7-1-3 Isolation Levels Explained

Key Concepts

• Read Uncommitted
• Read Committed
• Repeatable Read
• Serializable
• Dirty Reads
• Non-Repeatable Reads
• Phantom Reads

Read Uncommitted

Read Uncommitted is the lowest isolation level. It allows transactions to read data that has been modified by other transactions but not yet committed. This can lead to Dirty Reads, where a transaction reads data that may be rolled back later.

Example: Transaction A reads a value that Transaction B is in the process of updating. If Transaction B later rolls back, Transaction A has read an inconsistent state.

Analogy: Think of it as reading a draft document. The document is still being edited, and you might see changes that are not final.

Read Committed

Read Committed is a higher isolation level that ensures transactions only read data that has been committed by other transactions. This prevents Dirty Reads but can still result in Non-Repeatable Reads.

Example: Transaction A reads a value, and then Transaction B updates and commits the same value. When Transaction A reads the value again, it gets a different result.

Analogy: Think of it as reading a published book. You only see the final version, but if the book is updated, you might see different content on different reads.

Repeatable Read

Repeatable Read is an isolation level that ensures a transaction reads the same data consistently, even if other transactions modify and commit changes. This prevents Non-Repeatable Reads but can still result in Phantom Reads.

Example: Transaction A reads a set of rows, and then Transaction B inserts new rows that match the query. When Transaction A reads the same set of rows again, it sees the new rows.

Analogy: Think of it as reading a fixed edition of a book. You always see the same content, but new editions might introduce new chapters.

Serializable

Serializable is the highest isolation level. It ensures that transactions execute in a completely isolated manner, as if they were the only transaction running on the system. This prevents Dirty Reads, Non-Repeatable Reads, and Phantom Reads.

Example: Transaction A reads a set of rows, and Transaction B is not allowed to insert or update any rows that would affect the result of Transaction A's query until Transaction A completes.

Analogy: Think of it as reading a book in a library where no one else can borrow or return books while you are reading. You have exclusive access to the book and its content.

Dirty Reads

Dirty Reads occur when a transaction reads data that has been modified by another uncommitted transaction. If the other transaction rolls back, the data read by the first transaction is invalid.

Example: Transaction A reads a value that Transaction B is updating. If Transaction B later rolls back, Transaction A has read an inconsistent state.

Analogy: Think of it as reading a draft document. The document is still being edited, and you might see changes that are not final.

Non-Repeatable Reads

Non-Repeatable Reads occur when a transaction reads the same data twice but gets different results because another transaction has updated the data between the reads.

Example: Transaction A reads a value, and then Transaction B updates and commits the same value. When Transaction A reads the value again, it gets a different result.

Analogy: Think of it as reading a published book. You only see the final version, but if the book is updated, you might see different content on different reads.

Phantom Reads

Phantom Reads occur when a transaction reads a set of rows twice but finds new rows inserted by another transaction between the reads.

Example: Transaction A reads a set of rows, and then Transaction B inserts new rows that match the query. When Transaction A reads the same set of rows again, it sees the new rows.

Analogy: Think of it as reading a fixed edition of a book. You always see the same content, but new editions might introduce new chapters.