2/23/2004 load balancing february 23, 2004. 2/23/2004 assignments work on registrar assignment
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2/23/2004
Load Balancing
February 23, 2004
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2/23/2004
Assignments
• Work on Registrar Assignment
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2/23/2004
Pieces of the Puzzle
Load Balancing
Servers
Data Store
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2/23/2004
Load Balancing
• Distribute load across multiple servers – why?
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2/23/2004
Load Balancing
• Distribute load across multiple servers – why?– Service more requests per second– Reduce response time– Tolerate failures
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2/23/2004
Simple Algorithm
• Round-robin– N servers– Request 1 goes to server 1– Request 2 goes to server 2– …– Request N goes to server N– Request N+1 goes to server 1
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2/23/2004
Problems
• What are some problems with this strategy?
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2/23/2004
Goals/Challenges
• Determining least loaded server
• Determining closest server
• Detecting and deflecting requests from failed server
• Transparency
• What about user state?
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2/23/2004
Strategies
• DNS-based
• Dispatcher-based
• Server-based
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2/23/2004
DNS: Domain Name System
• How do I identify a computer?
• Hostname– ID for a computer– Examples?
• IP Address– Why is an IP needed?– Examples?
• Mapping from hostname to IP address?
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2/23/2004
DNS: Domain Name System
• Name servers– Store hostname to IP mapping– Organized in a hierarchy– Act as a distributed database
• Application-layer protocol– Defines communication between hosts and name
servers
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2/23/2004
Usage Scenario
• Which apps use DNS?
• HTTP – Browser extracts hostname– Sends hostname to DNS– DNS does lookup and returns IP address– Browser sends HTTP GET to IP address
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2/23/2004
Simple DNS Example
• Host surf.eurecom.fr wants IP address of gaia.cs.umass.edu1. contacts its local DNS
server, dns.eurecom.fr
2. dns.eurecom.fr contacts root name server, if necessary
3. root name server contacts authoritative name server, dns.umass.edu, if necessary
requesting hostsurf.eurecom.fr
gaia.cs.umass.edu
root name server
authorititive name serverdns.umass.edu
local name serverdns.eurecom.fr
1
23
4
5
6
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2/23/2004
Strategy 1: DNS-based
138.110.1.1
138.110.1.2
138.110.1.3
DNS server
138.110.1.1138.110.1.2
138.110.1.3
Web Servers
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2/23/2004
Strategy 1: DNS-based
• Pros– Easy
• Cons– Simple algorithm may not distribute load best– Slow to deal with failures– Not transparent to client
• Caching may disrupt algorithm
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2/23/2004
Strategy 2: Dispatcher-based
138.110.1.1
138.110.1.2
138.110.1.3
Web Servers
Load balancing switch
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2/23/2004
Strategy 2: Dispatcher-based
• Pros– Transparent (NAT)– Reacts more quickly to failure– Apply more advanced scheduling algorithms
• Number of active connections• Based on URI (/images, etc)
• Cons– Bottleneck/single point of failure– New piece of hardware
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2/23/2004
Strategy 3: Server-based
138.110.1.1
138.110.1.2
138.110.1.3
Web Servers
DNS server
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2/23/2004
Strategy 3: Server-based
• Pros– Reacts more quickly to failure– Apply more advanced scheduling algorithms
• Cons– Not transparent– Increased delay
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2/23/2004
Data Replication/Partitioning
• Replication– All servers can serve all data
• Partitioning– Servers keep track of specific set of data
• In reality, both
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2/23/2004
Example
A-Z A-Z A-Z A-I J-R S-Z
Replication Partitioning
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2/23/2004
Consistency
• All copies of same data should be the same– A change to one means a change to all
• Performance/consistency tradeoff