DBMS Concurrency Control

Concurrency Control is the management procedure that is required for controlling

concurrent execution of the operations that take place on a database.

But before knowing about concurrency control, we should know about concurrent
execution.

Concurrency Control

Concurrency Control is the working concept that is required for controlling and
managing the concurrent execution of database operations and thus avoiding the
inconsistencies in the database. Thus, for maintaining the concurrency of the
database, we have the concurrency control protocols.

Concurrency Control Protocols

The concurrency control protocols ensure the atomicity, consistency, isolation,

durability and serializability of the concurrent execution of the database transactions.
Therefore, these protocols are categorized as:

o Lock Based Concurrency Control Protocol

o Time Stamp Concurrency Control Protocol
o Validation Based Concurrency Control Protocol

We will understand and discuss each protocol one by one in our next sections.

Lock-Based Protocol

In this type of protocol, any transaction cannot read or write data until it acquires an
appropriate lock on it. There are two types of lock:

1. Shared lock:

o It is also known as a Read-only lock. In a shared lock, the data item can only
read by the transaction.
o It can be shared between the transactions because when the transaction holds
a lock, then it can't update the data on the data item.

2. Exclusive lock:

o In the exclusive lock, the data item can be both reads as well as written by the
transaction.
 o This lock is exclusive, and in this lock, multiple transactions do not modify the
same data simultaneously.

There are four types of lock protocols available:

1. Simplistic lock protocol

It is the simplest way of locking the data while transaction. Simplistic lock-based
protocols allow all the transactions to get the lock on the data before insert or delete
or update on it. It will unlock the data item after completing the transaction.

2. Pre-claiming Lock Protocol

o Pre-claiming Lock Protocols evaluate the transaction to list all the data items
on which they need locks.
o Before initiating an execution of the transaction, it requests DBMS for all the
lock on all those data items.
o If all the locks are granted then this protocol allows the transaction to begin.
When the transaction is completed then it releases all the lock.
o If all the locks are not granted then this protocol allows the transaction to rolls
back and waits until all the locks are granted.

3. Two-phase locking (2PL)

o The two-phase locking protocol divides the execution phase of the transaction
into three parts.
o In the first part, when the execution of the transaction starts, it seeks
permission for the lock it requires.
o In the second part, the transaction acquires all the locks. The third phase is
started as soon as the transaction releases its first lock.
o In the third phase, the transaction cannot demand any new locks. It only
releases the acquired locks.
There are two phases of 2PL:

Growing phase: In the growing phase, a new lock on the data item may be acquired
by the transaction, but none can be released.

Shrinking phase: In the shrinking phase, existing lock held by the transaction may
be released, but no new locks can be acquired.

In the below example, if lock conversion is allowed then the following phase can
happen:

1. Upgrading of lock (from S(a) to X (a)) is allowed in growing phase.

2. Downgrading of lock (from X(a) to S(a)) must be done in shrinking phase.

Example:

The following way shows how unlocking and locking work with 2-PL.
Transaction T1:

o Growing phase: from step 1-3

o Shrinking phase: from step 5-7
o Lock point: at 3

Transaction T2:

o Growing phase: from step 2-6

o Shrinking phase: from step 8-9
o Lock point: at 6

4. Strict Two-phase locking (Strict-2PL)

o The first phase of Strict-2PL is similar to 2PL. In the first phase, after acquiring
all the locks, the transaction continues to execute normally.
o The only difference between 2PL and strict 2PL is that Strict-2PL does not
release a lock after using it.
o Strict-2PL waits until the whole transaction to commit, and then it releases all
the locks at a time.
o Strict-2PL protocol does not have shrinking phase of lock release.

It does not have cascading abort as 2PL does.

Timestamp Ordering Protocol

o The Timestamp Ordering Protocol is used to order the transactions based on

their Timestamps. The order of transaction is nothing but the ascending order
of the transaction creation.
 o The priority of the older transaction is higher that's why it executes first. To
determine the timestamp of the transaction, this protocol uses system time or
logical counter.
o The lock-based protocol is used to manage the order between conflicting
pairs among transactions at the execution time. But Timestamp based
protocols start working as soon as a transaction is created.
o Let's assume there are two transactions T1 and T2. Suppose the transaction T1
has entered the system at 007 times and transaction T2 has entered the
system at 009 times. T1 has the higher priority, so it executes first as it is
entered the system first.
o The timestamp ordering protocol also maintains the timestamp of last 'read'
and 'write' operation on a data.

Basic Timestamp ordering protocol works as follows:

1. Check the following condition whenever a transaction Ti issues a Read

(X) operation:

o If W_TS(X) >TS(Ti) then the operation is rejected.

o If W_TS(X) <= TS(Ti) then the operation is executed.
o Timestamps of all the data items are updated.

2. Check the following condition whenever a transaction Ti issues

a Write(X) operation:

o If TS(Ti) < R_TS(X) then the operation is rejected.

o If TS(Ti) < W_TS(X) then the operation is rejected and Ti is rolled back
otherwise the operation is executed.

