Parallel and Distributed Computing Chapter 12
- 1. PARALLEL AND DISTRIBUTED COMPUTING FAULT TOLERANT DISTRIBUTED COMPUTING
- 2. FAULT TOLERANCE System ability to continue operating uninterrupted despite the failure of one or more of its components How an OS Responds to and allows malfunctions and failures It guarantees no break in service Recovers from failure completely and transparently
- 3. FAULT TOLERANCE Every achievement in fault tolerance leads to a drawback somewhere else The system will be slower, take more disk space, utilize more machines and also increase other costs There for fault tolerance is always a trad-off between cost and the degree of fault tolerance.
- 4. FAILUREVS ERROR System differs from expected behavior Failure might involve the system being unreachable or producing incorrect output Error is incorrectness of system that may lead to a failure. Error do not must create failures but can be detect in the system before they produce failure.
- 5. FAULT TOLERANCE Fault tolerance usually running through several phases. Error Detection: error has to be detect in order to avoid failure. Damage Confinement: it must prevent that the error spreads through other components Error recovery: error must be removed, otherwise system would run into failure
- 6. PROCESSOR FAULT Occur when the processor behaves in unexpected manner. It may be classified into three kinds. 1. Fail Stop: totally failed and will never respond, neighboring processors can detect the failed processor 2. Slowdown: processor might run in degraded form or might totally fail 3. Byzantine: processor can fail, run in degraded fashion for some time or execute at normal speed but tries to fail the computation
- 7. NETWORK FAULTS When processors are prevented from communicating with each other. Link faults can cause new kinds of problems like One way Links: one processor can send messages but other is not able to receive message. Network partition: network of portion is completely isolated with other
- 8. ATTRIBUTES OF FAULT TOLERANT SYSTEM Fault tolerance system is depended system which requires following attributes 1. Availability: when system is in a ready state and ready to deliver tis functions. Highly available systems works at a given instant in time. 2. Reliability: ability of computer to run continuously without failure, it is defined as time interval instead of instant time. Reliable system works constantly without interruption. 3. Safety: fails to carry out its corresponding processes correctly and operations are incorrect but no major disastrous happened and also doesn’t affect other system to be faulty 4. Maintainability: if failures can be notices and fixed easily.
- 11. FAULT TOLERANCE MECHANISM IN DISTRIBUTED SYSTEM Replication based fault tolerance technique Process level redundancy technique Fusion based redundancy technique
- 12. REPLICATION BASED FAULTTOLERANCE TECHNIQUE Replicate the data on other machine. It will not cause the whole system to stop. Replicate the data on different server.
- 13. Problems of replication Consistency: major problem of replication is consistency because of updating by any client. Consistency of data is ensured by some model such as sequential, causal memory consistency model Degree of replica: large number of replications are needed in order to achieve high fault tolerance.
- 14. PROCESS LEVEL REDUNDANCY TECHNIQUES Faults that disappears without anything been done is called transient faults.This type of faults are hard to identify Handling transient fault, software based fault tolerance technique are used PLR Compares processes to ensure correct execution Check point and roll back are popular technique in which the current state of system is done.
- 15. FUSION BASEDTECHNIQUE Replication: downside is multiple backups that increases cost This problem is solved by fusion based technique because it requires fewer backup Backup machines are fused to a given set of system (NP- Problem) Fusion based technique has very high overhead during recovery process and it’s acceptable in low probability of fault in a system.