

C. To the US data center regardless of health

D. To the Asia data center only

Solution

1. Step 1: Understand GSLB health check impact

   If the US data center is down, GSLB excludes it from routing decisions.

2. Step 2: Determine routing for European user

   The DNS directs the user to the closest healthy data center, which could be Europe or Asia depending on proximity and health.

3. Final Answer:

   To the closest healthy data center, Europe or Asia -> Option A

4. Quick Check:

   GSLB routes to closest healthy server [OK]

Hint: GSLB skips unhealthy centers, picks closest healthy [OK]

Common Mistakes:

• Routing to down data center ignoring health
• Choosing random data center without checks
• Always routing to Asia regardless of location

4. A GSLB system is not redirecting traffic away from a failed data center. What is the most likely cause?

medium

A. User's browser cache is cleared

B. DNS TTL is set to zero

C. Load balancer inside data center is overloaded

D. Health checks are not configured or failing silently

Solution

1. Step 1: Analyze GSLB failure to redirect

   If traffic is not redirected from a failed data center, health checks likely are missing or not reporting failures.

2. Step 2: Evaluate other options

   DNS TTL zero causes frequent lookups but doesn't prevent failover; local load balancer overload or browser cache do not affect global DNS routing.

3. Final Answer:

   Health checks are not configured or failing silently -> Option D

4. Quick Check:

   Health checks missing = no failover [OK]

Hint: Check health checks first if failover fails [OK]

Common Mistakes:

• Blaming DNS TTL for failover issues
• Confusing local load balancer issues with GSLB
• Assuming browser cache affects server routing

5. You need to design a GSLB system for a global video streaming service. Which combination best improves user experience and reliability?

hard

A. Single global load balancer without DNS + static IP routing

B. Local load balancers only with no global routing

C. Global DNS with health checks + local load balancers + geo-proximity routing

D. DNS-based routing without health checks or load balancing

Solution

1. Step 1: Identify key GSLB features for video streaming

   Global DNS with health checks ensures users reach healthy servers; local load balancers distribute load inside data centers.

2. Step 2: Consider geo-proximity routing benefits

   Routing users to nearest data center reduces latency, improving streaming quality and reliability.

3. Step 3: Eliminate less effective options

   Single load balancer or no health checks cause bottlenecks or failover issues; local only misses global optimization.

4. Final Answer:

   Global DNS with health checks + local load balancers + geo-proximity routing -> Option C

5. Quick Check:

   Combine global DNS, health checks, local balancers, geo-routing [OK]

Hint: Combine global DNS, health checks, and geo-routing for best results [OK]

Common Mistakes:

• Ignoring health checks in DNS routing
• Using single load balancer for global traffic
• Skipping geo-proximity routing for latency