Week 4, Election Algorithms
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Transcript of Week 4, Election Algorithms
8/8/2019 Week 4, Election Algorithms
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Election AlgorithmsCS-4513 D-term 2008 1
Election Algorithms
CS-4513
Distributed 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 E lection
All nodes currently involved get together to
choose a coordinator
If the coordinator crashes or becomesisolated, 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 thatit 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 ³bully´
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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 messagewith 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 messagewith 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
B IND ² Berkeley Internet Name Domain
http://www.bind9.net/
named ² the Name Daemon
± Implements local DNS service
± Multiple databases
Primary or secondary
Secondary database points back to primary
± Pointer to ³higher 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.edu
Server: cns.chelmsfdrdc2.ma.boston.comcast.net
Address: 68.87.71.226
Non-authoritative answer:
Name: cs.wpi.edu
Address: 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 ²
visible to Internet (DMZ) Database contains only a few names
Points to other internet DNS¶s for resolution of internet names
Internal ² 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