Reaching Consensus In Distributed Systems
DevOps Meetup
Saulius Grigaitis
Myself
- Informatics Bachelor (FreeBSD Security)
- Software Engineering Master (Extreme Programming Software Process Model)
- Doctoral Student (Blockchain Consensus Algorithms)
- Partnership Associate Professor at MIF VU
- Blockchain Technologies Course at MIF VU
- 12+ Years Startups Experience @ NECOLT
Consensus
A fundamental problem in distributed computing and multi-agent systems is to achieve overall system reliability in the presence of a number of faulty processes.
Failure Modes
- Crash stop
- Crash recovery
- Omission
- Byzantine (Arbitrary)
The Byzantine Generals' Problem
Byzantine Failures: Lieutenant Traitor
Byzantine Failures: Commander Traitor
No Solution With Fewer Than 3m+1 Generals for m Traitors
Byzantine Faults
Are Common
FLP Impossibility (1985)
-
In a fully asynchronous message-passing distributed system, in which one process may have a crash failure it is not possible to solve the consensus problem.
- Dijkstra award in 2001
FLP Outcome
-
In a fully asynchronous system with deterministic algorithms and the possibility of even one crash fault, it is impossible to guarantee:
Both safety and liveness at the same time.
- Safety - the nodes agree on a valid value that was proposed by one of the nodes.
- Liveness - the nodes eventually reach agreement
- Fault tolerance - the system can survive a failure
CAP Theorem: Only Two of The Three
- Consistency
- Availability
- Partition Tolerance
Timing Modes
- Synchronous model
- Asynchronous model
- Semi-synchronous model
Failure Detectors
(Semi-synchronous model)
Leadership Level
Strong Leadership
Leaderless
Raft
Egalitarian Paxos
Proof of Work*
VRR
Fast Paxos
Raft
Proof of Work
Common PoW Problems
- Probabilistic Finality
- Centralization
- Weak Privacy
- Low Performance
- High Energy Consumption
DevOps
vs.
Science
Distributed Systems Are (Mostly) Faulty
Bugs
vs.
No Proof
jepsen.io
GOTO 2018 • Jepsen 9: A Fsyncing Feeling • Kyle Kingsbury
https://www.youtube.com/watch?v=tRc0O9VgzB0
Define Your Needs
Look For Formal Specifications
Do Your Own Verification
( Vendors May Not Know )