System Design - Scalability: Load Balancing 1

Load Balancing

• Load balancing is critical for large-scale applications, distributing workload across multiple servers to optimize resource use, maximize throughput, and avoid overload.
• Primary goals: increase availability and reliability, improve response time and performance, and provide fault tolerance and redundancy.

Types of Load Balancers

• Layer 4 (Transport Layer) Load Balancers:
  • Operate at the transport layer (TCP/UDP)
  • Route traffic based on IP address and TCP/UDP port
  • Less CPU intensive as they do not inspect packet contents
• Layer 7 (Application Layer) Load Balancers:
  • Operate at the application layer (HTTP/HTTPS)
  • Route traffic based on detailed application-level data (e.g., URL, HTTP headers)
  • Can handle more sophisticated routing rules and provide features like SSL termination

Load Balancing Algorithms

• Round Robin:
  • Distributes requests sequentially across the pool of servers
  • Simple and works well when servers have similar capacity
• Weighted Round Robin:
  • Assigns a weight to each server based on its capacity
  • Servers with higher weights receive more requests
• Least Connections:
  • Directs traffic to the server with the fewest active connections
  • Useful when traffic is varied and some requests may take longer to process
• IP Hash:
  • Routes requests based on the hash of the client’s IP address
  • Ensures that the same client is always directed to the same server, useful for session persistence
• Least Response Time:
  • Sends requests to the server with the quickest response time
  • Ensures faster service delivery by choosing the least busy server
• Random:
  • Distributes traffic randomly
  • Simple and works well for evenly distributed traffic patterns

Implementation of Load Balancing

• Hardware Load Balancers:
  • Specialized devices designed for high-performance load balancing
  • Examples: F5 Networks, Citrix NetScaler
• Software Load Balancers:
  • Run on standard servers and can be deployed on-premises or in the cloud
  • Examples: Nginx, HAProxy, Apache Traffic Server
• Cloud-based Load Balancers:
  • Managed services provided by cloud providers
  • Examples: AWS Elastic Load Balancing (ELB), Google Cloud Load Balancing, Azure Load Balancer

Features of Modern Load Balancers

• Health Checks:
  • Regularly monitor the health of backend servers
  • Remove unresponsive servers from the pool and reintroduce them when they recover
• SSL Termination:
  • Decrypts incoming SSL traffic and forwards it as plain HTTP to backend servers
• Session Persistence (Sticky Sessions):
  • Ensures that a user is consistently directed to the same server for the duration of a session
• Automatic Scaling:
  • Integrates with auto-scaling groups to add or remove servers based on traffic load
  • Provides elasticity to handle varying traffic demands
• Content-based Routing:
  • Directs traffic based on the content of the request, such as URL path or query parameters

Benefits of Load Balancing

• Improved Performance:
  • Distributes workload evenly, reducing latency and improving user experience
• Increased Availability:
  • Ensures continuous availability by rerouting traffic if a server fails
• Scalability:
  • Allows easy addition of new servers to handle increased traffic
• Redundancy and Fault Tolerance:
  • Provides a failover mechanism in case of server outages

Challenges and Considerations

• Cost:
  • Hardware load balancers can be expensive
  • Cloud load balancing services have ongoing operational costs
• Complexity:
  • Configuration and management can be complex, especially for advanced features
• Latency:
  • Additional layer in the network can introduce latency, though typically minimal
• Single Point of Failure:
  • The load balancer itself can become a single point of failure if not properly managed