Conflict serializability orders any conflicting operations in the same way as some serial execution. A pair of operations is said to conflict if they operate on the same data item and one of them is a write operation.That means, Readi(x) readj(x) - non conflict read-read operationReadi(x) writej(x) - conflict read-write operation.Writei(x) readj(x) - conflict write-read operation.Writei(x) writej(x) - conflict write-write operation.Where, I and j denote two different transactions Ti and Tj.Precedence graphNow consider a precedence graph for testing conflict serializability. It is used to check conflict serializability.Steps for precedence graphFor each transaction T, put a node or vertex in the ... Read More

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

Check point refers to a particular point of time. Searching the entire log is time-consuming. We unnecessarily redo transactions which have already output their updates to the database. So, we use check-point records in log files.At the checkpoint the contents of the database are copied to some storage. That storage can be secondary storage like a hard disk or tertiary storage such as external hard-disk.After every 10 minutes the database administrator will look after the period of check point.Creation of check-pointsLet’s see the steps how check-points are created how it is helpful in data storage during transactions −Step 1 − ... Read More

Log is nothing but a file which contains a sequence of records, each log record refers to a write operation. All the log records are recorded step by step in the log file. We can say, log files store the history of all updates activities.Log contains start of transaction, transaction number, record number, old value, new value, end of transaction etc. For example, mini statements in bank ATMs.If within an ongoing transaction, the system crashes, then by using log files, we can return back to the previous state as if nothing has happened to the database.The log is kept on ... Read More