Introduction to Distributed Systems Seif Haridi

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1 Introduction to Distributed Systems Seif Haridi

2 What is a distributed system? A set of nodes, connected by a network, which appear to its users as a single coherent system p1 p2. pn send receive Network S. Haridi, KTHx ID2203x 2

3 Our focus in this course Concepts Models Given the model Which problems are solvable/ not solvable What are the core problems in distributed systems What are the algorithms How to reason about correctness S. Haridi, KTHx ID2203x 3

4 Why study distributed systems? It is important and useful Societal importance Internet WWW Cloud computing Edge computing Small devices (mobiles, sensors) S. Haridi, KTHx ID2203x 4

5 Why study distributed systems? Internet Edge Computing Cloud Computing S. Haridi, KTHx ID2203x 5

6 Why study distributed systems? p1 p4 p2 p10 p11 p12 p13 p3 p5 p6 p7 p8 p9 Internet Edge Computing Cloud Computing S. Haridi, KTHx ID2203x 6

7 Why study distributed systems? p1 p2 p3 p4 p5 p6 p7 p8 p9 p10 p11 p12 p13 Network S. Haridi, KTHx ID2203x 7

8 Why study distributed systems? It is important and useful Technical importance Improve scalability Improve reliability Inherent distribution S. Haridi, KTHx ID2203x 8

9 Why study distributed systems? It is very challenging Partial Failures Network (dropped messages, partitions) Node failures Concurrency Nodes execute in parallel Messages travel asynchronously Recurring core problems Parallel computing S. Haridi, KTHx ID2203x 9

10 Core Problems What types of problems are there?

11 Teaser: Two Generals Problem Two generals need to coordinate an attack Must agree on time to attack They ll win only if they attack simultaneously Communicate through messengers Messengers may be killed on their way S. Haridi, KTHx ID2203x 11

12 Teaser: Two Generals Problem Lets try to solve it for general g1 and g2 g1 sends time of attack to g2 Problem: how to ensure g2 received msg? Solution: let g2 ack receipt of msg Problem: how to ensure g1 received ack Solution: let g1 ack the receipt of the ack This problem is impossible to solve! S. Haridi, KTHx ID2203x 12

13 Teaser: Two Generals Problem 1 2 S. Haridi, KTHx ID2203x 13

14 Teaser: Two Generals Problem attack! 1 2 S. Haridi, KTHx ID2203x 14

15 Teaser: Two Generals Problem sure! please confirm yes.please confirm is 18:00 ok? 1 2 S. Haridi, KTHx ID2203x 15

16 Teaser: Two Generals Problem ok.confirmed? alright! confirm? sure! please confirm yes.please confirm is 18:00 ok? 1 2 S. Haridi, KTHx ID2203x 16

17 Teaser: Two Generals Problem ambushed! 1 2 Impossible to solve! S. Haridi, KTHx ID2203x 17

18 Teaser: Two Generals Problem Applicability to distributed systems Two nodes need to agree on a value before a specific time-bound Communicate by messages using an unreliable channel Agreement is a core problem S. Haridi, KTHx ID2203x 18

19 Consensus: agreeing on a number Consensus problem All nodes propose a value Some nodes might crash & stop responding The algorithm must ensure: All correct nodes eventually decide Every node decides the same Only decide on proposed values S. Haridi, KTHx ID2203x 19

20 Example: Agreeing on a Target 1 A B 2 3 attack A attack B attack B Consensus? Consensus? Consensus S. Haridi, KTHx ID2203x 20

21 Example: Agreeing on a Target 1 A B 2 3 A confirmed A confirmed Consensus A confirmed Consensus Consensus S. Haridi, KTHx ID2203x 21

22 Example: Agreeing on a Target attack! 1 A B 2 3 Consensus Consensus Consensus S. Haridi, KTHx ID2203x 22

23 Example: Agreeing on a Target 1 A B 2 3 attack A attack B attack B Consensus? Consensus? Consensus S. Haridi, KTHx ID2203x 23

