Next-Key Lock is a lock mechanism of the MySQL InnoDB storage engine.Combined record locks and gap locks, aiming to achieve more efficient concurrency control, especially in avoiding phantom reading. The following will be detailed analysis from the underlying principles, implementation mechanisms and related source code.
1. The definition and function of Next-Key Lock
Next-Key Lock is mainly used to prevent the occurrence of phantom reading phenomena. Its function is to lock a specific index record and its gap before and after, to ensure that during a transaction, other transactions cannot insert records located at that gap.
Features：
- Lock specific index records.
- Lock the gap before index record.
- Applicable to scope queries to prevent other transactions from inserting new records within the scope of the query.
The locking rules include two "principles" and two "optimizations".
1. Principle 1: The basic unit of locking is next-key lock. Remember, next-key lock is the front opening and rear closing interval.
2. Principle 2: Only objects accessed during the search process will be locked.
3. Optimization 1: When equal value query on the index, when locking the unique index, the next-key lock degenerates into a row lock.
4. Optimization 2: Equivalent query on the index. When traversing to the right and the last value does not meet the equal value condition, the next-key lock degenerates into a gap lock.
2. The underlying principle
Phantom reading phenomenon：
- Phantom reading refers to the same query being executed twice in the same transaction, and different rows appear in the result set, usually because other transactions insert new records within the query interval.
- Next-Key Lock prevents this from happening by locking the record and its gap.
Locking process：
- When a transaction executes similar to
SELECT ... FOR UPDATEWhen querying, InnoDB will look for index records that match the conditions and add Next-Key Lock to these records. - If the query condition is a range, e.g.
WHERE id BETWEEN 1 AND 10, InnoDB will lock records with ids 1 to 10, and all gaps between id=1 and id=10.
Locking mechanism：
- In InnoDB, Next-Key Lock is actually a composite mechanism that locks the index record and the gaps before and after it.
- When performing an insertion operation, Next-Key Lock can effectively prevent the insertion of the lock gap, thereby ensuring data consistency.
3. Source code analysis
The implementation of Next-Key Lock is mainly concentrated in the source code of InnoDB. The following are some key parts analysis:
Locking operation：
existIn the file,row_lock()Functions are responsible for locking logic. This function checks the required lock type and performs corresponding locking operations based on the current transaction state.
if (is_insert) {
// Lock record lock_record(record);
// Lock gap lock_gap(previous_record, record);
}
Unlock operation：
At the end of the lock, the relevant unlocking logic is defined in the same file.row_unlock()Functions are used to release Next-Key Lock.
unlock_record(record); unlock_gap(previous_record, record);
Lock conflict handling：
InnoDB bysrv_lock()andsrv_unlock()Functions manage lock conflicts. These functions are responsible for detecting lock compatibility and ensuring that deadlocks or improper release of locks are not occurring.
MVCC Combination：
Next-Key Lock is combined with multi-version concurrency control (MVCC) to ensure that read operations are not locked by write operations. existIn , the implementation of version control ensures that a consistent snapshot can be seen when queries are made.
Optimization strategy：
InnoDB also implements some optimization strategies, such as skipping the locking of gaps in some cases to improve performance, especially when conflict is not prone to occur.
4. Summary
Next-Key Lock is a very important locking mechanism in InnoDB. It avoids the phenomenon of illusion reading by locking index records and its gaps and ensures the isolation and consistency of transactions. From the source code level, the implementation of Next-Key Lock involves the collaborative work of multiple files, including locking, unlocking, conflict handling and optimization strategies.
This mechanism can effectively improve the concurrency performance and data security of the database when dealing with high concurrency scenarios. Understanding its underlying principles and implementation mechanisms will help developers make more effective optimization and adjustments when designing database applications.
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