Full Answer
What is recovery and atomicity?
Recovery and Atomicity It should check the states of all the transactions, which were being executed. A transaction may be in the middle of some operation; the DBMS must ensure the atomicity of the transaction in this case. It should check whether the transaction can be completed now or it needs to be rolled back.
What is recovery in DBMS?
Recovery is the rebuilding of a database or table space after a problem such as media or storage failure, power interruption, or application failure. If you have backed up your database, or individual table spaces, you can rebuild them should they become damaged or corrupted in some way.
What is DBMS atomicity?
Atomicity means that multiple operations can be grouped into a single logical entity, that is, other threads of control accessing the database will either see all of the changes or none of the changes.
What is recovery and its types?
What Are the Types of Recovery? There are three basic types of recovery: instance recovery, crash recovery, and media recovery. Oracle performs the first two types of recovery automatically at instance startup; only media recovery requires you to issue commands.
What is recovery with example?
Recovery is the regaining or returning of something. An example of recovery is someone getting healthy after being sick.
What is recovery explain?
Definition of recovery 1 : the act, process, or an instance of recovering especially : an economic upturn (as after a depression) 2 : the process of combating a disorder (such as alcoholism) or a real or perceived problem.
What is atomicity?
Atomicity is defined as the total number of atoms present in a molecule. For example, each molecule of oxygen (O2) is composed of two oxygen atoms. Therefore, atomicity of oxygen is 2. In older contexts, atomicity is sometimes equivalent to valency.
What is atomicity in database with example?
An example of an atomic transaction is a monetary transfer from bank account A to account B. It consists of two operations, withdrawing the money from account A and saving it to account B.
What is the use of atomicity?
Atomicity is a feature of databases systems dictating where a transaction must be all-or-nothing. That is, the transaction must either fully happen, or not happen at all. It must not complete partially.
What are the 3 recovery models?
Three recovery models exist: simple, full, and bulk-logged. Typically, a database uses the full recovery model or simple recovery model.
What are the two types of recovery?
There are two types of recovery: active and passive. Both recovery methods are important, and people may use one or the other at different points to suit their circumstances. In this article, we discuss the benefits of active recovery and how it differs from passive recovery.
What are two methods of recovery?
Methods to Enhance Recovery Some of the most popular recovery techniques for athletes include hydrotherapy, active recovery, stretching, compression garments, massage, sleep and nutrition.
What is recovery in system?
Restores your computer's system files to an earlier point in time without affecting your files, such as e‑mail, documents, or photos. If you use System Restore from the System Recovery Options menu, you cannot undo the restore operation.
What are the 4 types of data recovery?
And if you don't know that, you risk damaging your hardware permanently and losing your data.Hard Drive Recovery. Hard drive recovery is probably the most common type of data recovery, simply because you interact with hard drives every day. ... Tape Recovery. ... Optical Recovery. ... Removable Recovery. ... Digital Recovery.
What is backup and recovery in DBMS?
Backup and recovery describes the process of creating and storing copies of data that can be used to protect organizations against data loss. This is sometimes referred to as operational recovery.
Why We Use recovery in DBMS?
If such a failure affects the operation of a database system, you must usually recover the database and return to normal operation as quickly as possible. Recovery should protect the database and associated users from unnecessary problems and avoid or reduce the possibility of having to duplicate work manually.
What happens if a DBMS fails?
If it fails or crashes amid transactions, it is expected that the system would follow some sort of algorithm or techniques to recover lost data.
What is log based recovery?
Log-based Recovery. Log is a sequence of records, which maintains the records of actions performed by a transaction. It is important that the logs are written prior to the actual modification and stored on a stable storage media, which is failsafe.
Why does a transaction have to abort?
A transaction has to abort when it fails to execute or when it reaches a point from where it can’t go any further. This is called transaction failure where only a few transactions or processes are hurt. Reasons for a transaction failure could be −.
What happens when recovery system sees logs?
If the recovery system sees a log with <T n, Start> and <T n, Commit> or just <T n, Commit>, it puts the transaction in the redo-list.
What is disk failure?
Disk Failure. In early days of technology evolution, it was a common problem where hard-disk drives or storage drives used to fail frequently. Disk failures include formation of bad sectors, unreachability to the disk, disk head crash or any other failure, which destroys all or a part of disk storage.
What are some examples of volatile storage?
For example, main memory and cache memory are examples of volatile storage. They are fast but can store only a small amount of information. Non-volatile storage − These memories are made to survive system crashes. They are huge in data storage capacity, but slower in accessibility.
Can transactions leave DBMS?
No transactions would be allowed to leave the DBMS in an inconsistent state.
What is the process of updating a database before it reaches its commit point?
