Lecture 12 Synchronization

Lecture 12

Synchronization

EECE 411: Design of Distributed Software Applications

Roadmap for today Project logistics

Posted yesterday P01 due Wednesday Nov. 3rd

Apply for planetlab accounts Discuss quiz questions Synchronization in distributed systems


Before we start survey results


Useful Discussions structure

20 good thinking exercises; helps understand how knowledge is applied; good to discuss quizz-like questions

Assignments 9 closely related to class materials, usefull hands-on work, good that marking is done on coding style as much as functionality but *too few*!

Slides 8 Good summary of material, Useful for assignment, good overlap with previous week to tie in, I like the repetition, makes it more obvious what we have to learn

Real-world examples 7 Availability of TA / instructor || coding session 1 hmmmm' voting technique 1


Concerns Project not yet up 10

description / grading scheme / project expectations PlanetLab tutorial

Textbook is not a good reference 3 Epidemic 2

I would like to see this topic Replication 1 How do large things work 1 Distributed decision making 1 Cloud computing 1 Event-driven programming 1 Examples of pseudocode 1 Virtualization 1 Security 1


Suggestions Sample quizes/clearer idea on quiz expectations 8 More sample questions / more sample problems 7 Questions with answers (summarize discussions in slides) 6 More detailed explanation within the slides 3 Make slides available earlier 2 Discussion board 2 Fixed course structure 1 More structured relationships between topics 1 More coding sessions 1 More short assignments 1 Make sure that students without adequate coding experience can not take the class 1 Results of a design rather than covering a bit about each 1 Instructor-assigned groups (rather than based on student preference) 1 Good on quiz one: lots of questions 1 Provide grading scheme beforehand 1 Provide more of a big picture / Better organization of content / Roadmap 1 Tighter deadlines around project so that there is no cramming … 1 Assignments that include written parts 1 if we learn about gossip should we have a chance to implement? 1 More quantitative discussion questions 1 Go slower 1 Be more clear in describing concepts 1 Grading scheme: Best 2 out of 3 three quizes 1






Q5.) Consider a circular Distributed Hash Table (DHT) with identifiers in the range [0; 127]. Suppose there are eight participating nodes with identifiers 1, 13, 43, 51, 70, 83, 100 and 115. The DHT is configured so that the successor list has length 2. Also, the DHT is configured so that the finger table has size one: i.e., each peer maintains only one ‘shortcut’ (or ‘finger’) – this aims to reduce the search space in half. Questions:

a). Suppose that the following (key,value) pairs should be stored in the DHT: (0,’mama’), (3,’tata’), (7,’zaza’), (15,’bibi’), (110, ‘zizi’) and (125, ‘cici’). Which peers will store which (key,value) pair? Present your answer as a table.

b.) Assume a search launched at node 13 for key 0. Describe the search process.

c.) Suppose that peer 13 learns that peer 43 has left the DHT. How does peer 13 update its successor state information? Which peer is now its first successor? Its second successor? Is there any change in the set of keys each peer is responsible for?

c.) Suppose that a new peer with the identifier 5 wants to join the DHT and it initially only knows the IP address of the peer 53. What steps are taken for peer 6 to join the system? How does the system look like after peer 6 joins?


Key placement

CircularID Space

N13

N1

N70N51

N43

Node ID K110

K3, K7

K0, K125

K15

128 0

N83

N100

N115


Search launched at N13 for K0

Each node maintains Successor list (2) Shortcuts (1)

N13

N1

N70N51

N43

Node ID K110

K3, K7

K0, K125

K15N83

N100

N115




N13

N1

N70N51

N43

Node ID K110

K3, K7

K0, K125

K15N83

N100

N115




N13

N1

N70N51

N43

Node ID K110

K3, K7

K0, K125

K15N83

N100

N115




N13

N1

N70N51

N43

Node ID K110

K3, K7

K0, K125

K15N83

N100

N115


Peer 13 learns that peer 43 is dead. How does peer 13 update its successor state information? Which peer is now its first/second successor? Is there any change in the set of keys each peer holds?

N13

N1

N70N51

N43

Crashed

K110

K3, K7

K0, K125

K15N83

N100

N115


A new peer (6) wants to join the DHT and it initially only knows the IP address of the peer 51. What steps are taken for peer 6 to join the system? How does the system look like after peer 6 joins?

