Coordination in Distributed Systems

1
Coordination in Distributed Systems

• Lecture 8

2
Coordination Anecdotes

• Decentralized, no coordination
• Aloha 18
• Some coordinating Master
• Slotted Aloha 36
• Impossibility of Concensus
• If the ether dropped your packets, we will never
  have concensus (lynch)

3
Why is coordination important

• Resource Sharing
• Database
• Files
• Agreement
• Agree on an award for faculty
• Agree on whether to take a quiz
• Leader Election
• Choose Time server
• Choose Bus Master

4
Challenges

• Host Failures
• Link Failures
• Time and Causual Ordering

5
Distributed Mutual Exclusion

• Mutual Exclusion
• Atmost one process in the critical section
• What happens when processes are distributed
• Lets assume reliability first
• Three messages
• Enter() I want to enter
• ResourceAccess() OK Go ahead access the critical
  section
• Exit() I am done

6
Requirements for Mutex

• At most one process in the critical section
• Everyone gets to access the critical section
  Eventual grant
• Global ordering Difficult

7
Distributed Mutual Exclusion
P1
P4
P3
P2
8
Centralized Algorithm

• Single Coordinating Server
• P can enter Critical Section (CS) if it has a
  grant TOKEN
• Server Grants TOKEN
• Processes Request TOKEN
• GRANT if CS is free
• Else queue
• Grant to the oldest in the queue
• Problems
• Performance Bottleneck
• Server Crash
• Ordering?

9
Ring Based Coordination

• Logical Ring
• Typically based on network address
• Pass token clockwise (other otherwise)

10
Ring based Coorindation

• No Master, no Centralized Server
• Wait for TOKEN
• If you want to enter CS
• Grab Token
• Access CS
• Release and Pass along the TOKEN
• Else
• Pass along the Token
• Problems
• Continuous Overhead
• Must wait for N (worst case) before enter is
  granted
• Ordering?

11
RicartaAgarwal Algo

• Based on Reliable Multicast
• Multicast to All Processes when you want to enter
  CS
• If you hear an Ack from everyone
• Enter CS
• Else
• Repeat
• Ordering?
• Timestamp multicast with Lamport Clocks
• Use vector clocks for total ordering

12
Leader Election

• Elect Unique Master
• Typically after failure
• Simulataneous Requests Allowed
• Permissible States
• Either No Leader
• Or Well-known leader
• Everyone must know about leader
• All processes must participate in election and
  can discover leader

13
ChangRoberts Algo
14
Ring coordination

• Find the Master with the maximum UID
• Receive UID
• Pass max(Own UID, Nieghbor UID)
• If you hear your UID again
• You are the leader
• Annouce yourself as the leader
• Problems
• Two Iterations
• Unique IDs assumed
• Failure?

15
ItaiRodeh Algo

• Relaxes UID Requirement
• Again Ring Based (unidirectional)
• Election
• Assign youre an ID (from 1 k)
• Do the Chang algo
• If no leader
• Choose ID again
• Termination?
• Probablistic

16
Termination

• Larger the K, larger your probability of
  assigning yourself a unique ID
• N 4, K16
• Time to terminate 1.01 rounds
• Welcome to the power of randomization
• Used in everyday things to make problems
  tractable
• Think Ethernet

17
IEEE 1394 Firewire

• High-speed synchronous Bus
• Used on Sony Handycams
• Bus Master Election Algorithm
• Not a ring (actually a tree)
• Hot Plug-n-Play NO UIDs
• Must elect master after a device is attached or
  removed

18
Algo

• Start with Leaves
• Send adopt me
• Leave has only one egress link
• If you hear n-1 adopt me messages
• Forward the message to the Nth link
• Why is probability used again for root detection?