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)




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




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.




Election Algorithms

Wired systems

Bully algorithm

Ring algorithm

Wireless systems

Very large-scale systems




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

«




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





«

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´




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

«




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




Ring Algorithm (continued)

Coordinator=6





[2,3,4][2,3,4,5]

[5,6,0,1]





[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]




Ring Algorithm (concluded)

[5,6,0,1,2,3,4]

[1,2,3,4,5,6,0]

Coordinator=6

Coordinator=6




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




Questions?




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




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




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!




Digression

Domain Name Service




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




DNS Naming Hierarchy




Resolving DNS names to IP addresses

Two approaches:±

± Iterative

± Recursive




Iterative Resolution of Names




Recursive Resolution of Names




DNS Domain Registry Database

Text file containing records

Each record is {Name, Type, value(s)}




Example




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




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




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




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




Some Special Cases

Google

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




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




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




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




Questions?

week 4, election algorithms

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