A transaction is a logical unit of work that accesses and possibly modifies the database. It includes one or more database operations that must either all be completed or all rolled back together to maintain database consistency. Transactions must have ACID properties - Atomicity, Consistency, Isolation, and Durability to ensure data integrity during concurrent execution. Concurrency control techniques like locking and timestamps are used to isolate transactions and maintain serializability. Recovery techniques use a log to roll back or redo incomplete transactions and restore the database to a consistent state after failures.

1. What is a Database Transaction?
A transaction is a logical unit of processing in a DBMS which entails one or more database access
operation. In a nutshell, database transactions represent real-world events of any enterprise.
All types of database access operation which are held between the beginning and end transaction
statements are considered as a single logical transaction. During the transaction the database is
inconsistent. Only once the database is committed the state is changed from one consistent state to
another.
 A transaction is a program unit whose execution may or may not change the contents of a
database.
 The transaction is executed as a single unit
 If the database operations do not update the database but only retrieve data, this type of
transaction is called a read-only transaction.
 A successful transaction can change the database from one CONSISTENT STATE to another
 DBMS transactions must be atomic, consistent, isolated and durable
 If the database were in an inconsistent state before a transaction, it would remain in the
inconsistent state after the transaction.
 Hardware failure and system crashes
 Concurrent execution of the same transaction, deadlock, or slow performance
Concurrency control:-
 Concurrency control is the procedure in DBMS for managing simultaneous operations without
conflicting with each another. Concurrent access is quite easy if all users are just reading data.
There is no way they can interfere with one another. Though for any practical database, would have
a mix of reading and WRITE operations and hence the concurrency is a challenge.
 Concurrency control is used to address such conflicts which mostly occur with a multi-user system.
It helps you to make sure that database transactions are performed concurrently without violating
the data integrity of respective databases.
 Therefore, concurrency control is a most important element for the proper functioning of a system
where two or multiple database transactions that require access to the same data, are executed
simultaneously.
Lost Updates occur when multiple transactions select the same row and update the row based on
the value selected
Uncommitted dependency issues occur when the second transaction selects a row which is
updated by another transaction (dirty read)
Non-Repeatable Read occurs when a second transaction is trying to access the same row several
times and reads different data each time.
Incorrect Summary issue occurs when one transaction takes summary over the value of all the
instances of a repeated data-item, and second transaction update few instances of that specific
data-item. In that situation, the resulting summary does not reflect a correct result.
ACID Properties
A transaction is a very small unit of a program and it may contain several lowlevel tasks. A transaction in a database
system must maintain Atomicity, Consistency, Isolation, and Durability − commonly known as ACID properties − in
order to ensure accuracy, completeness, and data integrity.
 Atomicity − This property states that a transaction must be treated as an atomic unit, that is, either all of its operations
are executed or none.There mustbe no state in a database where a transaction is leftpartially completed.States should
be defined either before the execution of the transaction or after the execution/abortion/failure of the transaction.

2.  Consistency − The database must remain in a consistent state after any transaction. No transaction should have any
adverse effect on the data residing in the database. If the database was in a consistent state before the execution of a
transaction, it must remain consistent after the execution of the transaction as well.
 Durability − The database should be durable enough to hold all its latest updates even if the system fails or restarts. If a
transaction updates a chunk of data in a database and commits, then the database will hold the modified data. If a
transaction commits but the system fails before the data could be written on to the disk, then that data will be updated
once the system springs back into action.
 Isolation − In a database system where more than one transaction are being executed simultaneously and in parallel,
the property of isolation states that all the transactions will be carried out and executed as if it is the only transaction in
the system. No transaction will affect the existence of any other transaction.
Serializability
When multiple transactions are being executed by the operating system in a multiprogramming environment, there
are possibilities that instructions of one transactions are interleaved with some other transaction.
 Schedule − A chronological execution sequence of a transaction is called a schedule. A schedule can have many
transactions in it, each comprising of a number of instructions/tasks.
 Serial Schedule − It is a schedule in which transactions are aligned in such a way that one transaction is executed first.
When the firsttransaction completes its cycle, then the next transaction is executed. Transactions are ordered one after
the other. This type of schedule is called a serial schedule, as transactions are executed in a serial manner.
In a multi-transaction environment, serial schedules are considered as a benchmark. The execution sequence of an
instruction in a transaction cannot be changed, but two transactions can have their instructions executed in a random
fashion. This execution does no harm if two transactions are mutually independent and working on different
segments of data; but in case these two transactions are working on the same data, then the results may vary. This
ever-varying result may bring the database to an inconsistent state.
To resolve this problem, we allow parallel execution of a transaction schedule, if its transactions are either
serializable or have some equivalence relation among them.
Concurrency-control protocols : allow concurrent schedules,but ensure that the schedules are conflict/view serializable, and are
recoverable and maybe even cascadeless.
These protocols do not examine the precedence graph as it is being created, instead a protocol imposes a discipline that avoids
non-seralizable schedules.
Lock Based Protocols –
A lock is a variable associated with a data item that describes a status ofdata item with respect to possible operation that can be
applied to it. They synchronize the access by concurrent transactions to the database items. It is required in this protocol that all
the data items must be accessed in a mutually exclusive manner. Let me introduce you to two common locks which are used and
some terminology followed in this protocol.
1. Shared Lock (S): also known as Read-only lock. As the name suggests it can be shared between transactions because
while holding this lock the transaction does not have the permission to update data on the data item. S-lock is requested
using lock-S instruction.
2. Exclusive Lock (X): Data item can be both read as well as written.This is Exclusive and cannot be held simultaneously on
the same data item. X-lock is requested using lock-X instruction.
Deadlock – consider the above execution phase. Now, T1 holds an Exclusive lock over B, and T2 holds a Shared lock over A.
Consider Statement 7, T2 requests for lock on B, while in Statement 8 T1 requests lock on A. This as you may notice imposes
a Deadlock as none can proceed with their execution.
Two Phase Locking (2-PL)
Two-Phase locking protocol which is also known as a 2PL protocol. It is also called P2L. In this type of
locking protocol, the transaction should acquire a lock after it releases one of its locks.
This locking protocol divides the execution phase of a transaction into three different parts.
 In the first phase, when the transaction begins to execute, it requires permission for the locks it
needs.
 The second part is where the transaction obtains all the locks. When a transaction releases its first
lock, the third phase starts.

