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  Overview of recovery management

Overview of recovery management:

A transaction is started explicitly when a process is allocated or when an existing process issues BEGIN_TRANSACTION. When a transaction is initiated recovery administrator is invoked to allocate the recovery structure necessary to recover the transaction. This process places a capability for the COMMIT, SAVE, and BACKUP calls of recovery administrator in the transaction's capability list.

Afterwards all actions by the transaction on recoverable data are recorded in the recovery log using log manager. Generally each action performing an update operation should write an undo-log record and a redo-log record in the transaction’s log. The undo log record offers the old value of the object and the redo log record gives the new value.

At a transaction saves point, recovery manager records the save point identifier, as well as enough information so that all component of the system could be backed up to this point. In the event of a minor error, the transaction perhaps undone to a save point in which case the application (on its next or pending call) is given feedback indicating that the data base system has amnesia about all recoverable actions since that save point. If the transaction is completely backed-up (aborted), it may or mayn’t be restarted depending on the attributes of the transaction and of its initiating message.

If the transaction completes productively (commits) then (logically) it is always redone in case of a crash. Alternatively if it is in-progress at the time of the local or system failure then the transaction is logically undone (aborted).

Recovery manager should as well respond to the following kinds of failures:

• Action failure: a particular call can’t complete due to a foreseen condition. Generally the action undoes itself (cleans up its component) and then returns to the caller. Illustrations of this are bad parameters, resource limits as well as data not found.

• Transaction failure: a particular transaction cannot proceed and so is aborted. The transaction possibly reinitiated in some cases. Illustrations of such errors are deadlock, timeout, protection violation, as well as transaction-local system errors.

• System failure: a serious mistake is detected below the action interface. The system is stopped as well as restarted. Errors in critical tables wild branches by trusted processes, operating system failures as well as hardware failures are sources of system failure. Most non-volatile storage is presumed to survive a system failure.

• Media failure: a non-recoverable error is detected on some typically reliable (non-volatile) storage device. The recovery of recoverable data commencing a media failure is the responsibility of the component that implements it. If the device contained recoverable data the manager should reconstruct the data from an archive copy using the log and then place the result on an alternate device. Media failures do not usually force system failure. Parity error, head crash, dust on magnetic media, as well as lost tapes are typical media failures. Software errors that make the media unreadable are as well regarded as media errors, as are catastrophe such as fire, flood, and insurrection as well as operator error.

The system periodically makes copies of every recoverable object and keeps these copies in a safe place (archive). Lest the object suffers a media error each transaction with locks outstanding against the object are aborted. A special transaction (a utility) obtains the object in exclusive mode. (This takes the object “off-line".) This transaction combines an accumulation of changes to the object since the object copy was made as well as a recent archive version of the object to produce the most recent committed version. This accumulation of changes may perhaps take two forms- it possibly the REDO-log portion of the system log, or it may be a change accretion log that was constructed from the REDO-log portion of the system log when the system log is compressed. Subsequent to media recovery the data is unlocked and made public again.

The procedure of making an archive copy of an object has several varieties. Definite objects, notably IMS queue space, are recovered from scratch using an infinite redo log. Other objects, particularly databases get copied to some external media that is able to be used to restore the object to a consistent state if a failure occurs. (The resource may or mayn’t be off-line while the cozy is being made.)

Recovery manager as well periodically performs system checkpoint by recording critical parts of the system state in a safe spot in non-volatile storage (sometimes called the warm start file.)

Recovery administrator coordinates the process of system restart system shutdown. In performing system restart it prefer among

• Warm start- system shut-down in controlled manner. Recovery need merely locate last checkpoint record as well as rebuild its control structures.

• Emergency restart- system failed in unrestrained manner. Non-volatile storage contains recent state reliable with the log. Nevertheless some transactions were in progress at time of failure and should be redone or undone to obtain most recent consistent state.

• Cold start: the system is being brought up with amnesia about previous incarnations. The log isn’t examined to determine previous state.

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