Fault Tolerance

1. Fault Tolerance

2. Agenda • Overview • Introduction to Fault Tolerance • Process Resilience • Reliable Client-Server communication • Reliable group communication • Distributed commit • Recovery • Summary

3. Overview • Introduction to Fault Tolerance • Basic Concepts • Failure Modes • Failure Masking • Process Resilience • Design Issues • Reliable Communication • P2P Communication • Client Server Communication (RPC, RMI) • Group Communication (Multicasting) • Distributed Commit • Multi Phase Commit (Two & Three Phase) • Recovery Techniques • Check Pointing, Message Logging

4. Basic Concepts (1/3) • What is Failure? • System is said to be in failure state when it cannot meet its promise. • Why do Failure occurs? • Failures occurs because of the error state of the system. • What is the reason for Error? • The cause of an error is called a fault • Is there some thing ‘Partial Failure’? • Faults can be Prevented, Removed and Forecasted. • Can Faults be Tolerated by a system also?

5. Basic Concepts (2/3) • What characteristics makes a system Fault Tolerant? • Availability: System is ready to used immediately. • Reliability: System can run continuously without failure. • Safety: Nothing catastrophic happens if a system temporarily fails. • Maintainability: How easy a failed system can be repaired. • Dependability: ??? • What is the reliability and availability of following systems? • If a system goes down for one millisecond every hour • If a System never crashes but is shut down for two weeks every August.

6. Basic Concepts (3/3) • Classification of Faults • Transient: Occurs once and than disappears --A flying bird obstructing the transmitting waves signals • Intermittent: Occurs, vanishes on its own accord, than reappears and so on -- A loosely connected power plug • Permanent: They occurs and doesn’t vanish until fixed manually. -- Burnt out chips

7. Faults in Distributed Systems • If in a Distributed Systems some fault occurs, the error may by in any of • The collection of servers or • Communication Channel or • Even both • Dependency relations appear in abundance in DS. • Hence, we need to classify failures to know how serious a failure actually is.

8. Failure Models

9. Failure Masking by Redundancy (1/3) • A system to be fault tolerant, the best it can do is try to hide the occurrence of failure from other processes • Key technique to masking faults is to use Redundancy. • Information redundancy: Extra bits are added to allow recovery from garbled bits • Time redundancy: An action is performed, and then, if need be, it is performed again. • Physical redundancy: Extra equipment or processes are added

10. Failure Masking by Redundancy (2/3) • Some Examples of Redundancy Schemes • Hamming Code • Transactions • Replicated Processes or Components • Aircraft has four engines, can fly with only three • Sports game has extra referee.

11. Failure Masking by Redundancy (3/3) • Triple modular redundancy: • If two or three of the input are the same, the output is equal to that input. • If all three inputs are different, the output is undefined. Figure: Fault Tolerance in Electronic Circuits

12. Suppose that element Az fails. Each of the voters, VbVz, and V3 gets two good (identical) inputs and one rogue input, and each of them outputs the correct value to the second stage. • In essence, the effect of Az failing is completely masked, so that the inputs to B I, Bz, and B3 are exactly the same as they would have been had no fault occurred. • Now consider what happens if B3 and C1 are also faulty, in addition to Az· These effects are also masked, so the three final outputs are still correct.

13. Process Resilience • Problem: • How fault tolerance in distributed system is achieved, especially against Process Failures? • Solution: • Replicating processes into groups. • Groups are analogous to Social Organizations. • Consider collections of process as a single abstraction • All members of the group receive the same message, if one process fails, the others can take over for it. • Process groups are dynamic and a Process can be member of several groups. • Hence we need some management scheme for groups.

14. Process Groups (1/2) Flat Group vs. Hierarchical Group • Flat Group • Advantage: Symmetrical and has no single point failure • Disadvantage: Decision making is more complicated.  Voting • Hierarchical Group • Advantage: Make decision without bothering others • Disadvantage: Lost coordinator  Entire group halts

15. Process Groups (2/2) Group Membership • Group Server (Client Server Model) • Straight forward, simple and easy to implement • Major disadvantage  Single point of failure • Distributed Approach (P2P Model) • Broadcast message to join and leave the group • In case of fault, how to identify between a really dead and a dead slow member • Joining and Leaving must be synchronized  on joining send all previous messages to the new member • Another issue is how to create a new group?

16. Failure Masking & Replication • Replicate Process and organize them into groups • Replace a single vulnerable process with the whole fault tolerant Group • A system is said to be K fault tolerant if it can survive faults in Kcomponents and still meet its specifications. • How much replication is needed to support K Fault Tolerance? • K+1 or 2K+1 ? • Case: • If K processes stop, then the answer from the other one can be used. K+1 • If meet Byzantine failure, the number is 2K+1  Problem?

