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DEPARTMENT OF INFORMATION TECHNOLOGY
I.T.S ENGINEERING COLLEGE, GREATER NOIDA
UNIT- 4
Failure Recovery and Fault Tolerance Basic Concept
- A system consists of a set of hardware and software components and is designed to provide a specified service.
- Failure of a system occurs when the system does not perform its services in the manner specified.
- An erroneous state of the system is a state which could lead to a system failure by a sequence of valid state transitions
- A fault is an anomalous physical condition.
- An error is a manifestation of a fault in a system, which can lead to system failure.
- Failure recovery is a process that involves restoring an erroneous state to an error-free state
Failure A system is said to “fail” when it cannot meet its promises. A failure is brought about by the existence of “errors” in the system. The cause of an error is a “fault”. Fault may occur due to manufacturing problems, design errors, external disturbances, fatigue and deterioration or any combination of them.
DEPARTMENT OF INFORMATION TECHNOLOGY
I.T.S ENGINEERING COLLEGE, GREATER NOIDA
Type of Failure
Classification of Failure
1. Process failure: In a Process failure, the computation results in an incorrect outcome. The system state deviates from the specified result and may cause failure. Process failure may be occur due to deadlocks, timeouts, protection violation, wrong input from user or consistency violation. 2. System failure: A system failure occurs when the processor fails to execute. It is caused by software errors and hardware error such as CPU failure, main memory failure, bus failure, power failure etc. 3. Secondary storage failure: A secondary storage failure occurs when the stored data cannot be accessed. This failure is usually caused by parity error, head crash, or dust particles on medium. 4. Communication medium failure: A communication medium failure occurs when a site cannot be communicated with another operational site in the network. It is usually caused by the failure of switching nodes or links of communicating system.
DEPARTMENT OF INFORMATION TECHNOLOGY
I.T.S ENGINEERING COLLEGE, GREATER NOIDA
There are two ways to implement backward error recovery:
- The operation based approach
- The State based approach
The Operation based Approach: In this approach, the sufficient details of all the modifications made in the transaction are recorded so that the previous stable state of the system can be restored by reversing all the changes made in that state. The information in the logs contains the name of object, the old state of the object and the new state of the object. This can be done in two ways:
1. Updating-in-place: In Update-in-Place, every update or write operation to an object updates the object and create a log in the stable storage. The recoverable update operation can be implemented as a collection of operations as follows a. A ‘do’ operation, which does the action or update and write a log record. b. An ‘undo’ operation, which undoes the action performed by ‘do’ operation. c. A ‘redo’ operation, which redoes the action specified by ‘do’ operation. d. An optional ‘display’ operation, which displays the log record. The major problem with update-in-place that a ‘do’ operation cannot be undone if the system crashes after the update operation but before a log record is stored. 2. The Write-Ahead-Log Protocol: In Write-Ahead-Log Protocol, The recoverable update operation can be implemented as a collection of operations as follows a. Update an object only after the ‘undo’ log is recorded. b. Before committing the update, ‘redo’ and ‘undo’ logs are recorded.
State Based Approach
