Dynamic Load Balancing on Web-server Systems

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Dynamic Load Balancing on Web-server Systems

• Valeria Cardellini, Michele Colajanni, and Philip
  S. Yu
• Presented by Sui-Yu Wang

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Distributed Web-server System

• Consisting of multiple Web-server hosts,
  distributed on LANs and WANs
• Spread incoming requests among these servers
• Each server can respond to any request
• Successful load-balancing approaches make the
  distributed system appear as a single host

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Distributed Web-server Architectures

• Client-based
• Web clients
• Client-side proxies
• Web-server based
• DNS-based
• Dispatcher-based
• Server-based

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DNS-based approach

• Architecture transparency
• DNS has limited control on requests reaching the
  Web cluster
• TTL
• Scheduling algorithms
• Constant TTL
• Adaptive TTL

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Scheduling algorithms

• Constant TTL algorithms
• System-stateless algorithms
• Round-Robin DNS
• Server-state-based algorithms
• Client-state-based algorithms
• Hidden load weight
• Multitier round-robin policy
• Consider relative server-to-client topology and
  client-to-server link latency
• Network proximity information, round trip delays
• Server-and client-state-based algorithms

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Scheduling algorithms

• Adaptive TTL algorithms
• Select server similar to the hidden load weight
  algorithms
• Assign appropriate TTL
• Can scale from LANs to WANs

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Dispatcher-based Approach

• Centralized request scheduling
• Packet rewriting
• Single-rewriting
• Double-rewriting
• Packet forwarding
• HTTP redirection

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Packet single-rewriting
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Packet double-rewriting
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Packet forwarding

• Network Dispatcher
• Works with LANs and WANs
• Client and server transparent
• ONE-IP Address
• Routing-based dispatching
• Broadcast-based dispatching
• No dynamic load-balancing based on server load

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HTTP redirection

• Centralized dispatcher
• Does not require modification of packet IP
  address
• Server-state based dispatching
• Location-based dispatching

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Server-based approach

• Two-level dispatching
• No centralized dispatching
• HTTP redirection
• Packet redirection
• Static routing
• Load-balancing

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Comparison of different approaches
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Performance evaluation

• Exponential model the number of page requests
  per session and the time between two page
  requests from the same client were assumed to be
  exponentially distributed

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Performance evaluation

• Client load variability is represented by some
  heavy-tailed function