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Silberschatz Korth Sudarshan: Database System Concepts, Fourth Edition
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V Transaction Management
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The McGraw Hill Companies, 2001
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The problem with this scheme is that the updates of a committed transaction may not have made it to the backup site, when the backup site takes over processing Thus, the updates may appear to be lost When the primary site recovers, the lost updates cannot be merged in directly, since the updates may con ict with later updates performed at the backup site Thus, human intervention may be required to bring the database to a consistent state Two-very-safe A transaction commits as soon as its commit log record is written to stable storage at the primary and the backup site The problem with this scheme is that transaction processing cannot proceed if either the primary or the backup site is down Thus, availability is actually less than in the single-site case, although the probability of data loss is much less Two-safe This scheme is the same as two-very-safe if both primary and backup sites are active If only the primary is active, the transaction is allowed to commit as soon as its commit log record is written to stable storage at the primary site This scheme provides better availability than does two-very-safe, while avoiding the problem of lost transactions faced by the one-safe scheme It results in a slower commit than the one-safe scheme, but the bene ts generally outweigh the cost Several commercial shared-disk systems provide a level of fault tolerance that is intermediate between centralized and remote backup systems In these systems, the failure of a CPU does not result in system failure Instead, other CPUs take over, and they carry out recovery Recovery actions include rollback of transactions running on the failed CPU, and recovery of locks held by those transactions Since data are on a shared disk, there is no need for transfer of log records However, we should safeguard the data from disk failure by using, for example, a RAID disk organization An alternative way of achieving high availability is to use a distributed database, with data replicated at more than one site Transactions are then required to update all replicas of any data item that they update We study distributed databases, including replication, in 19
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A computer system, like any other mechanical or electrical device, is subject to failure There are a variety of causes of such failure, including disk crash, power failure, and software errors In each of these cases, information concerning the database system is lost In addition to system failures, transactions may also fail for various reasons, such as violation of integrity constraints or deadlocks An integral part of a database system is a recovery scheme that is responsible for the detection of failures and for the restoration of the database to a state that existed before the occurrence of the failure
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17 Recovery System
The McGraw Hill Companies, 2001
Summary
The various types of storage in a computer are volatile storage, nonvolatile storage, and stable storage Data in volatile storage, such as in RAM, are lost when the computer crashes Data in nonvolatile storage, such as disk, are not lost when the computer crashes, but may occasionally be lost because of failures such as disk crashes Data in stable storage are never lost Stable storage that must be accessible online is approximated with mirrored disks, or other forms of RAID, which provide redundant data storage Of ine, or archival, stable storage may consist of multiple tape copies of data stored in a physically secure location In case of failure, the state of the database system may no longer be consistent; that is, it may not re ect a state of the world that the database is supposed to capture To preserve consistency, we require that each transaction be atomic It is the responsibility of the recovery scheme to ensure the atomicity and durability property There are basically two different approaches for ensuring atomicity: log-based schemes and shadow paging In log-based schemes, all updates are recorded on a log, which must be kept in stable storage In the deferred-modi cations scheme, during the execution of a transaction, all the write operations are deferred until the transaction partially commits, at which time the system uses the information on the log associated with the transaction in executing the deferred writes In the immediate-modi cations scheme, the system applies all updates directly to the database If a crash occurs, the system uses the information in the log in restoring the state of the system to a previous consistent state To reduce the overhead of searching the log and redoing transactions, we can use the checkpointing technique In shadow paging, two page tables are maintained during the life of a transaction: the current page table and the shadow page table When the transaction starts, both page tables are identical The shadow page table and pages it points to are never changed during the duration of the transaction When the transaction partially commits, the shadow page table is discarded, and the current table becomes the new page table If the transaction aborts, the current page table is simply discarded If multiple transactions are allowed to execute concurrently, then the shadowpaging technique is not applicable, but the log-based technique can be used No transaction can be allowed to update a data item that has already been updated by an incomplete transaction We can use strict two-phase locking to ensure this condition Transaction processing is based on a storage model in which main memory holds a log buffer, a database buffer, and a system buffer The system buffer holds pages of system object code and local work areas of transactions
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