Distributed System Algorithms: Clock Synchronization, Snapshot, and Election, Slides of Computer Science

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Clock Synchronization

• Logical clocks
• Vector clocks
• Global state

Today: More Canonical Problems

• Distributed snapshot and termination detection
• Election algorithms
  • Bully algorithm
  • Ring algorithm

Distributed Snapshot Algorithm

• Assume each process communicates with another process using unidirectional point-to-point channels (e.g, TCP connections)
• Any process can initiate the algorithm
  • Checkpoint local state
  • Send marker on every outgoing channel
• On receiving a marker
  • Checkpoint state if first marker and send marker on outgoing channels, save messages on all other channels until:
  • Subsequent marker on a channel: stop saving state for that channel

Distributed Snapshot

• A process finishes when
  • It receives a marker on each incoming channel and processes them all
  • State: local state plus state of all channels
  • Send state to initiator
• Any process can initiate snapshot
  • Multiple snapshots may be in progress
    • Each is separate, and each is distinguished by tagging the marker with the initiator ID (and sequence number)

A C

M^ B M

Snapshot Algorithm Example

b) Process Q receives a marker for the first time and records its local state c) Q records all incoming message d) Q receives a marker for its incoming channel and finishes recording the state of the incoming channel

Termination Detection

• Detecting the end of a distributed computation
• Notation: let sender be predecessor , receiver be successor
• Two types of markers: Done and Continue
• After finishing its part of the snapshot, process Q sends a Done or a Continue to its predecessor
• Send a Done only when
  • All of Q ’s successors send a Done
  • Q has not received any message since it check-pointed its local state and received a marker on all incoming channels
  • Else send a Continue
• Computation has terminated if the initiator receives Done messages from everyone

Bully Algorithm

• Each process has a unique numerical ID
• Processes know the Ids and address of every other process
• Communication is assumed reliable
• Key Idea : select process with highest ID
• Process initiates election if it just recovered from failure or if coordinator failed
• 3 message types: election, OK, I won
• Several processes can initiate an election simultaneously
  • Need consistent result
• O(n 2 ) messages required with n processes

Bully Algorithm Details

• Any process P can initiate an election
• P sends Election messages to all process with higher Ids and awaits OK messages
• If no OK messages, P becomes coordinator and sends I won messages to all process with lower Ids
• If it receives an OK , it drops out and waits for an I won
• If a process receives an Election msg, it returns an OK and starts an election
• If a process receives a I won , it treats sender an coordinator

Bully Algorithm Example

d) Process 6 tells 5 to stop e) Process 6 wins and tells everyone

Ring-based Election

• Processes have unique Ids and arranged in a logical ring
• Each process knows its neighbors
  • Select process with highest ID
• Begin election if just recovered or coordinator has failed
• Send Election to closest downstream node that is alive
  • Sequentially poll each successor until a live node is found
• Each process tags its ID on the message
• Initiator picks node with highest ID and sends a coordinator message
• Multiple elections can be in progress
  • Wastes network bandwidth but does no harm

Comparison

• Assume n processes and one election in progress
• Bully algorithm
  • Worst case: initiator is node with lowest ID
    • Triggers n-2 elections at higher ranked nodes: O(n 2 ) msgs
  • Best case: immediate election: n-2 messages
• Ring
  • 2 (n-1) messages always