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What is Non-repeatable Read?
Non-repeatable read is a phenomenon that can occur in a database when a transaction reads the same data multiple times, but gets different results each time. This inconsistency in data is caused by concurrent transactions modifying the data in between the reads, leading to non-repeatable results.
In order to understand non-repeatable read, it is important to have a grasp of basic database and transaction concepts.
Basic Database Concepts
A database is a structured collection of data that is organized and accessible in a systematic way. It serves as a central repository for storing and managing data, enabling efficient data retrieval and manipulation.
In a database, data is typically organized into tables, where each table consists of rows and columns. Data is stored in the form of records, and each record represents a specific entity or object. Relationships between different tables can be established using keys, allowing for efficient data retrieval and analysis.
Transaction Concepts
A transaction represents a logical unit of work that is performed on a database. It consists of one or more database operations, such as reading, writing, or modifying data. The key characteristics of a transaction are commonly referred to as ACID properties:
1. Atomicity: A transaction is atomic, meaning that it can be treated as a single, indivisible unit of work. It either completes successfully or is rolled back, ensuring data integrity.
2. Consistency: A transaction brings the database from one consistent state to another consistent state. It follows certain predefined rules and constraints to ensure data validity and integrity.
3. Isolation: Each transaction is typically executed in isolation from other transactions. This ensures that the intermediate states of a transaction are not visible to other concurrent transactions until the transaction is committed.
4. Durability: Once a transaction is committed, the changes made by the transaction are permanent and will survive any subsequent failures, ensuring data durability.
Non-repeatable Read Explained
Now that we have a basic understanding of databases and transactions, let’s delve into non-repeatable read.
Imagine a scenario where Transaction A reads a certain record from a database. However, before it can make any changes, Transaction B modifies that same record. Now, when Transaction A reads the record again, it gets a different result compared to the initial read. This is the essence of the non-repeatable read problem.
Non-repeatable read occurs due to the lack of isolation between concurrent transactions. In other words, when multiple transactions are accessing and modifying the same data simultaneously, inconsistencies can arise. It is important to note that non-repeatable read is not a problem if the data remains unchanged between multiple reads.
To mitigate the non-repeatable read phenomenon, databases provide different isolation levels. Isolation levels determine the degree to which transactions are isolated from each other. Higher isolation levels, such as serializable, provide stronger isolation guarantees, but at the expense of concurrency performance. It is crucial to select an appropriate isolation level based on the specific requirements of the application.
In conclusion, non-repeatable read is a term used to describe the inconsistency in data that can occur when a transaction reads the same data multiple times and gets different results each time. Understanding this phenomenon is essential for developing robust and reliable database systems.
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