Paxos Made Moderately Complex Robert Van Renesse Cornell University - - PowerPoint PPT Presentation

Paxos Made Moderately Complex

Robert Van Renesse Cornell University

Problems Addressed

• Implementation of Multi-Decree Paxos
• Not a simple protocol!
• Liveness
• How to make Paxos live without overly simplifying?
• Practicality of Paxos
• How to make Paxos more pragmatic?

Comparison with the Required Paper

• What’s adopted?
• The multi-paxos protocol
• Basic state reduction optimizations
• What’s new?
• Very detailed pseudocode
• More cool optimizations!

Pseudocode

Working of the Pseudocode

• Leader uses scouts to get the next ballot number
• Clients bcast the requests to replicas
• Replicas propose the commands
• Leaders instruct commanders to carry out the

voting

• Decision is relayed back to the clients

Message Sequence Chart

Multi-decree paxos in action

Components and States

• Replicas
• Maintain a state, slot_num, proposal set, decision set
• Leaders
• Manage different proposals using different ballots
• Responsible for relaying a decision to the replicas
• Monitor other leaders when passive
• Acceptors
• They are the voters
• “Memory of Paxos”
• Scouts and Commanders
• Identify acceptors for a particular <ballot, slot, proposal> triplet - p1a message
• Preempt if higher ballots are running for the same proposal

Pragmatism with Paxos

• State Reduction
• Already discussed in the required paper
• Garbage Collection
• Read-only commands
• Collocation of replicas

• Garbage Collection
• When all replicas know of a decision for a slot, that information can be discarded from the

acceptor.

• Replica tells leader, leader informs acceptors
• 2f + 1 replicas to safely perform garbage collection
• Read-Only Command optimization
• Once a leader’s ballot is accepted, “attach” RO request to the update
• Use only one replica instead of all the replicas
• Using “leases”, a leader can become THE president.
• No need of jumping through all the hoops
• When the leaders and replicas are collocated…
• Mostly they are collocated
• Can send to local leaders instead of broadcasting to all leaders

Takeaways

• Exercise with 10 programming questions asking you to

do all the optimizations!

• Full fledged pseudocode
• Critical Optimizations for a practical implementation
• Garbage Collection
• Read-Only optimization
• Collocation