17. Agreement in Faulty Systems • Why we need Agreements? • Goal of Agreement • Make all the non-faulty processes reach consensus on some issue • Establish that consensus within a finite number of steps. • Problems of two cases • Good process, but unreliable communication • Example: Two-army problem • Good communication, but crashed process • Example: Byzantine generals problem

18. Byzantine generals problem The Byzantine generals problem for 3 loyal generals and1 traitor. • The generals announce their troop strengths (in units of 1 thousand soldiers). • The vectors that each general assembles based on (a) • The vectors that each general receives in step 3.

19. In Fig. 8-5 we illustrate the working of the algorithm for the case of N = 4 and k = 1. • For these parameters, the algorithm operates in four steps. In step 1, every nonfaulty process i sends Vi to every other process using reliable unicasting. • Faulty processes may send anything. Moreover, because we are using multicasting, they may send different values to different processes. Let Vi =i. • In Fig. 8-5(a) we see that process 1 reports 1, process 2 reports 2, process 3 lies to everyone, giving x, y, and z, respectively, and process 4 reports a value of 4. • In step 2, the results of the announcements of step 1 are collected together in the form of the vectors of Fig. 8-5(b).

20. Step 3 consists of every process passing its vector from Fig. 8-5(b) to every other process. • In this way, every process gets three vectors, one from every other process. Here, too, process 3 lies, inventing 12 new values, a through 1.The results of step 3 are shown in Fig. 8-5(c). • Finally, in step 4, each process examines the ith element of each of the newly received vectors. • If any value has a majority, that value is put into the result vector. If no value has a majority, the corresponding element of the result vector is marked UNKNOWN. From Fig. 8-5(c) we see that 1, 2, and 4 all come to agreement on the values for VI, v 2, and v 4, which is • the correct result. What these processes conclude regarding v 3 cannot be decided, but is also irrelevant. The goal of Byzantine agreement is that consensus is reached on the value for the nonfaulty processes only

21. Go forward one more step The same as in previous slide, except now with 2 loyal generals and one traitor. More than two-thirds  agreement Lamport proved that in a system with m faulty processes, agreement can be achieved only if 2m+1 correctly functioning processes are present, for a total of 3m+1.

22. Failure Detection • Failure detection is one of the cornerstones of fault tolerance in distributed systems. • What it all boils down to is that for a group of processes, nonfaulty members should be able to decide who is still a member, and who is not. • When it comes to detecting process failures, there are essentially only two • mechanisms. Either processes actively send "are you alive?" messages to each • other (for which they obviously expect an answer), or passively wait until messages come in from different processes. • The latter approach makes sense only when it can be guaranteed that there is enough communication between processes. • In practice, actively pinging processes is usually followed. • A timeout mechanism is used to check whether a process has failed

23. Reliable client-server communication • TCP masks omission failures • … by using ACKs & retransmissions • … but it does not mask crash failures ! • E.g.: When a connection is broken, the client is only notified via an exception What about reliable point-to-point transport protocols ?

24. Five classes of failures in RPC • Client is unable to locate server • Binding exception • … at the expense of transparency • Request message is lost • Is it safe to retransmit ? • Allow server to detect it is dealing with a retry • Server crashes after receiving a request • Reply message is lost • Client crashes after sending a request

25. Server Crashes (I) A server in client-server communication • Normal case • Crash after execution • Crash before execution

26. Server Crashes (II) • At-least-once semantics • Client keeps retransmitting until it gets a response • At-most-once semantics • Give up immediately & report failure • Guarantee nothing • Ideal would be exactly-once semantics • … no general way to arrange this !

27. Server Crashes (III) • Print server scenario: • M: server’s completion message • Server may send M either before or after printing • P: server’s print operation • C: server’s crash • Possible event orderings: • M  P  C • M  C ( P) • P  M  C • P  C ( M) • C ( P  M) • C ( M  P)

28. Server Crashes (IV) Different combinations of client & server strategies in the presence of server crashes. No combination of client & server strategy is correct for all cases !

29. Lost Reply Messages • Is it safe to retransmit the request ? • Idempotent requests • Example: Read a file’s first 1024 bytes • Counterexample: money transfer order • Assign sequence number to request • Server keeps track of client’s most recently received sequence # • … additionally, set a RETRANSMISSION bit in the request header

30. Client Crashes (I) • Orphan computation: • No process waiting for the result • Waste of resources (CPU cycles, locks) • Possible confusion upon client’s recovery • 4 alternative strategies proposed by Nelson (1981) • Extermination: • Client keeps log of requests to be issued • Upon recovery, explicitly kill orphans • Overhead of logging (for every RPC) • Problems with grand-orphans • Problems with network partitions

31. Client Crashes (II) • Reincarnation: • Divide time up into epochs (period of time)(sequentially numbered) • Upon reboot, client broadcasts start-of-epoch • Upon receipt, all remote computations on behalf of this client are killed • After a network partition, an orphan’s response will contain an obsolete epoch number  easily detected • Gentle reincarnation: • Upon receipt of start-of-epoch, each server checks to see if it has any remote computations • If the owner cannot be found, the computation is killed • Expiration: • Each RPC is given a time quantum T to complete • … must explicitly ask for another if it cannot finish in time • After reboot, client only needs to wait a time T … • How to select a reasonable value for T ?

