There are two types of serializability which are as follows −View serializabilityA schedule is view-serializability if it is view equivalent to a serial schedule.The rules it follows are given below −T1 is reading the initial value of A, and then T2 also reads the initial value of A.T1 is the reading value written by T2, and then T2 also reads the value written by T1.T1 is writing the final value, and then T2 also has the write operation as the final value.Conflict serializabilityIt orders any conflicting operations in the same way as some serial execution. A pair of operations is ... Read More

The characteristics of non-serializable schedules are as follows −The transactions may or may not be consistent.The transactions may or may not be recoverable.So, now let’s talk about recoverability schedules.We all know that recoverable and irrecoverable are non-serializable techniques, Irrecoverable schedulesIf a transaction does a dirty read operation from an uncommitted transaction and commits before the transaction from where it has read the value, then such a schedule is called an irrecoverable schedule.ExampleLet us consider a two transaction schedules as shown below −T1T2Read(A)Write(A)-Read(A) ///Dirty Read-Write(A)-Commit-RollbackThe above schedule is a irrecoverable because of the reasons mentioned below −The transaction T2 which is ... Read More

When many transactions are executed simultaneously then we call them concurrent transactions. Concurrency is required to increase time efficiency. If there are many transactions which are trying to access the same data, then inconsistency arises. Concurrency control is required to maintain consistency of data.In order to run transactions concurrently, we interleave their operations. Each transaction gets a share of computing time.Problems in transactionsThis leads to the following problems −Lost update problem. ( WW conflict)Dirty read / temporary update. ( WR conflict)Unrepeatable read / incorrect analysis problem. (RW conflict)All these arise because isolation is broken.Let us discuss the dirty read or ... Read More

A schedule is defined as an execution sequence of transactions. A schedule maintains the order of the operation in each individual transaction. A schedule is the arrangement of transaction operations. A schedule may contain a set of transactions. We already know that a transaction is a set of operations. To run transactions concurrently, we arrange or schedule their operations in an interleaved fashion.Schedules are divided into 2 categories, which are as follows −Serial ScheduleNon-serial ScheduleThe categories of schedules are given below in a form of chart −Serial scheduleTransactions present in this schedule are executed serially, after the instruction of Ti ... Read More

Redundant Array of Independent Disk (RAID) combines multiple small, inexpensive disk drives into an array of disk drives which yields performance more than that of a Single Large Expensive Drive (SLED). RAID is also called Redundant Array of Inexpensive Disks.Storing the same data in different disk increases the fault-tolerance.The array of Mean Time Between Failure (MTBF) = MTBF of an individual drive, which is divided by the number of drives in the array. Because of this reason, the MTBF of an array of drives are too low for many application requirements.Types of RAIDThe various types of RAID are explained below ... Read More

Magnetic disk is the secondary storage device used to support direct access to a desired location.Parts in Magnetic diskThe different parts that are present in magnetic disk or hard disk are explained below. All these parts are helpful to read, write and store the data in the hard disk.Disk blocks − The unit data transfer between disk and main memory is a block. A disk block is a contiguous sequence of bytes.Track − Blocks are arranged in concentric rings called tracks.Sectors − A sector is the smallest unit of information that can be read from or written to disk: for ... Read More

Shadow paging is one of the techniques that is used to recover from failure. We all know that recovery means to get back the information, which is lost. It helps to maintain database consistency in case of failure.Concept of shadow pagingNow let see the concept of shadow paging step by step −Step 1 − Page is a segment of memory. Page table is an index of pages. Each table entry points to a page on the disk.Step 2 − Two page tables are used during the life of a transaction: the current page table and the shadow page table. Shadow ... Read More

All data items are updated at the end of the transaction, at the end, if any data item is found inconsistent with respect to the value in, then the transaction is rolled back.Check for conflicts at the end of the transaction. No checking while the transaction is executing. Checks are all made at once, so low transaction execution overhead. Updates are not applied until end-transaction. They are applied to local copies in a transaction space.PhasesThe optimistic concurrency control has three phases, which are explained below −Read PhaseVarious data items are read and stored in temporary variables (local copies). All operations ... Read More

The timestamp-ordering protocol ensures serializability among transactions in their conflicting read and write operations. This is the responsibility of the protocol system that the conflicting pair of tasks should be executed according to the timestamp values of the transactions.A conflict occurs when an older transaction tries to read/write a value already read or written by a younger transaction. Read or write proceeds only if the last update on that data item was carried out by an older transaction.Otherwise, the transaction requesting read/write is restarted and gives a new timestamp. Here no locks are used so no deadlock.The timestamp of transaction ... Read More

There are three possible ways in which a transaction can be executed. These are as follows −Serial execution − In serial execution, the second transaction can begin its execution only after the first transaction has completed. This is possible on a uniprocessor system.Parallel execution − In parallel execution, two transactions can start their execution at exactly the same instant of time. For this, we require more than one processor.Concurrent execution − In concurrent execution, execution of the second process can begin even before the process has completed its execution.Let us consider two transactions T1 and T2. In concurrent execution, the ... Read More