Concurrency in XML

Concurrency occurs when two or more execution flows are able to run simultaneously. Concurrency control is a method used to ensure that database transactions are executed in a safe manner. ACID Properties • Atomicity refers to the ability of the DBMS to guarantee that either all of the tasks of a transaction are performed or none of them are. • Consistency refers to the database being in a legal state when the transaction begins and when it ends. • Isolation refers to the ability of the application to make operations in a transaction appear isolated from all other operations. • Durability refers to the guarantee that once the user has been notified of success, the transaction will persist, and not be undone Classic concurrency control mechanisms developed for relational, hierarchical, and object oriented databases are no longer adequate for semi-structured databases. Processing XML documents in multi-user database management environments requires: • a suitable storage model of XML data • support of typical XML document proccesing interfaces (DOM, SAX) • concurrency mechanisms specific to XML Node Locks While traversing or modifying an XML document, a transaction has to acquire a lock in an adequate mode for each node before accessing it. Because the nodes in an XML document are organized by a tree structure, the principles of multi-granularity locking schemes can be applied. NR An NR lock mode (node read) is requested for reading the context node. An NR lock has to be acquired for each node in the ancestor path. It only 1 locks the specified node, but not any descendant nodes. IX An IX lock mode (intention exclusive) indicates the intent to perform write operations somewhere in the subtree, but not on a direct-child node of the node being locked. LR An LR lock mode (level read) locks the context node together with its direct-child nodes for shared access. SR An SR lock mode (subtree read) is requested for the context node c as the root of subtree s to perform read operations on all nodes belonging to s. Hence, the entire subtree is granted for shared access. CX A CX lock mode (child exclusive) on context node c indicates the existence of an X lock on some direct-child node and prohibits inconsistent locking states by preventing LR and SR lock modes. U A U lock mode (update option) supports a read operation on context node c with the option to convert the mode for subsequent write access