Lecture 12 Synchronization

Lecture 12Synchronization

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 • Assignments9 • closely related to class materials, usefull hands-on work, • good that marking is done on coding style as much as functionality • but *too few*! • Slides8 • 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?

K0, K125 K110 K3, K7 K15 Key placement N1 Node ID N115 0 N13 N100 128 Circular ID Space N83 N43 N70 N51

K0, K125 K110 K3, K7 K15 Search launched at N13 for K0 Each node maintains • Successor list (2) • Shortcuts (1) N1 Node ID N115 N13 N100 N83 N43 N70 N51

K0, K125 K110 K3, K7 K15 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? N1 N115 N13 N100 N83 N43 Crashed N70 N51

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? • Lookup for 6?  N13 • Predecessor of N13?  N1 • Announce yourself to N1 • N1 updates successor • N1 notifies predecesor • N115 updates successor • N6 joins • Creates successor list • Splits keys with N13 • N13 updates predecesor Two invariants to maintain for correctness • Key to node assignment • Successor lists K0, K125 N1 K110 N115 N13 N100 K3, K7 N83 N70 N51 K15 N6 joins

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: useUniversal 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 broadcastthrough short wave radio and satellite. • Accuracy ± 1ms (but if weather conditions considered ±10ms)

Physical clocks - underlying model Suppose 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 interruptH 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: clockdrift, 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 two nodes 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)

Lecture 12 Synchronization