System Architecture and XDP Interfaces

XTC database engine (XTC server) adheres to the widely used five-layer DBMS architectureIn Figure 1, we concentrate on the representation and mapping of XML documents. The file-services layer operates on the bit pattern stored on external, non-volatile storagedevices. In collaboration with the OS file system, the i/o managers store the physicaldata into extensible container files; their uniform block length is configurable to thecharacteristics of the XML documents to be stored. A buffer manager per container filehandles fixing and unfixing of pages in main memory and provides a replacement algorithmfor them which can be optimized to the anticipated reference locality inherent inthe respective XDP applications. Using pages as basic storage units, the record, index,and catalog managers form the access services. The record manager maintains in a setof pages the tree-connected nodes of XML documents as physically adjacent records.Each record is addressed by a unique life-time ID managed within a B-tree by the indexmanager. This is essential to allow for fine-grained concurrency control which requireslock acquisition on unique identifiable nodes. The catalog manager provides for the database metadata. The node manager implementing the navigational access layer transforms the records from their internal physical into an external representation, thereby managing the lock acquisition to isolate the concurrent transactions.The XML-services layer contains the XML manager responsible for declarative document access, e. g., evaluation of XPath queries or XSLT transformations.

Figure 1 XTC architecture overview

The agents of the interface layer make the functionality of the XML and node services
available to common internet browsers, ftp clients, and the XTC driver thereby
achieving declarative / set-oriented as well as navigational / node-oriented
interfaces. The XTCdriver linked to client-side applications provides for methods to
execute XPath-like queries and to manipulate documents via the SAX or DOM API. Each
API accesses the stored documents within a transaction to be started by the XTC
driver. Transactions can be processed in the well-known isolation levels
uncommitted, committed, repeatable, and serializable


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