3.  In this third phase, the transaction cannot demand any new locks. Instead, it only releases the
acquired locks.
The Two-Phase Locking protocol allows each transaction to make a lock or unlock request in two steps:
 Growing Phase: In this phase transaction may obtain locks but may not release any locks.
 Shrinking Phase: In this phase, a transaction may release locks but not obtain any new lock
Timestamp-based Protocols
The timestamp-based algorithm uses a timestamp to serialize the execution of concurrent transactions.
This protocol ensures that every conflicting read and write operations are executed in timestamp order. The
protocol uses the System Time or Logical Count as a Timestamp.
The older transaction is always given priority in this method. It uses system time to determine the time
stamp of the transaction. This is the most commonly used concurrency protocol.
Lock-based protocols help you to manage the order between the conflicting transactions when they will
execute. Timestamp-based protocols manage conflicts as soon as an operation is created.
Advantages:
 Schedules are serializable just like 2PL protocols
 No waiting for the transaction, which eliminates the possibility of deadlocks!
Disadvantages:
Starvation is possible if the same transaction is restarted and continually aborted
Database Recovery Techniques in DBMS
Database systems, like any other computer system,are subject to failures but the data stored in it must be available as and when
required. When a database fails it must possess the facilities for fast recovery. It must also have atomicity i.e. either transactions
are completed successfully and committed (the effect is recorded permanently in the database)or the transaction should have no
effect on the database.
There are both automatic and non-automatic ways for both,backing up of data and recovery from any failure situations.The
techniques used to recover the lost data due to systemcrash, transaction errors, viruses, catastrophic failure, incorrect commands
execution etc. are database recovery techniques.So to prevent data loss recovery techniques based on deferred update and
immediate update or backing up data can be used.
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. This information is needed to recover from transaction failure.
Crash Recovery
DBMS is a highly complex system with hundreds of transactions being executed every second. The durability and
robustness of a DBMS depends on its complex architecture and its underlying hardware and system software. 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.
Failure Classification
To see where the problem has occurred, we generalize a failure into various categories, as follows −
Transaction failure
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.

4. Reasons for a transaction failure could be −
 Logical errors − Where a transaction cannot complete because it has some code error or any internal error condition.
 System errors − Where the database system itself terminates an active transaction because the DBMS is not able to
execute it, or it has to stop because of some system condition. For example, in case of deadlock or resource
unavailability, the system aborts an active transaction.
System Crash
There are problems − external to the system − that may cause the system to stop abruptly and cause the system to
crash. For example, interruptions in power supply may cause the failure of underlying hardware or software failure.
Examples may include operating system errors.
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.
Storage Structure
We have already described the storage system. In brief, the storage structure can be divided into two categories −
 Volatile storage − As the name suggests,a volatile storage cannotsurvive system crashes.Volatile storage devices are
placed very close to the CPU; normallythey are embedded onto the chipsetitself.For example,main memoryand 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. Examples may include hard-disks, magnetic tapes, flash memory, and non-volatile (battery
backed up) RAM.
Optimistic Methods of ConcurrencyControl :
The optimistic method of concurrency control is based on the assumption that conflicts of databaseoperations arerareand th at
it is better to let transactions run to completion and only check for conflicts beforethey commit.
An optimistic concurrency control method is also known as validation or certification methods.No checkingis done while the
transaction is executing. The optimistic method does not requirelockingor timestamping techniques. Instead, a transaction is
executed without restrictions until itis committed. In optimistic methods, each transaction moves through the following
phases:
a. Read phase.
b. Validation or certification phase.
c. Write phase.
Advantages of Optimistic Methods for Concurrency Control :
i. This technique is very efficient when conflicts arerare.The occasional conflictsresultin the transaction roll back.
ii. The rollback involves only thelocal copy of data,the databaseis not involved and thus there will notbe any cascading
rollbacks.
Problems of Optimistic Methods for Concurrency Control :
i. Conflicts areexpensiveto deal with, sincethe conflictingtransaction mustbe rolled back.
ii. Longer transactions aremorelikely to have conflicts and may be repeatedly rolled back becauseof conflicts with short
transactions.