5 Questions
What is the main problem that can occur in the context of liveness in consensus protocol?
Protocol may never end
What is the Impossibility result mentioned in the text and how is it related to consensus?
Impossibility result not violated; Consensus can be reached if things go well sometime in the future.
What is the scenario described in the example involving simultaneous proposers, prepare, and accept requests?
Livelock
What happens in the scenario where multiple promises and prepares are made but no progress towards accepting occurs?
Highest prepare keeps increasing, but no actual progress towards acceptance is made.
How is eventual liveness demonstrated in the context of the scenarios described?
Even though progress may be slow, there is eventual progress towards reaching a consensus.
Study Notes
Paxos Consensus Algorithm
- Provides safety (only a proposed value can be chosen, and only one value can be chosen) and eventual liveness (consensus is eventually reached, but not guaranteed)
FLP Impossibility Result
- Proves that it's impossible to guarantee consensus in an asynchronous system where even one host might fail
- In an asynchronous system, there's no upper bound on processing time, clock drift rate, or networking delay
- It's impossible to detect reliably whether a host has failed or is just slow
Paxos Algorithm Phases
- Phase 1: Leader election
- Phase 2: Leader proposes a value and receives acknowledgments
- Phase 3: Leader multicasts final value
Paxos Roles
- Proposer: receives client requests and tries to get a quorum of acceptors to agree
- Acceptor: participant in maintaining distributed storage, agrees on values with a quorum
- Learner: learns the agreed-upon value and can be queried later
Synchronous vs. Asynchronous Systems
- Synchronous systems have simultaneous steps, can detect faults, and are resilient to faults
- Asynchronous systems have no upper bounds on processing time, clock drift rate, or networking delay
Explore the safety and eventual liveness guarantees provided by the Paxos consensus protocol. Learn about the key principles such as only chosen values, single value selection, and eventual consensus. Understand the implications of the FLP result on Paxos' ability to reach consensus.
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