which schedule can be categorized as a strict schedule?

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Cascading Rollback | Cascadeless Schedule

Database Management System

Recoverability-

Before you go through this article, make sure that you have gone through the previous article on Recoverability in DBMS.

We have discussed-

Non-serial schedules which are not serializable are called as non-serializable schedules.

Non-serializable schedules may be recoverable or irrecoverable.

Recoverable Schedules-

If in a schedule,

A transaction performs a dirty read operation from an uncommitted transaction

And its commit operation is delayed till the uncommitted transaction either commits or roll backs

then such a schedule is called as a Recoverable Schedule.

Types of Recoverable Schedules-

A recoverable schedule may be any one of these kinds-

Cascading Schedule

Cascadeless Schedule

Strict Schedule

Cascading Schedule-

If in a schedule, failure of one transaction causes several other dependent transactions to rollback or abort, then such a schedule is called as a Cascading Schedule or Cascading Rollback or Cascading Abort.

It simply leads to the wastage of CPU time.

Example-

Here,

Transaction T2 depends on transaction T1.

Transaction T3 depends on transaction T2.

Transaction T4 depends on transaction T3.

In this schedule,

The failure of transaction T1 causes the transaction T2 to rollback.

The rollback of transaction T2 causes the transaction T3 to rollback.

The rollback of transaction T3 causes the transaction T4 to rollback.

Such a rollback is called as a Cascading Rollback.

NOTE-

If the transactions T2, T3 and T4 would have committed before the failure of transaction T1, then the schedule would have been irrecoverable.

Cascadeless Schedule-

If in a schedule, a transaction is not allowed to read a data item until the last transaction that has written it is committed or aborted, then such a schedule is called as a Cascadeless Schedule.

In other words,

Cascadeless schedule allows only committed read operations.

Therefore, it avoids cascading roll back and thus saves CPU time.

Example-

NOTE-

Cascadeless schedule allows only committed read operations.

However, it allows uncommitted write operations.

Example-

Strict Schedule-

If in a schedule, a transaction is neither allowed to read nor write a data item until the last transaction that has written it is committed or aborted, then such a schedule is called as a Strict Schedule.

In other words,

Strict schedule allows only committed read and write operations.

Clearly, strict schedule implements more restrictions than cascadeless schedule.

Example-

Remember-

Strict schedules are more strict than cascadeless schedules.

All strict schedules are cascadeless schedules.

All cascadeless schedules are not strict schedules.

Next Article- Equivalence of Schedules

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DBMS Schedules and the Types of Schedules

We know that transactions are set of instructions and these instructions perform operations on database. When multiple transactions are running

DBMS Schedules and the Types of Schedules

BY CHAITANYA SINGH | FILED UNDER: DBMS

We know that transactions are set of instructions and these instructions perform operations on database. When multiple transactions are running concurrently then there needs to be a sequence in which the operations are performed because at a time only one operation can be performed on the database. This sequence of operations is known as Schedule.

Lets take an example to understand what is a schedule in DBMS.

DBMS Schedule example

The following sequence of operations is a schedule. Here we have two transactions T1 & T2 which are running concurrently.

This schedule determines the exact order of operations that are going to be performed on database. In this example, all the instructions of transaction T1 are executed before the instructions of transaction T2, however this is not always necessary and we can have various types of schedules which we will discuss in this article.

T1 T2 ---- ---- R(X) W(X) R(Y) R(Y) R(X) W(Y)

Types of Schedules in DBMS

We have various types of schedules in DBMS. Lets discuss them one by one.

Serial Schedule

In Serial schedule, a transaction is executed completely before starting the execution of another transaction. In other words, you can say that in serial schedule, a transaction does not start execution until the currently running transaction finished execution. This type of execution of transaction is also known as non-interleaved execution. The example we have seen above is the serial schedule.

Lets take another example.

Serial Schedule example

Here R refers to the read operation and W refers to the write operation. In this example, the transaction T2 does not start execution until the transaction T1 is finished.

T1 T2 ---- ---- R(A) R(B) W(A) commit R(B) R(A) W(B) commit

Strict Schedule

In Strict schedule, if the write operation of a transaction precedes a conflicting operation (Read or Write operation) of another transaction then the commit or abort operation of such transaction should also precede the conflicting operation of other transaction.

Lets take an example.

Strict Schedule example

Lets say we have two transactions Ta and Tb. The write operation of transaction Ta precedes the read or write operation of transaction Tb, so the commit or abort operation of transaction Ta should also precede the read or write of Tb.

Ta Tb ----- ----- R(X) R(X) W(X) commit W(X) R(X) commit

Here the write operation W(X) of Ta precedes the conflicting operation (Read or Write operation) of Tb so the conflicting operation of Tb had to wait the commit operation of Ta.

Cascadeless Schedule

In Cascadeless Schedule, if a transaction is going to perform read operation on a value, it has to wait until the transaction who is performing write on that value commits.

