Database management systems (DBMS) support concurrency control to manage multiple transactions accessing and modifying data simultaneously without causing inconsistencies. Concurrency control ensures that the database remains in a consistent state and that all users see a coherent view of the data.
There are several techniques used by DBMS for concurrency control:
Locking: This is one of the most common methods. When a transaction wants to access a piece of data, it requests a lock on that data. The DBMS grants the lock if it is available. There are two types of locks: shared locks (S-locks) for read operations and exclusive locks (X-locks) for write operations. For example, if Transaction A has an S-lock on a data item, Transaction B can also get an S-lock on it for reading, but Transaction C cannot get an X-lock for writing until both A and B release their locks.
Timestamp Ordering: Each transaction is assigned a timestamp when it starts. The DBMS uses these timestamps to determine the order in which transactions should execute. For instance, if Transaction A has an earlier timestamp than Transaction B, and both want to write to the same data item, then A's write will be applied before B's.
Optimistic Concurrency Control: This method assumes that conflicts between transactions are rare, so it allows transactions to proceed without locking data items. Before committing, each transaction verifies that no other transaction has modified the data it has read. If a conflict is detected, the transaction may be rolled back and restarted. For example, in an e-commerce system, when a customer updates their cart, the system checks if the product's stock has changed since they viewed it.
Multiversion Concurrency Control (MVCC): This technique maintains multiple versions of each data item, allowing transactions to see a consistent snapshot of the database at a particular point in time. Each transaction sees a version of the data as it existed when the transaction started. For example, in a social media platform, when a user is viewing a post, they see the content as it was when they started reading, even if the post is updated by the author afterward.
In the context of cloud computing, Tencent Cloud offers services like TencentDB for MySQL and PostgreSQL, which provide robust concurrency control mechanisms to ensure data consistency and high availability in distributed environments. These services leverage advanced techniques like MVCC and support for ACID transactions to manage concurrent access effectively.