The transaction can be expressed as a group of tasks. A Simple task which cannot be divided in to further. A transaction is a small unit of a program. It may include multiple low-level tasks. A transaction is a database system which should maintain atomicity consistency isolation and durability. A transaction within the context of a database is a logical unit which is independently executed for data retrieval or updates.
A transaction is said to be a set of logically related operations. The primary operations of a transaction are:
- Read(A): Read operations Read(A), or R(A) reads the value of A from the database and stocks it in a buffer in primary memory.
- Write(A): Write operation Write(A) or W(A) writes the value back to the database from the buffer.
- Commit: After all instructions of a transaction are successfully executed, the changes made by a transaction are made permanent in the database.
- Rollback: If a transaction is not able to execute all operations successfully, all the changes made by a transaction are undone.
PROPERTIES OF TRANSACTION:
- Atomicity: A transaction should be fully complete, committed (saved) or rollback (completely undone)
- Consistency: The transaction should be fully compliant with the state of the database since it was before the transaction.
- Isolation: Transaction data should be unavailable to other transactions until the original transaction is committed or rolled back.
- Durability: Though in the event of a database failure, Transaction data changes must be available.
In a database, transaction could be in the following states,
- Active - This is the initial state of every transaction, and it is being executed.
- Partially Committed - If a transaction performs its final operation, it is said to be in a partially committed state.
- Failed - If any check made by a database recovery system falls to fail then a transaction is said to be in the collapsed state. A transaction if it is failed it can no longer proceed further.
- Aborted - If any of the checks collapse and the transaction has reached a failed state, then the recovery manager deals in rolling back all its write operations on the database and brings the database back to its initial state where it was before the execution of the transaction. Transactions in this state are called aborted.
The database recovery module can select any one from the following two operations after a transaction aborts,
- Re-start the transaction
- Kill the transaction
COMMITTED - If a transaction executes all its operations, then it is said to be committed successfully. All its effects are now completely and permanently established on the database system.
The multiple transactions can be executed simultaneously in a multiprogramming environment; hence it is highly important to control the concurrency of transactions. There are concurrency control protocols to ensure atomicity, isolation, and serializability of concurrent transactions.
There are several other problems which arise with concurrent execution of transactions which operate on the same data item. The "Concurrency control" system manages all of them in a database. When multiple transactions are executed concurrently, it is essential to note that data are updated correctly.
Concurrency controlling techniques plays a vital role in multiple programming transactions and makes sure that various transactions are executed simultaneously while maintaining the ACID properties of the transactions and serializability in the schedules.
LOCKING BASED CONCURRENCY CONTROL PROTOCOLS:
Locking methodology in concurrency control protocols uses the concept of securing the data items. A lock is a variable mainly associated with a data item that purely determines whether it could perform read/write operations on a particular data item.
A lock compatibility matrix is employed that states whether or not an information item is fast by two transactions at constant time.
Locking-based concurrency control systems have the following two locking protocol techniques.
One-phase Locking Protocol:
The one phase locking protocol ensures that each transaction locks an item before it starts using it and then releases the lock as soon as it has finished using it. A maximum concurrency is provided by this one phase locking protocol but does not always enforce serializability.
Two-phase Locking Protocol:
The two-phase locking protocols concentrate on two major phases.
- Growing Phase
- Shrinking phase
In the growing period, a transaction is meant to acquire the locks and need not release any locks.
In the shrinking phase, a transaction is meant to release the locks and could not request any new locks.
Every transaction which follows two-phase locking protocol is guaranteed to be serializable.
However, this approach mainly needed which provides low parallelism between two conflicting transactions.
IMPORTANCE OF CONCURRENCY CONTROL:
Concurrency control is used mainly for addressing the conflicts which occur mostly with a multi-user system. Therefore concurrency control is more critical for the proper functioning of a system where two or more multiple database transactions are executed.
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