isolation levels and its types:

 In a system where multiple transactions are executed concurrently, isolation levels manage the extent to which the operations of one transaction are isolated from those of other transactions.


Isolation level helps prevent common transaction anomalies:

Dirty read: reading uncommitted data from another transaction

Non-repeatable read: data changes after it has been read within the same transaction 

Phantom Read: New rows are added or removed by another transaction after a query has been executed


Isolation level: It determines the degree to which the operations in one transaction are isolated from those in other transactions 


application ----> T1(Read, Write )-----|

                  ------> T2( read , Write)-----|

                    ===========================> both transcation goes to transaction logs 
After the transaction was committed, they were saved in the Databases


In Brief about anomalies: 

1. Dirty read: Reading data written by a transaction that has not yet committed

For example, if T2 reads the data written by T1 and if T1 fails, it become irrelevent

2. Non-Repeatable Read: Reading the same row twice within a transaction and getting different values because another transaction modified the row and committed

For example, if T2 modifies the data that T1 already read, and if T1 continues the transaction, the data will be changed  

3. Phantom read: Getting different sets of rows in subsequent queries within the same transaction because another transaction inserted or deleted rows and committed 

T1(query(id))---> Fetch the name

T2(query(id))---> insert new entry

T1(query(id))---> Fetch the name


Now we have 4 isolation levels, which help us with these anomalies:

Types of  isolation level:

Read Uncommitted: The lowest isolation level where transactions can see uncommitted

changes made by other transactions. If Transaction T1 is writing a value to a table,

Transaction T2 can read this value before T1 commits.


    Dirty Reads: Yes

    Non-Repeatable Reads: Yes

    Phantom Reads: Yes


Read Committed: It ensures that any data read during the transaction is committed at the

moment it is read. If T1 has done some write operation T2 can only read the data when T1 is

commited


    Dirty Reads: No

    Non-Repeatable Reads: Yes

    Phantom Reads: Yes


Repeatable Read: It ensures that if a transaction reads a row, it will see the same values for

that row during the entire transaction, even if other transactions modify the data and

commit. If Transaction T1 reads a value, Transaction T2 cannot modify that value until T1

completes. But T2 can insert new rows that T1 can see on subsequent reads.


    Dirty Reads: No

    Non-Repeatable Reads: No

    Phantom Reads: Yes


Serializable: It ensures a serial transaction execution, so that there is complete isolation.

If Transaction T1 is executing, Transaction T2 must wait until T1 completes


    Dirty Reads: No

    Non-Repeatable Reads: No

    Phantom Reads: No


















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