What is Schedule in DBMS? - Ducat Tutorials

Quick Contact

DBMS Tutorial

DBMS ER Model

DBMS Relational Data Model

DBMS Normalization

DBMS Transactions

What is Schedule?

The schedule is defined as the chronological order in which the instruction of the transactions will be executed.

Example of Schedule

The following sequence of operations is a schedule. Moreover, we find two transactions T1 and T2, that are executing concurrently.

This schedule decides the actual series of operations which are prevailing to be implemented on the database.

T1	T2
Read (P)
Write (P)
Read (Q)
	Read (Q)
	Read (P)
	Write (Q)

Types of Schedule

Serial Schedule

A schedule is said to be serial if transactions are executed serially one after the other.

Let us suppose that T1 and T2 are the two transactions. A schedule containing T1 and T2 is said to be serial if either of the two conditions is true.

Executes all operations of T1 and then performs all operations of T2, as shown in figure (a).
Executes all operations of T2 and then performs all operations of T1, as shown in figure (b).

Non-Serial Schedule

A schedule is said to be non-serial if the operation of each transaction is executed non-consecutively with inter-leaved operation from the other transaction.

Consider the schedule given in figure (c), which is a non-serial schedule.

Difference between Serial Schedule and Non-Serial Schedule

Serial Schedule	Non-Serial Schedule
A serial schedule is a series of operations by a set of concurrent transactions that maintains the order of operations in each of the single transactions.	A non-serial schedule is a schedule wherever the functioning of a set of concurrent dealing are interlaced.
Transactions are implemented in serial order.	Transactions are implemented in non-serial order, but the outcome should be similar to serial.
There is no interference among transactions.	Concurrency problems can arise here.
The serial schedule provides the advantage of concurrent execution without any problem.	In this schedule, there is no advantage of concurrent execution.
Serial schedule that does interleave the aspect of different transactions.	The non-serial schedule has no only fixed aspect of any transaction.
Example: If some transaction T is long, another transaction should wait for T to finish all its operations.	Example: In this schedule, the execution of another transaction goes on without waiting for the implementation of T.

What is Serializability?

The serializable schedule is a schedule that succeeds a fixed of transactions to perform in a selected order like that the consequences are similar to imposing them in a few serial order like a serial schedule. The serial execution of transactions usually forgets the database in a consistent state.

Serializability defines the concurrent functions of various dealings. The goals of serializability are to determine the non-serial schedules that permit dealings to perform concurrently beyond interference with each other and there generating a record state that should be provided by the serial execution. Serializability should be authenticated to ensure inconsistency from a business dealing with interference with each other. The order of read and write process is essential in serializability.

Testing of Serializability

The serialization diagram is utilized to check the serializability of the schedule.

Suppose a schedule S. For schedule S, generate a graph known as a precedence graph. It has a pair G = (V, E), where E contains a collection of edges, and V includes a selection of vertices. The collection of vertices includes all the transactions available within the S schedule. The set of edges consists of all edges T_i → T_j for which one of the following three conditions satisfy:

Generate a node T_i → T_j given T_i transaction executes write (Q) before T_j transaction executes read (Q).
Generate a node T_i → T_j given T_i transaction executes read (Q) before T_j transaction executes write (Q).
Generate a node T_i → T_j given T_i transaction executes write (Q) before T_j transaction executes write (Q).

Schedule S

Time	Transaction T1	Transaction T2
T1	Read (P)
T2	P:=P+50
T3	Write (P)
T4		Read (P)
T5		P:=P+100
T6		Write (P)

In the above precedence graph of schedule S, contains two vertices T_i and T_j, and a single edge T_i → T_j, because all the instructions of T_i are executed before the first instruction of T_j is executed.

Given a precedence graph for each schedule includes a cycle, further that schedule is non-serializable. During the precedence, the structure has no cycle, and here the schedule is serializable.

The schedule S is serializable (i.e., serial schedule) during the precedence diagram has no cycle.

Schedule S1

Time	Transaction T1	Transaction T2
T1	Read (P)
T2		Read (P)
T3		Write (P)
T4	P:=P+50
T5	Write (P)

In above precedence graph of schedule S1, contains two vertices T_i and T_j, and edges T_i → T_j and T_j → T_i. In this Schedule S1, operations of T_i and T_j transactions are present in an interleaved manner.

The precedence graph contains a cycle; that is to say, schedule S1 is non-serializable.

Enroll Yourself in Live Training: DBMS Training

Previous Next