Course: Decentralized Systems (12min): Low-Latency Multi-Writer Atomic Registers. Specific tasks: Present a paper of interest in class

1. Towards Feasible Implementations
of Low-Latency Multi-Writer Atomic
Registers
Chryssis Georgiou, Nicolas Nicolaou,
Alexander C. Russell, Alexander A. Shvartsman
Maria Stylianou – marsty5@gmail.com
34335 DS – Decentralized Systems

2. 2
Overview
• Problem, Theory & Motivation
• Contributions
▫ APRX-SFW algorithm
▫ CWFR algorithm
• Experiments & Results
• Conclusions
34335 DS – Decentralized Systems

3. 3
Problem
Distributed
Storage System
• Data Replication
• Servers Failures
• Read/Write
Operations
• Consistency?
34335 DS – Decentralized Systems

4. 4
Conditions for Consistency
Safety
Regularity
Atomicity
34335 DS – Decentralized Systems

5. 5
Model Characteristics
• MWMR
• Asynchrony
• Concurrency
• Fault Tolerance
• Availability 3 Sets of Processes
• R: readers
• Reliability • W: writers
• 1 operation / 1 process • S: servers
34335 DS – Decentralized Systems

6. 6
Model Characteristics
Quorum Systems
Efficiency
Read/Write ops
Latency
Communication cost
Communication cost FAST:1 comm-round
+ SLOW: 2 comm-round
Computation cost
34335 DS – Decentralized Systems

7. 7
Model Characteristics
Communication Round
1. User sends msg to a set of
servers
2. Each server that receives msg
 replies with ACK
3. User receives ACK from a
quorum of servers
Tag <timestamp, wid>
• timestamp: counter
• wid: last writer’s id
34335 DS – Decentralized Systems

8. 8
Motivation & Contributions
• Motivation – SFW algorithm
▫ Use of predicates
▫ Pros: Fast READ/WRITE ops
▫ Cons#1: Too much computation
▫ Cons#2: Dependent on the QS construction
• Contribution – APRX-SFW algorithm
▫ Polynomial log-approximation complexity
▫ Speeds up computation of SFW
34335 DS – Decentralized Systems

9. 9
Contributions: CWFR Algorithm
• Independent from the QS construction!
• Writer – 2 rounds
▫ 1st round: Send a READ to find the most recent tag
▫ 2nd round: Increase tag & Send a WRITE msg
• Reader – 1 or 2 round(s)
▫ 1st round: Send a READ to find the most recent tag
▫ Use of Quorum Views  Decide if 2nd round is needed
• Server – passive role
▫ Answers to received messages
34335 DS – Decentralized Systems

10. 10
Quorum Views
Qj
• 1st check: All servers of Qj keep the latest tag
 qView(1) (a)
▫ WRITE is complete READ FAST!
34335 DS – Decentralized Systems

11. 11
Quorum Views
Qj
• 1st check: All servers of Qj keep the latest tag
 qView(1) (a)
▫ WRITE is complete READ FAST!
• 2nd check: There is at least one intersection
where all servers keep the latest tag
 qView(3) (c + d)
▫ No info if WRITE is complete  READ SLOW!
34335 DS – Decentralized Systems

12. 12
Quorum Views
Qj
• 1st check: All servers of Qj keep the latest tag
 qView(1) (a)
▫ WRITE is complete READ FAST!
• 2nd check: There is at least one intersection where all servers
keep the latest tag
 qView(3) (c + d)
▫ No info if WRITE is completed  READ SLOW!
• 3rd check: Each intersection has an older tag
 qView(2) (b)
▫ Remove servers with latest tag and repeat checks
34335 DS – Decentralized Systems

13. 13
QS with small intersection degree
SIMPLE
APRX-SFW
CWFR
34335 DS – Decentralized Systems

14. 14
QS with large intersection degree
SIMPLE
CWFR
APRX-SFW
34335 DS – Decentralized Systems

15. 15
Conclusions
• Performance of APRX-SFW
▫ Number of writers
▫ Intersection degree
• CWFR has more stable performance
34335 DS – Decentralized Systems

16. 16
Web References
• Towards Feasible Implementations of Low-
Latency Multi-Writer Atomic Registers,
http://www.cs.ucy.ac.cy/fastMWMR/pubs/MW
MR_NCA11_IEEE.pdf
• Towards Feasible Implementations of Low-
Latency Multi-Writer Atomic Registers,
Technical Report,
http://www.cs.ucy.ac.cy/fastMWMR/MWMR-
TR.pdf
34335 DS – Decentralized Systems

17. Towards Feasible Implementations
of Low-Latency Multi-Writer Atomic
Registers
Chryssis Georgiou, Nicolas Nicolaou,
Alexander C. Russell, Alexander A. Shvartsman
Maria Stylianou – marsty5@gmail.com
34335 DS – Decentralized Systems