Elections in a Distributed Computing System

Hector Garcia-Molina

Presenter: Srinath Rao

Introduction to Elections

• Strategies to deal with a node failure– Have s/w which can operate continuously

even as failures occur– Halt temporarily, reorganize the system

• Need for the coordinator and hence election

• Election protocols can be used to start up a system, add/remove nodes

• Issues ?

Issues

• Constituent nodes may fail after election• What does it mean to be a coordinator?• How to cope up with failures during the

election itself?• Always possible to select a unique

coordinator?• Might wish to have more than one

coordinator

Outline

• Assumptions• Elections with no commn. failures

– The Bully Election Algorithm

• Elections with commn. failures– The Invitation Election Algorithm

• Related Work• Conclusions

Assumptions

1. All nodes cooperate2. Election algorithm makes use of

“bug-free” software facilities3. Communication subsystem will not

spontaneously generate messages4. Nodes have “safe” storage cells5. Node halts processing when it fails6. No transmission errors

Assumptions (contd..)

7. Messages are processed in the order they are received

8. Communication subsystem does not fail

9. Node never pauses

State Vector of node

• A collection of safe storage cells• Principal components of Vector S(i)

– Status of node i: S(i).s• Down, Election, Reorganization, Normal

– Coordinator according to node i: S(i).c– Definition of the Task being

performed: S(i).d

Outline

• Assumptions• Elections with no commn. failures

– The Bully Election Algorithm

• Elections with commn. failures– The Invitation Election Algorithm

• Related Work• Conclusions

Desired Characteristics

• Assertion 1: For two nodes i and j– S(i).c = S(j).c if nodes i and j are in one

of the states “Normal” or “Reorganization”

– S(i).d = S(j).d if both i and j are in “Normal” state

• States what it means to be a coordinator

Desired Characteristics (contd..)

• Assertion 2: If no failures, election will eventually transform a system in any state to a state:– There exists node i with S(i).s

=“Normal” and S(i).c = i– Other active nodes j have S(j).s =

“Normal” and S(j).c = i

The Bully Election Algorithm

• Each node has an unique id no.• Algorithm uses id no. as priorities• Two step algorithm

1. Node i tries to contact all nodes with higher priorities. If no reply received, then assume the role of coordinator

2. Inform all the lower priority nodes– Send “halt” message, force state of j to “Election”– Send “I am elected” message, node j sets S(j).c=I

and S(j).s = “Reorganization”– Distribute new algorithms to nodes, all status

changed to “Normal”

Bully (contd..)

• Let the recovering node k attempt to become the coordinator using the same algorithm– Halts all lower priority nodes which may

be in the process of becoming coordinators

– Step 1 ensures no conflict with higher priority nodes

Outline

• Assumptions• Elections with no commn. failures

– The Bully Election Algorithm

• Elections with commn. failures– The Invitation Election Algorithm

• Related Work• Conclusions

Discussion

• Failures– Partitioning of nodes– A node can only send/receive message– Node i and node j can talk to node k but not

with each other– Node may pause and then resume

• Observation: Impossibility of consensus in the event of failure of commn. subsystem or node pausing.

• Inference: Redefine the meaning of an election

Discussion (contd..)

• Notion of a group of nodes and group id

• Node i stores group id in its state vector: S(i).g

• Nodes are free to change groups• Identify messages with group id• Coordinator is unique within a group

New Desired Characteristics

• Assertion 3: For two nodes i and j– S(i).c = S(j).c if nodes i and j are in the

same group and are in one of the states “Normal” or “Reorganization”

– S(i).d = S(j).d if both i and j are in “Normal” state and are in the same group

Desired Characteristics (contd..)

• No requirements for nodes with only one-way communication

• Assertion 4: If no failures, election will eventually transform a set of nodes R that have two way communication in any state to a state:– There exists node i with S(i).s =“Normal”

and S(i).c = i– Other active nodes j in R have S(j).s =

“Normal” and S(j).c = i and S(j).g = S(i).g

The Invitation Election Algo.

• A node “invites” other nodes to join it in forming a new group

• A node may accept or decline an invitation• Make a receiving node form a new group

with itself the coordinator and the only member

• Objective: to merge groups• Coordinators periodically send “invite”

message• Can be used instead of the Bully algorithm

Related Work

• Scott D. Stoller (2000)– Modifies Bully algorithm to work with

crash failures– Points out a flaw and proposes a new

specification

• Gurdip Singh (1996)– Proposes an algorithm for leader

election in the presence of link failures

Conclusions

• Meaning of an election depends on the possible types of failures

• Paper studied elections in two representative failure environments

• Postulated assertions that define concept of an election

• Presented an election algorithm for each environment