The InnoDB storage engine is a transactional engine that provides ACID (Atomicity, Consistency, Isolation, and Durability) compliance. It is the default engine used by MySQL since version 5.5.
#### File Structure:
InnoDB stores data and indexes in tablespace files (*.ibd) located in the data directory. The data dictionary and other metadata are stored in system tablespaces (ibdata files) also located in the data directory.
InnoDB uses a multi-versioning scheme to store the data in its tablespace files. This means that for each row modification, a new version is created keeping the old version intact. This is done to support read consistency and transaction isolation levels.
#### Transaction Handling:
InnoDB provides full ACID compliance by supporting transactions. In a transactional system, a set of SQL statements is considered a single logical unit of work. Either all the statements in the set are executed successfully, or the entire transaction is rolled back. This ensures data integrity and consistency.
InnoDB uses a technique called “change buffering” to improve transaction throughput. Change buffering groups together small changes made to a page and writes them in a batch to disk later. This reduces the number of disk write operations and makes the transactional process more efficient.
#### Locking Mechanisms:
InnoDB uses a combination of shared and exclusive locks to maintain data consistency and isolation. The lock types can be applied at different granularities, such as table-level, row-level, or even at the page-level.
InnoDB also provides a feature called “locking reads” that allows transactions to acquire shared locks on rows they read. This ensures that other transactions cannot modify the same rows concurrently.
InnoDB also has the capability to allow multiple transactions to access the same row concurrently, depending on the isolation level. For example, at the default isolation level of REPEATABLE READ, a single transaction can hold shared locks on various rows, while another transaction can read but not modify those same rows.
In summary, the InnoDB storage engine has a robust file structure that supports multi-versioning, a transaction handling mechanism that provides ACID compliance, and locking mechanisms that ensure data consistency and isolation.