While doing my High-Performance Java Persistence training, I came to realize that it’s worth explaining how a relational database works, as otherwise, it is very difficult to grasp many transaction-related concepts like atomicity, durability, and checkpoints.
In this post, I’m going to give a high-level explanation of how a relational database works internally while also hinting some database-specific implementation details.
Continue reading “How does a relational database work”
Relational database systems employ various Concurrency Control mechanisms to provide transactions with ACID property guarantees. While isolation levels are one way of choosing a given Concurrency Control mechanism, you can also use explicit locking whenever you want a finer-grained control to prevent data integrity issues.
As previously explained, there are two types of explicit locking mechanisms: pessimistic (physical) and optimistic (logical). In this post, I’m going to explain how explicit pessimistic locking interacts with non-query DML statements (e.g. insert, update, and delete).
Continue reading “How does database pessimistic locking interact with INSERT, UPDATE, and DELETE SQL statements”
Unlike SQL Server which, by default, relies on the 2PL (Two-Phase Locking) to implement the SQL standard isolation levels, Oracle, PostgreSQL, and MySQL InnoDB engine use MVCC (Multi-Version Concurrency Control).
However, providing a truly Serializable isolation level on top of MVCC is really difficult, and, in this post, I’ll demonstrate that it’s very difficult to prevent the Phantom Read anomaly without resorting to pessimistic locking.
Continue reading “A beginner’s guide to the Phantom Read anomaly, and how it differs between 2PL and MVCC”
As previously explained, every SQL statement must be executed in the context of a database transaction. For modifying statements (e.g.
DELETE), row-level locks must be taken to ensure recoverability and avoid the data anomalies.
Next, I’ll demonstrate what can happen when a database transaction is not properly ended.
Continue reading “MySQL metadata locking and database transaction ending”
In my article about ACID and database transactions, I introduced the three phenomena described by the SQL standard:
- dirty read
- non-repeatable read
- phantom read
While these are good to differentiate the four isolation levels (Read Uncommitted, Read Committed, Repeatable Read and Serializable), in reality, there are more phenomena to take into consideration as well. The 1995 paper (A Critique of ANSI SQL Isolation Levels) introduces the other phenomena that are omitted from the standard specification.
In my High-Performance Java Persistence book, I decided to insist on the Transaction chapter as it is very important for both data-access effectiveness and efficiency.
Continue reading “A beginner’s guide to read and write skew phenomena”