Global server load balancing (GSLB) in HLD - Scalability & System Analysis

Scalability Analysis - Global server load balancing (GSLB)

Growth Table: Global Server Load Balancing (GSLB)

Users	Traffic Characteristics	GSLB Impact	Infrastructure Changes
100 users	Low requests, regional access	Simple DNS-based routing sufficient	Single data center, basic load balancer
10,000 users	Moderate requests, some geo diversity	Need multiple regional data centers, basic GSLB	Deploy multiple data centers, DNS with geo-location
1,000,000 users	High requests, global access, peak traffic	GSLB must handle failover, latency optimization	Multiple global data centers, health checks, latency-based routing
100,000,000 users	Very high requests, global with spikes	GSLB must scale massively, integrate CDN, DDoS protection	Global distributed data centers, multi-layer load balancing, advanced monitoring

First Bottleneck

The first bottleneck in GSLB systems is usually the DNS resolution and health check system.

At scale, DNS servers can be overwhelmed by queries, and stale or slow health checks can cause poor routing decisions.

Also, network latency and inconsistent health data can cause traffic to route to unhealthy or distant servers, increasing user latency.

Scaling Solutions

• DNS Scaling: Use Anycast DNS and multiple authoritative DNS servers worldwide to handle query load.
• Health Checks: Implement distributed, frequent, and fast health checks with caching to reduce load and improve accuracy.
• Latency-based Routing: Use real-time latency measurements and geo-IP databases to route users to the closest healthy server.
• Failover: Automatic failover to backup data centers when primary ones are down.
• Integration with CDN: Offload static content to CDN to reduce load on origin servers and improve global performance.
• Load Balancer Hierarchy: Combine global load balancers (GSLB) with local load balancers for efficient traffic distribution.
• DDoS Protection: Deploy network-level protections to handle large traffic spikes and attacks.

Back-of-Envelope Cost Analysis

• Requests per second (RPS): At 1M users, assuming 1 request per second per 10 users, ~100,000 RPS globally.
• DNS Queries: Each user may generate multiple DNS queries; DNS servers must handle millions of queries per second at large scale.
• Bandwidth: For 100M users, assuming 100 KB per request, bandwidth needed is ~1 TB/s globally, requiring multi-region data centers and CDNs.
• Storage: Mainly for logs and health data; scalable with cloud storage solutions.
• Infrastructure: Multiple global data centers, Anycast DNS, CDN subscriptions, monitoring and security services.

Interview Tip

Start by explaining the user growth and traffic patterns globally.

Identify the first bottleneck (DNS and health checks) and explain why.

Discuss scaling solutions step-by-step: DNS scaling, health checks, latency routing, failover, CDN integration.

Use real numbers to justify your choices and show understanding of global infrastructure challenges.

Self Check Question

Your DNS servers handle 1000 queries per second. Traffic grows 10x. What do you do first?

Answer: Deploy additional authoritative DNS servers with Anycast IPs to distribute query load and reduce latency. Also, implement DNS caching and optimize TTL values to reduce query frequency.

Key Result

GSLB scales by distributing DNS and traffic globally, but DNS query load and health check accuracy are the first bottlenecks; scaling requires Anycast DNS, distributed health checks, latency-based routing, and CDN integration.

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