24 Example: Agreeing on a Target 1 A B 2 3 ambush! Consensus? Consensus? Consensus S. Haridi, KTHx ID2203x 24

25 Example: Agreeing on a Target 1 A B 2 A confirmed A confirmed 3 Consensus Consensus Consensus S. Haridi, KTHx ID2203x 25

26 Example: Agreeing on a Target attack! 1 A B 2 3 Consensus Consensus Consensus S. Haridi, KTHx ID2203x 26

27 Is Consensus is Solvable? Consensus problem All nodes propose a value Some nodes might crash & stop responding The algorithm must ensure: All correct nodes eventually decide Every node decides the same Only decide on proposed values S. Haridi, KTHx ID2203x 27

28 Consensus is Important Databases Concurrent changes to same data Nodes should agree on changes Use a kind of consensus: atomic commit Only two proposal values {commit, abort} S. Haridi, KTHx ID2203x 28

29 Broadcast Problem Atomic Broadcast A node broadcasts a message If sender correct, all correct nodes deliver msg All correct nodes deliver same messages Messages delivered in the same order S. Haridi, KTHx ID2203x 29

30 Atomic broadcast is Important Replicated services Multiple servers (processes) Execute the same sequence of commands Replicated State Machines RSM Use atomic broadcast Provide fault tolerance S. Haridi, KTHx ID2203x 30

31 Atomic Broadcast Consensus Given Atomic broadcast Can use it to solve Consensus Every node broadcasts its proposal Decide on the first received proposal Messages received in same order All nodes will decide the same Given Consensus Can use it to solve Atomic broadcast [d] Atomic Broadcast equivalent to Consensus S. Haridi, KTHx ID2203x 31

32 Atomic Broadcast Consensus attack A attack B attack B Consensus Consensus Consensus attack A Atomic Broadcast attack B Atomic Broadcast?? attack B Atomic Broadcast S. Haridi, KTHx ID2203x 32

33 Atomic Broadcast Consensus attack A attack A attack A Consensus Consensus Consensus attack A attack A attack A Atomic Broadcast Atomic Broadcast Atomic Broadcast S. Haridi, KTHx ID2203x 33

34 Atomic Broadcast Consensus 1 attacking A 2 3 attacking A attacking A Consensus Consensus Consensus attack B attack B attack B Atomic Broadcast Atomic Broadcast Atomic Broadcast S. Haridi, KTHx ID2203x 34

35 Models How to reason about them?

36 Modeling a Distributed System Timing assumptions Processes bounds on time to make a computation step Network Bounds on time to transmit a message between a sender and a receiver Clocks: Lower and upper bounds on clock drift rate S. Haridi, KTHx ID

37 Modeling a Distributed System Failure assumptions Processes What kind of failure a process can exhibit? Crashes and stops Behaves arbitrary (Byzantine) Network Can a network channel drop messages? S. Haridi, KTHx ID

38 Modeling a Distributed System p1 p2 p3 Network S. Haridi, KTHx ID

39 Modeling a Distributed System p1 p2 p3 Network S. Haridi, KTHx ID

40 Modeling a Distributed System p1 p2 p3 Network S. Haridi, KTHx ID

41 Network Failures p1 p2 p3 Network S. Haridi, KTHx ID

42 Network Failures p1 p2 p3 Network dropped S. Haridi, KTHx ID

43 Process Failures p1 p2 p3 Network S. Haridi, KTHx ID

44 Process Failures p1 p2 p3 Network S. Haridi, KTHx ID

45 Process Failures p1 p3 p2 Network S. Haridi, KTHx ID

46 Process Failures p1 p3 p2 Network S. Haridi, KTHx ID

47 Byzantine Processes p1 p2 p3 Network S. Haridi, KTHx ID

48 Byzantine Processes p1 p2 p3 Network S. Haridi, KTHx ID

49 Byzantine Processes?? p1 p2 p3 Network S. Haridi, KTHx ID

50 Modeling a Distributed System Asynchronous system No bound on time to deliver a message No bound on time to compute Clocks are not synchronized Internet essentially asynchronous S. Haridi, KTHx ID

