1 / 16

Concurrency control in XML databases

Concurrency control in XML databases. Ali Abbasi. Concurrency in XML Bases. XML : standard format of data exchange on the internet XML docs is stored in databases Concurrency control is an important function of a DB Traditional CC protocols are not tailored to XML database systems

jola
Download Presentation

Concurrency control in XML databases

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Concurrency control in XML databases Ali Abbasi

  2. Concurrency in XML Bases • XML : standard format of data exchange on the internet • XML docs is stored in databases • Concurrency control is an important function of a DB • Traditional CC protocols are not tailored to XML database systems • New protocols designed based on different access methods : DOM, XPath, ...

  3. XPath • XPath models an XML document as a tree of nodes. • XPath expressions are used to indicate the requested nodes in the XML tree • An Xpath exp : /S1,j /S2,j /S3,j /.../Sm,j • Si,j : Axis::NodeTest[Predicate]

  4. Non-conflicting conditions • A transaction T is a sequence of pairs of operations Oj(x) and location path Lj • Operations are : pass-by, read, write, insert, delete • For any x,y, <Pi(x),Pj(y)>, <Pi(x),Rj(y)> and <Ri(x),Rj(y)>, • For any x,y, <Ri(x),Ij(y)>, <Wi(x),Ij(y)> • For any y, if there exists a location step S in location path L in Ti such that x is not a member of R(S) and RI(S), <Pi(x),Wj(y)>, <Pi(x),Ij(y)>, <Pi(x),Dj(y)>

  5. Conflicting conditions • <Ri(x),Wj(x)>, <Ri(x),Dj(x)> • <Wi(x),Wj(x)>, <Wi(x),Dj(x)> • <Ii(x),Ij(x)>, <Ii(x),Dj(x)>, <Di(x),Dj(x)> • For any x, if there exists a location step S in location path L such that x is a member of R(S) or RI(S), <Pi(x),Wj(x)>, <Pi(x),Ij(x)>, <Pi(x),Dj(x)>

  6. XLP • XLP is a lock-based protocol with five lock modes, denoted by P-, R-, W-, I- and D-locks • XLP is 2PL • Nodes in the M-set of Si,j (M(Si,j) and MI(Si,j)) are all locked by P-locks before performing the Node-Test and Predicate • Granularity Rules : • (1) Lock granularity of P-, R-, I-, or W-locks on a node is only the node itself. • (2) Lock granularity of D-locks on a node includes the whole subtree rooted at the node.

  7. XLP • Compatibility Rule. • A particular type of lock on location step Si can be granted as long as the compatibility matrix is respected. • Release Rules • R-, W-, I- or D-locks on Nd(Lj) can only be released in the shrinking phase of a transaction. • P-locks on nodes in the set are released only in the shrinking phase; • Other P-locks on are released after location step Si,j finishes

  8. Properties • XLP ensures conflict serializability. • XLP outperforms 2PL owing to the earlier release of P-locks and lower lock conflicts according to its lock compatibility matrix. • XLP fully supports most XPath’s axis operations • phantom problem may exist in some schedules under XLP

  9. LWMGL • A lock-based CC protocol • XML documents have a hierarchical structure • Two hierarchical locking protocol: TL, MGL • None of them a is an appropriate solution • LWMGL is a hybrid of MGL and TL • It’s goal is preventing phantoms and providing high concurrency

  10. pseudo-conflict: T1 retrieves all the /description elements T2 inserts price element Pseudo-conflict

  11. phantom T1 retrieves all the elements in /Books/Book T2 inserts Discount

  12. XIET data model • The element table schema E has a set of element records E = {indexing group, elements, rowID} • The data table schema D has a set of data records D = {rowID, TextString} • indexing group I = {G,S,D}

  13. LWMGL Protocol • Find the target element by using GSD indices • Acquire S_locks for reading each element where the target element traverses subelements • Acquire X_locks on the leaf element in order to update the value in the data table • Acquire IX_lock on the parent for inserting a node for prohibiting phantom

  14. LWMGL Protocol • Acquire IX locks from the root element to target element in order to delete an element in the element table so as to prohibit parent element deletion from other transactions during deletion. • Follow the order in locking from the parent element of the subelements to their child. In an unlocking fashion, follow it in the opposite direction, i.e., from the child element to the parent of the subelements.

  15. References • Kuen-Fang Jea, Shih-Ying Chen, A High concurrency XPath-based locking protocol for XML Databases, In: Elsevier Science ,2005. • Yonggoo Choi, Songchum Moon, Lightweight multigranularity locking for transaction management in XML database systems, In: Elsevier Science ,2005. • Grabs, T., Bo¨hm, K., Schek, H.-J., XMLTM: efficient transaction management for XML documents. In: Proceedings of the ACM International Conference on Information and Knowledge Management, CIKM 2002

  16. L :/child::a/descendant::d[position()=2] • M(S1,1)=R(S1,1)= {a} , MI(S1,1) and RI(S1,1) are empty • MI(S2,1)={b2, b3, c1, c2} , M(S2,1)={d1, d2, d3} • RI(S2,1)={b2, c1} , R(S2,1)={d2}.

More Related