Immediate update – In the immediate update, the database may be updated by some operations of a transaction before the transaction reaches its commit point. However, these operations are recorded in a log on disk before they are applied to the database, making recovery still possible. If a transaction fails to reach its commit point, the effect of its operation must be undone i.e. the transaction must be rolled back hence we require both undo and redo. This technique is known as undo/redo algorithm.
What is checkpoint in DBMS?
checkpoint: Checkpoint is a mechanism where all the previous logs are removed from the system and stored permanently in a storage disk. Checkpoint declares a point before which the DBMS was in consistent state, and all the transactions were committed.
What happens if a transaction fails before reaching its commit point?
If a transaction fails before reaching its commit point, it will not have changed the database in any way so UNDO is not needed. It may be necessary to REDO the effect of the operations that are recorded in the local transaction workspace, because their effect may not yet have been written in the database.
How to backup a database?
Some of the backup techniques are as follows : 1 Full database backup – In this full database including data and database, Meta information needed to restore the whole database, including full-text catalogs are backed up in a predefined time series. 2 Differential backup – It stores only the data changes that have occurred since last full database backup. When same data has changed many times since last full database backup, a differential backup stores the most recent version of changed data. For this first, we need to restore a full database backup. 3 Transaction log backup – In this, all events that have occurred in the database, like a record of every single statement executed is backed up. It is the backup of transaction log entries and contains all transaction that had happened to the database. Through this, the database can be recovered to a specific point in time. It is even possible to perform a backup from a transaction log if the data files are destroyed and not even a single committed transaction is lost.
What is the log file used for recovery?
Recovery techniques are heavily dependent upon the existence of a special file known as a system log. It contains information about the start and end of each transaction and any updates which occur in the transaction. The log keeps track of all transaction operations that affect the values of database items.
What is differential backup?
Differential backup – It stores only the data changes that have occurred since last full database backup. When same data has changed many times since last full database backup, a differential backup stores the most recent version of changed data. For this first, we need to restore a full database backup.
What is a full database backup?
Full database backup – In this full database including data and database, Meta information needed to restore the whole database, including full-text catalogs are backed up in a predefined time series.
Why is it important to have atomicity in a database?
It is essential that a database system that claims to offer atomicity be able to do so even in the face of failure in power supply or the underlying operating system or application that uses the database.
What constitutes an atomic transaction?
The definition of what constitutes an atomic transaction is decided by its context or the environment in which it being implemented. For example, in an Online airline-booking system, a booking may consist of 2 separate actions that together form a transaction -- paying for the seat, and reserving the seat for the customer who’s just paid. Business logic dictates that these two, though distinct and separate actions, must occur together. If one happens without the other, problems can occur. For example, the system may reserve the same seat for two separate customers.
What is ACID in database?
The ACID properties, in totality, provide a mechanism to ensure correctness and consistency of a database in a way such that each transaction is a group of operations that acts a single unit, produces consistent results, acts in isolation from other operations and updates that it makes are durably stored.
What happens if a transaction fails after T1?
( say, after write (X) but before write (Y) ), then amount has been deducted from X but not added to Y. This results in an inconsistent database state. Therefore, the transaction must be executed in entirety in order to ensure correctness of database state.
What is a transaction in a database?
A transaction is a single logical unit of work which accesses and possibly modifies the contents of a database. Transactions access data using read and write operations. In order to maintain consistency in a database, before and after the transaction, certain properties are followed. These are called ACID properties. Atomicity.
Crash Recovery
Failure Classification
- To see where the problem has occurred, we generalize a failure into various categories, as follows −
Storage Structure
- We have already described the storage system. In brief, the storage structure can be divided into two categories − 1. Volatile storage− As the name suggests, a volatile storage cannot survive system crashes. Volatile storage devices are placed very close to the CPU; normally they are embedded onto the chipset itself. For example, main memory and cache memory are examples …
Recovery and Atomicity
- When a system crashes, it may have several transactions being executed and various files opened for them to modify the data items. Transactions are made of various operations, which are atomic in nature. But according to ACID properties of DBMS, atomicity of transactions as a whole must be maintained, that is, either all the operations are executed...
Log-Based Recovery
- Log is a sequence of records, which maintains the records of actions performed by a transaction. It is important that the logs are written prior to the actual modification and stored on a stable storage media, which is failsafe. Log-based recovery works as follows − 1. The log file is kept on a stable storage media. 2. When a transaction enters the system and starts execution, it writes a l…
Recovery with Concurrent Transactions
- When more than one transaction are being executed in parallel, the logs are interleaved. At the time of recovery, it would become hard for the recovery system to backtrack all logs, and then start recovering. To ease this situation, most modern DBMS use the concept of 'checkpoints'.