Transaction Isolation Levels JDBC Transaction Your Questions on Java Example

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What are the JDBC Transaction isolation levels while using Database and explain different types of reads resulting from these isolation levels? To my understanding, Transaction comes into picture when there is a need to allow many clients to access system concurrently, without really bothering about data integrity and consistency, all or nothing behavior and durable against server crash, power failure, or something that is beyond controller or developer, system architect and or any stakeholder to this system. One question comes to my mind, if Transaction manages all these properties as mentioned above, then what are isolation levels and what for these are needed. In case of multiple transactions running in server container or within server transaction context/boundary, special care is to be taken on database side as Database is a shared resource, across multiple transactions.

So Isolation level is related to how transactions are isolated from data store/database. Before starting to discuss isolation levels, let us discuss possible facts about a system where different threads working on same or different table and rows within table. Suppose we are dealing with two threads, T1 and T2 and both are trying to work on a single row of a database table. If T1, reads this row, and T2 either changes the same row, adds some more rows, something similar to what T1 has already read or deleted the same row. Then in this case how T1 will know what has happened to the data that it has already fetched and probably would be working on. Many possible cases will be as follows: 1. T1 reads a row, T2 changes the same row, now if T1 reads the same row of table, it won't find the same data again. So is a kind of no same data or no repeatable data. 2. T1 reads the data, that T2 has inserted into table but not yet committed. Now T1 is working on this data, but somehow, data inserted by T2 got roll backed. This way T1 is working on the data that now doesn't existed in data store, isn't it dirty data avaliable with T1? 3. T1 reads some numbers of rows by some where clause in SQL statement, but in the mean time T2 inserted some more records, and the condition comes out to be same as that of T1 used to read initially. If again T1 reads data with same where clause, and found many more rows, those are something like phantom data to T1. So it can be phantom read. So we have three types of reads discussed here as follows:

Dirty read

Non-repeatable read

Phantom read Four types of isolation levels as follows:

TRANSACTION_READ_COMMITTED In this case, all transactions read committed data only, so by using this isolation level, dirty read can be stopped from occuring. As it cannot stop other transactions from changing to adding rows, so non repeatable and phantom reads can occur.

TRANSACTION_READ_UNCOMMITTED As it is not committed, so all three types of reads can occur.

TRANSACTION_REPEATABLE_READ In this isolation level, dirty and non repeatable reads cannot occur, but phantom read can occur.

TRANSACTION_SERIALIZABLE Transaction serializable type of Isoclation Level can stop all three types of reads from occurring, but at the expense of performance, as at a time only on transaction can have access to a table as one point of time. This makes all other transactions to wait, so this is not good performance oriented option.

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