What types of data inconsistencies may occur during concurrent operations?
Data inconsistencies caused by concurrent operations include three categories: lost modifications, non-repeatable reads, and reading "dirty" data. The method and technology to avoid inconsistency is concurrency control. The most commonly used technology is blocking technology; other technologies can also be used. For example, in distributed database systems, the timestamp method can be used for concurrency control.
Data inconsistencies caused by concurrent operations include three categories: lost modifications, non-repeatable reads, and reading "dirty" data.
1. Lost Update
Two transactions T1 and T2 read the same data and modify it, and the result submitted by T2 is destroyed. It overwrites (overwrites) the results submitted by T1, causing the modifications of T1 to be lost.
2. Non-Repeatable Read
Non-Repeatable Read means that after transaction T1 reads data, transaction T2 performs an update operation, making T1 unable to Reproduce the results of the previous read.
3. Read "dirty" data (Dirty Read)
Reading "dirty" data means that transaction T1 modifies a certain data and writes it Back to the disk, after transaction T2 reads the same data, T1 is revoked for some reason. At this time, the modified data of T1 is restored to its original value. The data read by T2 is inconsistent with the data in the database, so the data read by T2 It is "dirty" data, that is, incorrect data.
What methods can be used to avoid various inconsistencies?
The method and technology to avoid inconsistency is concurrency control. The most commonly used technology is blocking technology; other technologies can also be used. For example, in distributed database systems, the timestamp method can be used for concurrency control. .
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