Schedule and its types

 Schedule: it refers to the sequence in which the set of concurrent/multiple transactions is executed.

- It is the sequence in which the operations (such as read, write, commit, abort) of multiple transcation executed 

- so it helps in data consistency and integrity

so for example, if we have T1, T2, T3 .... TN transcations then the possible schedule = n!

Incomplete schedule: An incomplete schedule is one where not all transactions have reached their final state, either commit or abort


T1: Read(A)

T1: Write(A)

T2: Read(B)\

T2: Write(B)

T2: commit

Here, T1 is  still in progress, as there is no commit for transaction T1


Complete schedule: A complete schedule is one where all transactions  in the schedule have either committed or aborted 

T1: Read(A)

T1: Write(A)

T1:commit

T2: Write(B)

T2: commit


Types of schedule:

1. serial schedule: transaction executed one after another. menas wait for T1 to commit/ abort then t2 start

Advantage:

    - It follows the ACID properties. transactions are isolated

    - it mantaine consitency

challenges

-    can be inefficient as there is poor throughput ( no of trans. completed per unit time)

-    since wait time is high, less no of transcation are completed


2.  Non-Serial/ Concurrent Schedule: where multiple transactions can execute simultaneously.

- We can say it as if there are two transcation T1 and T2, so T2 doesn't need T1 to commit, it can start at any point 

Advantage:

    - Better resource utilization

    - throughput(no of transcation completed per unit time) is high

challenge

- A consistency issue may arise because of non-serial execution. so need roboust concurrency control mechanism to ensure data consistency and  

-    We can use serializability and concurrency control mechanisms to ensure consistency

3. Conflict- serializable Schedule :  

4. View-serializable schedule : 

5. Recoverable Schedule: If a transaction reads data from another transaction, it should not commit until the transaction from which it read has committed. this help in maintaining the integrity of the database in case a transaction fails 

6. Irrecoverable Schedule: Allows a transaction to commit even if it has read data from another uncommitted transaction. This can lead to inconsistency and make it impossible to recover from certain failures 

7. Cascading schedule: When the failures or abort of one transaction cause a series of other transcation also to also abort 

8. Cascadeless Schedule: It ensures transactions only read committed data, such that the abort of one transaction does not lead to the other transaction that have alredy read its uncommitted changes 

9. Strict Schedule: It ensures transcation is not allowed to read or write a data item that another transaction has written until the first transaction has either committed or aborted. it prevents cascading aborts


 

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