Document databases handle concurrency by using various techniques that enable multiple users or applications to read and write data simultaneously without causing conflicts or inconsistencies. One common approach is optimistic concurrency control, where the database allows multiple transactions to proceed without locking the documents. When a transaction is ready to commit changes, the database checks if the document's version has changed since it was read. If it has, the transaction is rejected, and the developer can decide how to handle the situation, such as prompting the user to refresh or retry the operation. This method is efficient because it reduces waiting time and locks, allowing for higher throughput in scenarios where conflicts are rare.
Another method is the use of explicit locking, where a document can be locked during a write operation. This means that while one user is editing the document, others are prevented from making changes until the lock is released. While this ensures data integrity, it can lead to delays if multiple users need to access the same document at the same time. Some document databases provide configurable locking options to strike a balance between availability and consistency based on the application's needs. For example, MongoDB offers mechanisms for locking at different levels, such as collection-level locks or more granular document-level locks.
Finally, some document databases implement a system of versioning, where each document keeps track of its own version or timestamp. When a document is updated, the system records a new version rather than overwriting the existing one. This allows multiple versions to coexist and enables features like conflict resolution, where the application can determine how to handle different changes made by various users. For instance, a collaborative application might allow users to see different versions of a document and choose which changes to merge. Overall, the strategies document databases use for concurrency aim to optimize both performance and data integrity, allowing developers to create responsive applications with accurate data.