Locking in Databases

Locking in databases refers to the practice of securing specific tables or rows within a database to prevent interference from other transactions. When a particular table or row is locked, other transactions are unable to reference or update that data. This mechanism is essential in managing concurrent database transactions and maintaining the integrity of data, ensuring that the same data isn't modified by multiple transactions simultaneously.

Types of Locks

Shared Locks

A shared lock is a type of database lock that allows for concurrency in data reading while ensuring data integrity. Once a shared lock is applied on a table or a row, other transactions can still read (reference) the data, but they cannot modify (update) it. This mechanism ensures that while the data is being read, its state remains consistent and unaffected by other transactions.

Multitransaction Accessibility

A distinctive feature of shared locks is their ability to be acquired by multiple transactions simultaneously. This capability enables higher concurrency by allowing multiple transactions to read the same data at the same time, without risking data inconsistency due to concurrent updates.

Exclusive Locks

Exclusive locks represent a higher level of data protection in a transactional environment. When an exclusive lock is applied to a table or row, it restricts all other transactions from both reading (referencing) and writing (updating) the locked data. An exclusive lock ensures that the transaction holding the lock has exclusive access to the data for the duration of the lock, thus preventing any potential conflicts or inconsistencies.

Automatic Locking with Update, Delete, and Insert Operations

In many database management systems, certain operations such as updating, deleting, or inserting data automatically apply an exclusive lock to the affected tables or rows. This automatic application of exclusive locks ensures that the data being modified remains in a consistent state throughout the operation, providing reliable and predictable results.

Lock Levels

Row-Level Locking

Row-level locking is a strategy where individual rows within a database table are locked, allowing for detailed control over database access. In essence, it provides a mechanism to secure specific entries within a table, while leaving other entries open for reading or writing by other transactions. This level of granularity means that even while a row is locked, operations can still be conducted on other rows within the same table. The main advantage of row-level locking is that it minimizes the chances of contention between transactions, enabling a higher degree of concurrent database operations.

Table-Level Locking

Table-level locking, as the name suggests, involves the locking of an entire table within the database. During this type of lock, no transaction can access the locked table for either reading or writing operations. The key benefit of table-level locking is its simplicity and lower overhead, as the system only needs to keep track of a single lock for the entire table, rather than multiple locks for individual rows. However, this type of lock also increases the likelihood of contention, particularly in multi-user systems, as an entire table becomes off-limits during the duration of the lock. This type of lock is typically useful when large amounts of data in a table need to be updated or read at once.

Locking and Transactions

Transaction locking is a critical component in ensuring the consistency and reliability of database operations. When a transaction begins and executes operations such as updates, deletes, or insertions, the involved tables or records are locked. The lock remains in effect until the transaction is completed, either through commit or rollback.

The application of locks during a transaction ensures that no other transaction interferes with the data being manipulated, maintaining the atomicity and isolation properties of the transaction. This means that every transaction is treated as a single, indivisible unit of work, and its operations are isolated from all other transactions.

Impact on Table and Record Access

Locks implemented during a transaction can have a significant effect on the accessibility of a table or a record. While a lock is in place, the locked data cannot be accessed for writing by other transactions, and in the case of exclusive locks, for reading as well.

This transactional locking mechanism can lead to a temporary halt in the accessibility of certain data, preventing other transactions from executing conflicting operations. However, it is a necessary process to ensure data integrity and consistency in the database. The system has to balance between maintaining this consistency and optimizing for performance, preventing long-lasting locks when possible to minimize the impact on access and usability.

Deadlocks

A deadlock in databases is a state where two or more sessions lock each other's resources and neither can proceed because each is waiting for the other to release its lock. This can lead to a system hang, causing a delay in the processing of transactions.

Scenario and Consequences of Deadlocks

Consider a scenario where Session A has locked Table 1 and needs access to Table 2, which is locked by Session B. Meanwhile, Session B, while still holding its lock on Table 2, also needs access to Table 1. This circular dependency where each session is waiting for the other to release its lock results in a deadlock. In such situations, a deadlock resolution strategy needs to be applied, usually by a DBMS automatically detecting and resolving the deadlock, often by aborting one of the transactions.