Multi-Version Concurrency Control (MVCC) is a technique used to allow multiple concurrent transactions to read and write data in a database without interfering with each other. This approach is used in many databases, including MySQL.
In MVCC, each transaction sees a snapshot of the database at the time of the query’s start, rather than the current state of the database. This means that each transaction sees a consistent view of the database, even when other transactions are modifying the same data.
MVCC works by keeping multiple versions of each row in the database. Each version of a row is marked with a timestamp, and when a transaction reads a row, it sees the version with the highest timestamp that is older than the transaction’s start time. If a transaction modifies a row, it creates a new version of the row with a new timestamp.
The different transaction isolation levels in MySQL determine the degree to which one transaction can see changes made by other transactions. The four standard transaction isolation levels in MySQL are:
1. READ UNCOMMITTED: This level allows transactions to read data that has been modified by other transactions but not yet committed. This level can result in dirty reads, which means that a transaction can read data that ends up being rolled back by another transaction, causing the first transaction to read invalid data.
2. READ COMMITTED: This level allows transactions to read only data that has been committed by other transactions. This level avoids dirty reads but can produce non-repeatable reads, which means that the same query may return different results at different times because the data may have been modified by other transactions.
3. REPEATABLE READ: This level ensures that a transaction sees the same data throughout its execution. This means that the transaction will not see changes made by other transactions that occur after the transaction starts. This level avoids both dirty reads and non-repeatable reads.
4. SERIALIZABLE: This level provides the highest degree of isolation and ensures that transactions are executed in a serializable order. This means that even though multiple transactions may attempt to modify the same data simultaneously, the results will be the same as if they were executed in a serial order. This level avoids all consistency problems but can be slower because transactions must wait for each other to complete.
In summary, MVCC allows concurrent transactions to operate on the same data without interfering with each other by keeping multiple versions of each row in the database. The different transaction isolation levels in MySQL determine the degree to which one transaction can see changes made by other transactions. Each level has different trade-offs in terms of consistency and performance.