Concurrency control in a distributed database refers to the methods used to manage simultaneous access to data by multiple transactions across a network of database servers, ensuring data integrity and consistency. As stated in Distributed Concurrency Control on SpringerLink, distributed concurrency control provides concepts and technologies to synchronize distributed transactions in a way that their interleaved execution does not violate the ACID properties.
Understanding the Need
In a distributed database environment, data can be fragmented or replicated across multiple interconnected servers. Distributed transactions involve operations that span across these various servers. When multiple such transactions execute concurrently, their operations can become interleaved, leading to potential issues like:
- Lost Updates: One transaction's changes are overwritten by another without being considered.
- Dirty Reads: A transaction reads data that has been written by another transaction that has not yet committed (and might later abort).
- Non-Repeatable Reads: A transaction reads the same data twice and gets different values because another transaction modified it between the reads.
- Phantom Reads: A transaction re-executes a query and gets a different number of rows because another transaction inserted or deleted rows.
These anomalies can violate the fundamental ACID properties (Atomicity, Consistency, Isolation, Durability) essential for reliable database systems.
The Role of Distributed Concurrency Control
The primary goal of distributed concurrency control is to ensure that despite transactions executing concurrently across different servers, the final state of the database is the same as if the transactions had executed serially (one after another). This is primarily achieved by enforcing the Isolation property of ACID.
It involves coordinating operations across the participating data servers to maintain a globally consistent view of the data. This coordination is significantly more complex than in a centralized database due to network delays, potential site failures, and the need for communication between different nodes.
Concepts and Technologies
Implementing distributed concurrency control requires specific concepts and technologies designed for the distributed environment. These include extensions of traditional concurrency control techniques adapted for the distributed setting, such as:
- Distributed Locking (e.g., Two-Phase Locking adapted for distributed systems)
- Distributed Timestamping
- Optimistic Concurrency Control methods
These mechanisms synchronize the actions of distributed transactions, which are executed in a distributed database environment, where a set of connected data servers host related data. By coordinating operations across these servers, distributed concurrency control prevents the anomalies mentioned earlier, preserving the integrity and consistency of the data in the face of concurrent access.
