1. Chapter 8 Fault Tolerance
Introduction
Process resilience
Reliable communication
Failure recovery
Distributed commit

2. Dependability
Dependability is the ability to avoid service failures that are more frequent or severe than desired. It is an important goal of distributed systems.
Requirements for dependable systems
Availability: the probability that the system is available to perform its functions at any moment
% availability (five 9s)  5 minutes of downtime per year
Reliability: the ability of the system to run continuously without failure
Down for 1ms every hour  % availability but highly unreliable
Down for two weeks every year  high reliability but only 96% availability
Safety: when a system temporarily fails to operate correctly, nothing catastrophic happens
Maintainability: how easily a failed system can be repaired
Security: will cover in Chapter 9
Availability - Readiness for usage, Reliability - Continuity of service delivery. Example: control system for airplanes, nuclear power plants. Safety - Very low probability of catastrophes, Maintainability - How easy can a failed system be repaired
Dependability is the trustworthiness of a computing system which allows reliance to be justifiably placed on the service it delivers
Dependability attributes
Worthy of confidence, confident about relying on its service
Attributes - A way to assess the Dependability of a system
Need to prevent failure which is caused by faults. Fault tolerance means that a system can provide its services even in the presence of faults

3. Failures and Faults
Building a dependable system comes down to preventing failures
A failure of a system occurs when the system cannot meet its promises
Failures are caused by faults
A fault is an anomalous condition. There are three categories of faults:
Transient faults: occur once and never reoccur (e.g., wireless communication being interrupted by external interference)
Intermittent faults: reoccur irregularly (e.g., a loose contact on a connector)
Permanent faults: persist until the faulty component is replaced (e.g., software bugs)

4. Types of Failures
Fail-stop: server will stop in a way that clients can tell that it has halted
Fail-silent: clients do not know server has halted
State transition failure: Execution of component brings it into a wrong state
Arbitrary failures are also known as Byzantine failures

5. Fault Tolerance
In a single-machine system, a failure is almost always total
All components are affected and entire system may be brought down (e.g., OS crash, disk failures)
Partial failures are possible in distributed systems
When one component fails, it may affect some components, while leaving other components unaffected
Fault tolerance means that a system can provide its services even in the presence of faults
Fault tolerance requires
preventing faults and failures from affecting other components of the system
automatically recovering from partial failures
DS: multiple independent nodes, Prob(failure) = Prob(any one component fails)

6. Failure Masking
Failure masking is a fault tolerance technique that hides occurrence of failures from other processes
The most common approach to failure masking is redundancy
Three types of redundancy:
Information redundancy: add extra bits to allow recovery from garbled bits
Time redundancy: repeat an action if needed
Physical redundancy: add extra equipment or processes so that the system can tolerate the loss or malfunctioning of some components

7. An Example of Physical Redundancy
Triple modular redundancy: the effect of a single component failing is completely masked.

8. Process Resilience
Protection against process failures can be achieved by organizing several identical processes into a group
Flat group: all process are equal; the processes make decisions collectively
No single point of failure, but decision making is more complicated
Hierarchical group: a single coordinator makes all decisions
Decision making is simpler, but coordinator is a single point of failure
Group is transparent to its users, the whole group is dealt with as a single process

9. Fault Tolerance in Process Groups
Having a group of identical processes allows us to mask one or more faculty processes in that group
A group of replicated processes is said to be k fault tolerant if it can survive k faults and still meet its specifications
With crash failures, K+1 processes are sufficient to survive k faults
With Byzantine failures, processes may produce erroneous, random, or malicious results  2k+1 processes are required to survive k faults (group output is defined by voting)
Assumption: All requests arrive at all members in the group in the same order (this requires atomic multicast)  only then are we sure that all members do exactly the same thing
processes run even if sick

10. Agreement in Faulty Systems
The goal of distributed agreement algorithms is to have all the nonfaulty processes reach consensus on some issue within a finite number of steps
Q1: Can consensus be reached with nonfaulty processes and unreliable communication channel?
A: Two nonfaulty processes can never reach agreement in presence of unreliable channel
Q2: Can consensus be reached with faulty (Byzantine) processes and reliable channel?
A: Depends
Two-army problem: two blue armies must agree to attack simultaneously in order to defeat the white army
Each blue army coordinates with a messenger
Messenger can be captured by the white army
Can the two blue armies reach agreement?

11. Conditions for Consensus
Process behavior
Message Order
Communication delay
Unordered
Ordered
Asynchronous
Yes
Unbounded
Bounded
Synchronous
Unicast
Multicast
Message Transmission
Assume processes may be faulty and communication is reliable.
A system is synchronous iff the processes operate in a lock-step mode (i.e., there is a constant c≥1, such that if any process has taken c+1 steps, every other process has taken at least one step).

12. Byzantine Agreement Problem
Byzantine agreement problem: Can N generals reach consensus about each other’s troop strengths when communication channel is perfect but some of the generals are traitors and will lie to prevent agreement?
Formally, there are N processes, each process i will provide a value vi to the others. The goal is to let each process construct a vector V of length N, such that if process i is nonfaulty, V[i]= vi. Otherwise V[i] is undefined.
Assume processes are synchronous, messages are unicast while preserving ordering, and communication delay is bounded, with k faulty processes, agreement can be achieved if there are 2k+1 nonfaulty processes [Lamport et al., 1982].
In lamport’s paper, byzantine generals problem requires two conditions to be met: 1) all loyal lieutenants obey the same order 2) if the commanding general is loyal, then every loyal lieutenants obeys the order he sends

13. Byzantine Agreement Problem: An Example
The Byzantine agreement problem for 3 nonfaulty processes and 1 faulty process with vi=i. Consensus is reached for the nonfaulty processes. (a) Each process sends its value to the others. (b) The vectors that each process assembles based on (a). (c) The vectors that each process receives after each process passes its vector from (b) to every other process.

14. Byzantine Agreement Problem: Another Example
The Byzantine agreement problem for 2 nonfaulty processes and 1 faulty process. The algorithm fails to produce agreement.