Election AlgorithmsCS-4513 D-term 2008 1
Election Algorithms
CS-4513Distributed Computing Systems
(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne, Distributed Systems: Principles & Paradigms, 2nd ed. By Tanenbaum and Van Steen, and
Modern Operating Systems, 2nd ed., by Tanenbaum)
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Election Algorithms
• If we are using one process as a coordinator for a shared resource …
• …how do we select that one process?
• Often, there is no owner or master that is automatically considered as coordinator
• E.g., Grapevine, there is no owner for a Registry• By contrast:–DNS has a master for every domain
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Solution – an Election
• All nodes currently involved get together to choose a coordinator
• If the coordinator crashes or becomes isolated, elect a new coordinator
• If a previously crashed or isolated node, comes on line, a new election may have to be held.
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Election Algorithms
• Wired systems• Bully algorithm
• Ring algorithm
• Wireless systems
• Very large-scale systems
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Bully Algorithm
• Assume • All processes know about each other
• Processes numbered uniquely
• Suppose P notices no coordinator• Sends election message to all higher numbered
processes
• If none response, P takes over as coordinator
• If any responds, P yields
• …
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Bully Algorithm (continued)
• …
• Suppose Q receives election message• Replies OK to sender, saying it will take over
• Sends a new election message to higher numbered processes
• Repeat until only one process left standing• Announces victory by sending message saying that
it is coordinator
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Bully Algorithm (continued)
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Bully Algorithm (continued)
• …
• Suppose R comes back on line• Sends a new election message to higher numbered
processes
• Repeat until only one process left standing• Announces victory by sending message saying that
it is coordinator (if not already coordinator)
• Existing (lower numbered) coordinator yields• Hence the term “bully”
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Alternative – Ring Algorithm
• All processed organized in ring• Independent of process number
• Suppose P notices no coordinator• Sends election message to successor with own
process number in body of message
• (If successor is down, skip to next process, etc.)
• Suppose Q receives an election message• Adds own process number to list in message body
• …
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Alternative – Ring Algorithm
• Suppose P receives an election message with its own process number in body
• Changes message to coordinator message, preserving body
• All processes recognize highest numbered process as new coordinator
• If multiple messages circulate …• …they will all contain same list of processes
(eventually)
• If process comes back on-line• Calls new election
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Ring Algorithm (continued)
Coordinator=6
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Ring Algorithm (continued)
[2,3,4][2,3,4,5]
[5,6,0,1]
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Ring Algorithm (continued)
[5,6,0,1,2,3,4][2,3,4,5]
[1,2,3,4,5,6,0]
[5,6,0,1,2]
[2,3,4,5,6]
[2,3,4,5,6,0]
[5,6,0,1,2,3]
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Ring Algorithm (concluded)
[5,6,0,1,2,3,4]
[1,2,3,4,5,6,0]
Coordinator=6
Coordinator=6
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Ring Algorithm (concluded)
• Suppose P receives an election message with its own process number in body
• Changes message to coordinator message, preserving body
• All processes recognize highest numbered process as new coordinator
• If multiple messages circulate …• …they will all contain same list of processes
(eventually)
• If process comes back on-line• Calls new election
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Questions?
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Wireless Networks
• Different assumptions• Message passing is less reliable
• Network topology constantly changing
• Expanding ring of broadcast• Election messages
• Decision rules for when to yield
• Not very well developed.• Topic of current research
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Very Large Scale Networks
• Sometimes more than one node should be selected
• Nodes organized as peers and super-peers• Elections held within each peer group
• Super-peers coordinate among themselves
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Reading Assignment
• Tanenbaum & van Steen (2nd ed.)• §6.5.2 – Elections in Wireless Systems
• §6.5.3 – Elections in Large Scale Systems
• Potential topics for quiz or test!
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Digression
Domain Name Service
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DNS
• Maps names of the form
www.cs.wpi.edu
to IP addresses
• Maps aliases to names
• Maps mailbox requests to names
• Maps service requests to names
• Maps IP addresses to names• I.e., reverse mapping
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DNS Naming Hierarchy
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Resolving DNS names to IP addresses
• Two approaches:–– Iterative– Recursive
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Iterative Resolution of Names
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Recursive Resolution of Names
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DNS Domain Registry Database
• Text file containing records• Each record is {Name, Type, value(s)}
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Example
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DNS Implementation
• One master copy per domain or subdomain• Edited manually by system administrator
– Using text editor or GUI tool
• Multiple slave copies• Automatically copied / updated periodically from
master• Stored in file on slave server, reloaded up restart
• Caching in DNS clients• Lots and lots of caching• Entries include TTL (time-to-live) specification
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Implementation in Linux/Unix
• BIND — Berkeley Internet Name Domain• http://www.bind9.net/
• named — the Name Daemon– Implements local DNS service– Multiple databases
• Primary or secondary
• Secondary database points back to primary
– Pointer to “higher level” service• For resolving names not in own database
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Example
• Want to find www.cs.wpi.edu• My DNS contacts DNS server 68.87.71.226
• A Comcast server specified in my DHCP lease
• Comcast DNS service• Almost certainly has root (global) domain in cache
• Probably has many .edu entries in cache (very large)
• Possibly has .wpi.edu in cache (many local users)
• May have .cs.wpi.edu
• Consults cache or official server for IP address• nslookup
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Example (continued)
C:\>nslookup cs.wpi.eduServer: cns.chelmsfdrdc2.ma.boston.comcast.netAddress: 68.87.71.226
Non-authoritative answer:Name: cs.wpi.eduAddress: 130.215.28.181
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Some Special Cases
• Yahoo
• MSN• Need to distribute names geographically
• Need to distribute different addresses for same name
• Special handling of replicated databases
• More (perhaps) later in term
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Naming Privacy
• Problem:– corporations need to have own domains
• www.merl.com
• Some public hosts – mail server, web server, etc.
• Does not want to expose names of internal hosts to outside world
• E.g., proprietary stuff
• But wants to make them visible internally• hotspur.merl.com
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Solution
• Two name services for same domain name!• Internal
• External
• External — visible to Internet (DMZ)• Database contains only a few names
• Points to other internet DNS’s for resolution of internet names
• Internal — seen only by internal hosts• Database contains all internal names
• Points to external version for resolution of internet names
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Result
• Internal names can be resolved internally, not externally
• hotspur.merl.com
• Internal names and IP addresses are invisible on Internet
• All external names can be resolved internally
• Two levels of indirection
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Questions?
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