1. Lookup for 6? N132. Predecessor of N13?

N13. Announce yourself to

N1 4. N1 updates successor5. N1 notifies predecesor 6. N115 updates

successor 7. N6 joins8. Creates successor list 9. Splits keys with N1310. N13 updates

predecesor

N13

N1

N70N51

K110

K3, K7

K0, K125

K15

N83

N100

N115

N6 joins

Two invariants to maintain for correctness

Key to node assignment

Successor lists


6.) What are the criteria to choose between a system based on consistent hashing and one based on a distributed hash table.

Where are the differences? Key to node assignment? No difference Lookup? Yes (logN hops

for DHT vs. 1 hop consistent

hashing) Information used for lookup? Yes (logN vs. N) Impact of failures? Yes (logN vs. N) Ability to scale? Yes






Summary so far …

A distributed system is: a collection of independent computers that

appears to its users as a single coherent system

Components need to: Communicate

Point to point: sockets, RPC/RMI Point to multipoint: multicast, epidemic

Cooperate Naming to enable some resource sharing

Naming systems for flat (unstructured) namespaces: consistent hashing, DHTs

Naming systems for structured namespaces: EECE456 for DNS

Synchronization


Synchronization to support coordination Examples

Distributed make Printer sharing Monitoring of a real world system Agreement on message ordering

Why is synchronization more complex than in a single-box system No global views, multiple clocks, failures


Roadmap Physical clocks

Provide actual / real time ‘Logical clocks’

Where only ordering of events matters Leader election

How do I choose a coordinator?


Physical clocks (I) Problem: How to achieve agreement on time in a

distributed system?

A possible solution: use Universal Coordinated Time (UTC):

Based on the number of transitions per second of the cesium 133 atom (pretty accurate).

At present, the real time is taken as the average of some 50 cesium-clocks around the world.

Introduces a leap second from time to time to compensate for days getting longer.

UTC is broadcast through short wave radio and satellite. Accuracy ± 1ms (but if weather conditions considered ±10ms)


Physical clocks - underlying modelSuppose we have a distributed system with a UTC-

receiver somewhere in it. Problem: we still have to distribute time to each

machine.

Internal mechanism at each node Each machine has a timer Timer causes an interrupt H times a second Interrupt handler adds 1 to a software clock Software clock keeps track of the number of ticks

since agreed-upon time in the past.

Notation Value of clock on machine p at real time t is Cp(t)


Physical clocks – main problem: clock drift

Notation: Value of clock on machine p at real time t is Cp(t)Ideally: Cp(t) == t and dCp(t) = dt

Real world: clock drift, i.e., |Cp(t) - t | > 0

Clock value (Cp) guaranteed to progress:1 - ρ ≤ (dC/dt) ≤ 1 + ρ ρ -- maximum drift rate

Goal: Never let clocks in any twonodes in the system differ by more than x time units synchronize at least every x/(2ρ) seconds.


Building a complete system … Option I: Every machine asks a time server for the accurate

time at least once every x/(2ρ) seconds (Network Time Protocol).

Okay, but need to account for network delays, including interrupt handling and processing of messages.

Client updates time to:Tnew=CUTC+(T2-T1)/2

Fundamental: You’ll have to take into account that setting the time back is never allowed smooth adjustments.

Option II: Let the time server scan all machines periodically, calculate an average, and inform each machine how it should adjust its time relative to its present time.

Note: you don’t even need to propagate UTC time.


Building a complete system … Option I: Every machine asks a time server

for the accurate time at least once every x/(2ρ) seconds (Network Time Protocol).

Okay, but need to account for network delays, including interrupt handling and processing of messages.

Client updates time to


Real world: Network Time Protocol (NTP) Stratum 0 NTP servers – receive time from external

sources (cesium clocks, GPS, radio broadcasts) Stratum N+1 servers synchronize with stratum N

servers and between themselves Self-configuring network

User configured to contact local NTP server

Survey (N. Minar’99) > 175K NTP servers 90% of the NTP servers have <100ms offset fro

synchronization peer 99% are synchronized within 1s


Uses of (synchronized) physical clocks in the real world NTP Global Positioning Systems Using physical clocks to implement at-

most-once semantics


Summary so far Synchronization solutions

Physical time synchronization Often costly, imperfect But with real applications (NEXT TIME)

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Lecture 12 Synchronization