Cascadeless Schedule example

For example, lets say we have two transactions Ta and Tb. Tb is going to read the value X after the W(X) of Ta then Tb has to wait for the commit operation of transaction Ta before it reads the X.

Ta Tb ----- ----- R(X) W(X) W(X) commit R(X) W(X) commit

Recoverable Schedule

In Recoverable schedule, if a transaction is reading a value which has been updated by some other transaction then this transaction can commit only after the commit of other transaction which is updating value.

Recoverable Schedule example

Here Tb is performing read operation on X after the Ta has made changes in X using W(X) so Tb can only commit after the commit operation of Ta.

Ta Tb ----- ----- R(X) W(X) R(X) W(X) R(X) commit commit ❮ Previous Next ❯

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Types of Schedules in DBMS

A Computer Science portal for geeks. It contains well written, well thought and well explained computer science and programming articles, quizzes and practice/competitive programming/company interview Questions.

Types of Schedules in DBMS

Difficulty Level : Medium

Last Updated : 04 Feb, 2020

Schedule, as the name suggests, is a process of lining the transactions and executing them one by one. When there are multiple transactions that are running in a concurrent manner and the order of operation is needed to be set so that the operations do not overlap each other, Scheduling is brought into play and the transactions are timed accordingly. The basics of Transactions and Schedules is discussed in Concurrency Control (Introduction), and Transaction Isolation Levels in DBMS articles. Here we will discuss various types of schedules.

Serial Schedules:

Schedules in which the transactions are executed non-interleaved, i.e., a serial schedule is one in which no transaction starts until a running transaction has ended are called serial schedules.

Example: Consider the following schedule involving two transactions T1 and T2.

T1 T2 R(A) W(A) R(B) W(B) R(A) R(B)

where R(A) denotes that a read operation is performed on some data item ‘A’

This is a serial schedule since the transactions perform serially in the order T1 —> T2

Non-Serial Schedule:

This is a type of Scheduling where the operations of multiple transactions are interleaved. This might lead to a rise in the concurrency problem. The transactions are executed in a non-serial manner, keeping the end result correct and same as the serial schedule. Unlike the serial schedule where one transaction must wait for another to complete all its operation, in the non-serial schedule, the other transaction proceeds without waiting for the previous transaction to complete. This sort of schedule does not provide any benefit of the concurrent transaction. It can be of two types namely, Serializable and Non-Serializable Schedule.

The Non-Serial Schedule can be divided further into Serializable and Non-Serializable.

Serializable:

This is used to maintain the consistency of the database. It is mainly used in the Non-Serial scheduling to verify whether the scheduling will lead to any inconsistency or not. On the other hand, a serial schedule does not need the serializability because it follows a transaction only when the previous transaction is complete. The non-serial schedule is said to be in a serializable schedule only when it is equivalent to the serial schedules, for an n number of transactions. Since concurrency is allowed in this case thus, multiple transactions can execute concurrently. A serializable schedule helps in improving both resource utilization and CPU throughput. These are of two types:

Conflict Serializable:

A schedule is called conflict serializable if it can be transformed into a serial schedule by swapping non-conflicting operations. Two operations are said to be conflicting if all conditions satisfy:

They belong to different transactions

They operate on the same data item

At Least one of them is a write operation

View Serializable:

A Schedule is called view serializable if it is view equal to a serial schedule (no overlapping transactions). A conflict schedule is a view serializable but if the serializability contains blind writes, then the view serializable does not conflict serializable.

Non-Serializable:

The non-serializable schedule is divided into two types, Recoverable and Non-recoverable Schedule.

Recoverable Schedule:

Schedules in which transactions commit only after all transactions whose changes they read commit are called recoverable schedules. In other words, if some transaction Tj is reading value updated or written by some other transaction Ti, then the commit of Tj must occur after the commit of Ti.

Example – Consider the following schedule involving two transactions T1 and T2.

T1 T2 R(A) W(A) W(A) R(A) commit commit

This is a recoverable schedule since T1 commits before T2, that makes the value read by T2 correct.

There can be three types of recoverable schedule:

Cascading Schedule:

Also called Avoids cascading aborts/rollbacks (ACA). When there is a failure in one transaction and this leads to the rolling back or aborting other dependent transactions, then such scheduling is referred to as Cascading rollback or cascading abort. Example:

Cascadeless Schedule:

Schedules in which transactions read values only after all transactions whose changes they are going to read commit are called cascadeless schedules. Avoids that a single transaction abort leads to a series of transaction rollbacks. A strategy to prevent cascading aborts is to disallow a transaction from reading uncommitted changes from another transaction in the same schedule.

In other words, if some transaction Tj wants to read value updated or written by some other transaction Ti, then the commit of Tj must read it after the commit of Ti.

Example: Consider the following schedule involving two transactions T1 and T2.

T1 T2 R(A)

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