What is a Dirty Read
By definition, a dirty read occurs when a transaction reads data from another transaction that has not yet been committed. The “dirty” moniker refers to the potential unstable state of the data, as it is possible that the changes made by the uncommitted transaction may be rolled back, hence making the read data invalid.
Consider a simple scenario to illustrate this: Imagine you are using a banking application where two transactions T1 and T2 are simultaneously operating on the same account balance. Let’s assume the initial account balance is $1000. Transaction T1 is trying to deduct $200 and T2 is reading the balance.
- T1 begins and deducts $200, making the balance $800. But it has not yet committed this change.
- Meanwhile, T2 starts and reads the balance as $800.
- Now, for some reason, T1 encounters an error and has to rollback the operation, returning the balance to the original $1000.
- However, T2 has already read the balance as $800 – a dirty read.
In this scenario, T2 made a decision based on inaccurate and uncommitted data, which may lead to misinformation and invalid operations.
Preventing Dirty Reads
The prevention of dirty reads primarily involves the judicious use of transaction isolation levels in SQL-based databases. These isolation levels determine how and when the changes made by one transaction are visible to others. The SQL standard defines four isolation levels: Read Uncommitted, Read Committed, Repeatable Read, and Serializable. To avoid dirty reads, it’s recommended to use a transaction isolation level of “Read Committed” or higher.
The “Read Committed” isolation level ensures that a transaction only sees data that has been committed before it begins. Any changes made by other transactions, which are not yet committed, will not be visible, thereby avoiding the possibility of a dirty read. This isolation level can be set in SQL-based databases using the SET TRANSACTION command.
For example, in a PostgreSQL database, you can set the isolation level to Read Committed as follows:
However, it is important to understand that increasing the isolation level can lead to other phenomena like non-repeatable reads or phantom reads and can potentially affect concurrency performance. Therefore, the choice of the appropriate isolation level requires careful consideration of the specific application requirements and trade-offs.