51 Impossibility of Consensus Consensus cannot be solved in asynchronous system If a single node may crash Implications on Atomic broadcast Atomic commit Leader election S. Haridi, KTHx ID

52 Modeling a Distributed System Synchronous system Known bound on time to deliver a message (latency) Known bound on time to compute Known lower and upper bounds in physical clock drift rate Examples: Embedded systems Multicore computers S. Haridi, KTHx ID

53 Possibility of Consensus Consensus solvable in synchronous system with up to N-1 crashes Intuition behind solution Accurate crash detection Every node sends a message to every other node If no msg from a node within bound, node has crashed Not useful for Internet, how to proceed? S. Haridi, KTHx ID

54 Modeling a Distributed System But Internet is mostly synchronous Bounds respected mostly Occasionally violate bounds (congestion/failures) How do we model this? Partially synchronous system Initially system is asynchronous Eventually the system becomes synchronous S. Haridi, KTHx ID

55 Possibility of Consensus Consensus solvable in partially synchronous system with up to N/2 crashes Useful for Internet? S. Haridi, KTHx ID

56 Failure detectors Let each node use a failure detector Detects crashes Implemented by heartbeats and waiting Might be initially wrong, but eventually correct Consensus and Atomic Broadcast solvable with failure detectors How? Attend rest of course! S. Haridi, KTHx ID

57 Modeling a Distributed System Timed Asynchronous system No bound on time to deliver a message No bound on time to compute Clocks have known clock-drift rate Realistic model Internet S. Haridi, KTHx ID

58 Conclusions

59 Topics not covered

60 Processes always crash? Other types of failures Not just crash stops Byzantine faults Self-stabilizing algorithms S. Haridi, KTHx ID2203x 60

61 Byzantine Faults Some processes might behave arbitrarily Sending wrong information Omit messages Byzantine algorithms that tolerate such faults Only tolerate up to 1/3 Byzantine processes Non-Byzantine algorithms can often tolerate ½ nodes in the asynchronous model S. Haridi, KTHx ID2203x 61

62 Self-stabilizing Algorithms Robust algorithms that run forever System might temporarily be incorrect But eventually always becomes correct System can either by in a legitimate state or an illegitimate state Self-stabilizing algorithm iff Convergence Given any illegitimate state, system eventually goes to a legitimate state Closure If system in a legitimate state, it remains in a legitimate state S. Haridi, KTHx ID2203x 62

63 Self-stabilizing Algorithms System can either by in a legitimate state or an illegitimate state Self-stabilizing algorithm iff Convergence Closure S. Haridi, KTHx ID2203x 63

64 Self-stabilizing Algorithms Advantages Robust to transient failures Don t need initialization Can be easily composed A service composed of two self-stabilizing services is self-stabilizing service S. Haridi, KTHx ID2203x 64

65 Self-stabilizing Example Token ring algorithm Wish to have one token at all times circulating among processes Self-Stabilization Error leads to 2,3, tokens Ensure always 1 token eventually S. Haridi, KTHx ID2203x 65

66 Content of the Course

67 Content I Formal Models of Asynchronous Systems Basic Abstractions Reliable Broadcast Algorithms Distributed Shared Store and Consistency Models S. Haridi, KTHx ID2203x 67

68 Content II Single Value Consensus Paxos algorithm Sequence Consensus Multi-Paxos Replicated State Machines (RSM) Dynamic Reconfiguration Physical Clocks Leader election (timed asynchronous model) More efficient RSM) Shared stores with Strong Consistency Relaxed consistency models CAP theorem S. Haridi, KTHx ID2203x 68

69 Summary Distributed systems everywhere Set of processes (nodes) cooperating over a network Few core problems reoccur Consensus, Broadcast, Leader election, Shared Memory Different failure scenarios important Crash stop, Byzantine, self-stabilizing algorithms Interesting research directions Large scale dynamic distributed systems S. Haridi, KTHx ID2203x 69

70 Let s start S. Haridi, KTHx ID2203x 70

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