Isolation

In database systems, particularly those that handle concurrent transactions, conflicts can arise due to the interference of multiple transactions accessing the same data. This can lead to various inconsistencies and anomalies, such as dirty reads, nonrepeatable reads, and phantom reads. These issues are significant because they can violate the ACID (Atomicity, Consistency, Isolation, Durability) properties that a database transaction should maintain.

Here is a description of each type of read anomaly:

Dirty Read: A dirty read occurs when a transaction reads data that has been written (updated) by a concurrent uncommitted transaction. Because the concurrent transaction has not yet committed, its updates are not finalized, and it could still roll back and change the data again. If it does, the data read by the first transaction will be "dirty," or incorrect. For example, if Transaction 1 updates a row, and Transaction 2 reads the updated row before Transaction 1 commits the changes, then Transaction 2 is said to have performed a dirty read.
Non-repeatable Read: A non-repeatable read occurs when a transaction reads the same row twice but gets different data each time. This happens when another transaction modifies and commits the row between the two reads. For example, if Transaction 1 reads a row, Transaction 2 updates or deletes that row and commits, and then Transaction 1 reads the same row again, it gets different data, causing a non-repeatable read.
Phantom Read: A phantom read occurs when a transaction executes a query twice, and the second result set includes rows that weren't visible in the first result set. This anomaly happens when another transaction inserts or deletes a row between the execution of the two queries. For example, if Transaction 1 retrieves a set of rows that satisfy some condition, Transaction 2 then inserts a new row that meets the same condition, and Transaction 1 re-executes the first query, it finds the newly inserted "phantom" row.

Various concurrency control techniques, like two-phase locking and multiversion concurrency control, are used in databases to avoid these conflicts and maintain the ACID properties. Also, different levels of isolation in transaction management (Read Uncommitted, Read Committed, Repeatable Read, Serializable) help deal with these anomalies. The level of isolation impacts the concurrency and performance, thus should be decided carefully based on the requirements.