32. Reliable group communication

33. Basic Reliable-Multicasting Schemes • A simple solution to reliable multicasting when all receivers are known & are assumed not to fail • Message transmission • Reporting feedback

34. Scalability in Reliable Multicasting • The scheme described above can not support large numbers of receivers . • Reason:  Feedback Implosion  Receivers are spread across a wide-area network • Solution: Reduce the number of feedback messages that are returned to the sender. • Model: Feedback suppression

35. Nonhierarchical Feedback Control Several receivers have scheduled a request for retransmission, but the first retransmission request leads to the suppression of others.

36. Hierarchical Feedback Control • The essence of hierarchical reliable multicasting: • Each coordinator forwards the message to its children. • A coordinator handles retransmission requests.

37. Atomic Multicast • We need to achieve reliable multicasting in the presence of process failures. • Atomic multicast problem: a message is delivered to either all processors or to none at all all messages are delivered in the same order to all processes • Virtually synchronous reliable multicasting offering totally-ordered delivery of messages is called atomic multicasting

38. Virtual Synchrony (I) The logical organization of a distributed system to distinguish between message receipt and message delivery

39. Virtual Synchrony (II) • Reliable multicast guarantees that a message multicast to group view G is delivered to each nonfaulty process in G. • If the sender of the message crashes during the multicast, the message may either be delivered to all remaining processes, or ignored by each of them. • A reliable multicast with this property is said to be virtually synchronous • All multicasts take place between view changes. A view change acts as a barrier across which no multicast can pass

40. Virtual Synchrony (III) The principle of virtual synchronous multicast.

41. Implementing Virtual Synchrony • Process 4 notices that process 7 has crashed, sends a view change • Process 6 sends out all its unstable messages, followed by a flush message • Process 6 installs the new view when it has received a flush message from everyone else

42. Message Ordering (I) • 1. Unordered multicast • 2. FIFO-ordered multicast

43. Message Ordering (II) • Reliable causally-ordered multicast delivers messages so that potential causality between different messages is preserved • Total-ordered delivery

44. Agenda Introduction to Fault Tolerance Process Resilience Reliable Client-Server communication Reliable group communication Distributed commit Recovery Summary

45. Two-phase Commit (I) • The finite state machine for the coordinator in 2PC. • The finite state machine for a participant. • Process crashes  other processes may be indefinite waiting for a message  This protocol can easily fail •  timeout mechanisms are used

46. Failure handling in 2PC • Participant times out waiting for coordinator’s Request-to-prepare • It decide to abort. • Coordinator times out waiting for a participant’s vote • It decides to abort. • A participant that voted Prepared times out waiting for the coordinator’s decision • It’s blocked. • Use a termination protocol to decide what to do. • Native termination protocol – wait until coordinator recovers. • The coordinator times out waiting for ACK message • It must resolicit them, so it can forget the decision

47. Participant Wait • In INIT -timeout -abort

48. Participant waits in Ready State

49. Actions by coordinator while START _2PC to local log;multicast VOTE_REQUEST to all participants;while not all votes have been collected { wait for any incoming vote; if timeout { while GLOBAL_ABORT to local log; multicast GLOBAL_ABORT to all participants; exit; } record vote;}if all participants sent VOTE_COMMIT and coordinator votes COMMIT{ write GLOBAL_COMMIT to local log; multicast GLOBAL_COMMIT to all participants;} else { write GLOBAL_ABORT to local log; multicast GLOBAL_ABORT to all participants;}

50. Actions by participant write INIT to local log;wait for VOTE_REQUEST from coordinator;if timeout { write VOTE_ABORT to local log; exit;}if participant votes COMMIT { write VOTE_COMMIT to local log; send VOTE_COMMIT to coordinator; wait for DECISION from coordinator; if timeout { multicast DECISION_REQUEST to other participants; wait until DECISION is received; /* remain blocked */ write DECISION to local log; } if DECISION == GLOBAL_COMMIT write GLOBAL_COMMIT to local log; else if DECISION == GLOBAL_ABORT write GLOBAL_ABORT to local log;} else { write VOTE_ABORT to local log; send VOTE ABORT to coordinator;}