Where,

TS(TI) denotes the timestamp of the transaction Ti.

R_TS(X) denotes the Read time-stamp of data-item X.

W_TS(X) denotes the Write time-stamp of data-item X.

Advantages and Disadvantages of TO protocol:

o TO protocol ensures serializability since the precedence graph is as follows:

 o TS protocol ensures freedom from deadlock that means no transaction ever
waits.
o But the schedule may not be recoverable and may not even be cascade- free.

Validation Based Protocol

Validation phase is also known as optimistic concurrency control technique. In the

validation based protocol, the transaction is executed in the following three phases:

1. Read phase: In this phase, the transaction T is read and executed. It is used to
read the value of various data items and stores them in temporary local
variables. It can perform all the write operations on temporary variables
without an update to the actual database.
2. Validation phase: In this phase, the temporary variable value will be validated
against the actual data to see if it violates the serializability.
3. Write phase: If the validation of the transaction is validated, then the
temporary results are written to the database or system otherwise the
transaction is rolled back.

Here each phase has the following different timestamps:

Start(Ti): It contains the time when Ti started its execution.

Validation (Ti): It contains the time when Ti finishes its read phase and starts its
validation phase.

Finish(Ti): It contains the time when Ti finishes its write phase.

o This protocol is used to determine the time stamp for the transaction for
serialization using the time stamp of the validation phase, as it is the actual
phase which determines if the transaction will commit or rollback.
o Hence TS(T) = validation(T).
 o The serializability is determined during the validation process. It can't be
decided in advance.
o While executing the transaction, it ensures a greater degree of concurrency
and also less number of conflicts.
o Thus it contains transactions which have less number of rollbacks.

Deadlock in DBMS

A deadlock is a condition where two or more transactions are waiting indefinitely for
one another to give up locks. Deadlock is said to be one of the most feared
complications in DBMS as no task ever gets finished and is in waiting state forever.

For example: In the student table, transaction T1 holds a lock on some rows and
needs to update some rows in the grade table. Simultaneously, transaction T2 holds
locks on some rows in the grade table and needs to update the rows in the Student
table held by Transaction T1.

Now, the main problem arises. Now Transaction T1 is waiting for T2 to release its
lock and similarly, transaction T2 is waiting for T1 to release its lock. All activities
come to a halt state and remain at a standstill. It will remain in a standstill until the
DBMS detects the deadlock and aborts one of the transactions.

Deadlock Avoidance

o When a database is stuck in a deadlock state, then it is better to avoid the

database rather than aborting or restating the database. This is a waste of
time and resource.
 o Deadlock avoidance mechanism is used to detect any deadlock situation in
advance. A method like "wait for graph" is used for detecting the deadlock
situation but this method is suitable only for the smaller database. For the
larger database, deadlock prevention method can be used.

Deadlock Detection

In a database, when a transaction waits indefinitely to obtain a lock, then the DBMS
should detect whether the transaction is involved in a deadlock or not. The lock
manager maintains a Wait for the graph to detect the deadlock cycle in the
database.

Wait for Graph

o This is the suitable method for deadlock detection. In this method, a graph is
created based on the transaction and their lock. If the created graph has a
cycle or closed loop, then there is a deadlock.
o The wait for the graph is maintained by the system for every transaction which
is waiting for some data held by the others. The system keeps checking the
graph if there is any cycle in the graph.

The wait for a graph for the above scenario is shown below:

Deadlock Prevention

o Deadlock prevention method is suitable for a large database. If the resources

are allocated in such a way that deadlock never occurs, then the deadlock can
be prevented.
 o The Database management system analyzes the operations of the transaction
whether they can create a deadlock situation or not. If they do, then the DBMS
never allowed that transaction to be executed.

Wait-Die scheme

In this scheme, if a transaction requests for a resource which is already held with a
conflicting lock by another transaction then the DBMS simply checks the timestamp
of both transactions. It allows the older transaction to wait until the resource is
available for execution.

Let's assume there are two transactions Ti and Tj and let TS(T) is a timestamp of any
transaction T. If T2 holds a lock by some other transaction and T1 is requesting for
resources held by T2 then the following actions are performed by DBMS:

1. Check if TS(Ti) < TS(Tj) - If Ti is the older transaction and Tj has held some
resource, then Ti is allowed to wait until the data-item is available for
execution. That means if the older transaction is waiting for a resource which
is locked by the younger transaction, then the older transaction is allowed to
wait for resource until it is available.
2. Check if TS(Ti) < TS(Tj) - If Ti is older transaction and has held some resource
and if Tj is waiting for it, then Tj is killed and restarted later with the random
delay but with the same timestamp.

Wound wait scheme

o In wound wait scheme, if the older transaction requests for a resource which is
held by the younger transaction, then older transaction forces younger one to
kill the transaction and release the resource. After the minute delay, the
younger transaction is restarted but with the same timestamp.
o If the older transaction has held a resource which is requested by the Younger
transaction, then the younger transaction is asked to wait until older releases
it.