Transaction Isolation Levels With PostgreSQL as an example

Summary

In this video, the concept of transaction isolation levels in PostgreSQL (version 13) is explained in detail, with practical examples using the pgcli utility. The video covers the importance of isolation in database transactions, and highlights four isolation levels: Read Uncommitted, Read Committed, Repeatable Read, and Serializable. The potential phenomena like dirty reads, lost updates, non-repeatable reads, and serialization anomalies are discussed, with focus on how higher isolation levels prevent such issues. It emphasizes the trade-off between performance and concurrency, and provides a clear understanding of when to use explicit locks or higher isolation levels in development.

Takeaways

• 😀 ACID stands for Atomicity, Consistency, Isolation, and Durability, with 'Isolation' being crucial in determining how concurrent transactions interact in a database.
• 😀 Transaction isolation levels in PostgreSQL define the visibility and interaction of concurrent transactions, ensuring that one transaction doesn’t interfere with another.
• 😀 The four classic isolation levels in PostgreSQL (version 13) are: Read Uncommitted, Read Committed, Repeatable Read, and Serializable.
• 😀 PostgreSQL prevents 'Dirty Reads' at all isolation levels, ensuring transactions can’t read uncommitted data from others.
• 😀 A 'Lost Update' occurs when one transaction's update is overwritten by another, without accounting for the changes made in between.
• 😀 'Non-repeatable Reads' happen when data read by one transaction is modified by another, leading to inconsistent results if the transaction re-queries.
• 😀 'Phantoms' are new rows added or removed by another transaction during the execution of a query, causing an inconsistent result when the query is re-run.
• 😀 'Serialization Anomaly' occurs when the final state of a set of transactions is inconsistent with any sequential order of their execution.
• 😀 PostgreSQL's 'Repeatable Read' isolation level prevents non-repeatable reads, lost updates, and phantoms, but still allows some concurrency issues.
• 😀 The 'Serializable' isolation level in PostgreSQL provides the highest level of transaction isolation, preventing serialization anomalies, but may result in transaction errors that require code retries.

Q & A

What is the ACID acronym in database transactions, and how does it relate to isolation?

ACID stands for Atomicity, Consistency, Isolation, and Durability. Isolation is one of the key properties of transactions, ensuring that concurrent transactions do not interfere with each other in ways that would lead to data inconsistencies or anomalies.

What is a dirty read, and does PostgreSQL allow it?

A dirty read occurs when a transaction reads uncommitted changes made by another transaction. In PostgreSQL, dirty reads are not allowed at any isolation level, ensuring that only committed data is read by a transaction.

What is a lost update phenomenon, and how does it occur in PostgreSQL?

A lost update happens when two transactions read and modify the same data concurrently, but only the changes from the last transaction are preserved. PostgreSQL prevents lost updates at the Repeatable Read and Serializable isolation levels by locking the data or throwing an error.

What is a non-repeatable read, and how does PostgreSQL handle it?

A non-repeatable read occurs when a transaction reads the same data multiple times, but the data changes between reads due to another transaction. In PostgreSQL, the Repeatable Read isolation level prevents non-repeatable reads by ensuring data consistency throughout the transaction.

What are phantoms, and how do they impact transactions?

Phantoms are rows that appear or disappear between different reads of a dataset within a transaction due to changes made by other transactions. PostgreSQL's Repeatable Read isolation level prevents phantoms by locking the set of rows being read.

Can you explain the concept of a serialization anomaly in database transactions?

A serialization anomaly occurs when the final result of a set of transactions is inconsistent with any possible sequence of executing those transactions individually. PostgreSQL’s Serializable isolation level prevents serialization anomalies by ensuring a strict order of transactions.

What are the four transaction isolation levels supported by PostgreSQL, and how do they differ?

The four isolation levels in PostgreSQL are: Read Uncommitted, Read Committed (default), Repeatable Read, and Serializable. These levels differ in how they handle concurrency and phenomena like dirty reads, non-repeatable reads, lost updates, and phantoms, with higher isolation levels providing more strict guarantees but potentially affecting performance.

What happens when a transaction in Repeatable Read tries to update a record modified by another transaction?

In the Repeatable Read isolation level, if a transaction tries to update a record that has already been modified by another transaction, it will receive an error. This prevents lost updates and ensures consistency by enforcing data integrity across transactions.

What is the behavior of the Serializable isolation level in PostgreSQL when two transactions try to commit with conflicting changes?

At the Serializable isolation level, if two transactions try to commit with conflicting changes, one of them will fail. This prevents serialization anomalies by ensuring that the transactions are executed in a serial order, ensuring no inconsistencies arise.

Why is it important to consider transaction isolation levels when developing backend software?

Transaction isolation levels are crucial when developing backend software because they determine how concurrent transactions interact. Understanding and choosing the right isolation level helps prevent issues such as data corruption, inconsistencies, and logical errors in business processes.