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Using of XML for object store

Using of XML for object store. S. Linev, GSI. Content. XML and existing packages Concept of XML I/O for ROOT Possible implementations Problems and questions Conclusion. eXtensible Markup Language (XML). Tree like structure (not ROOT tree) of text tags Each tag opened should be closed

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Using of XML for object store

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  1. Using of XML for object store S. Linev, GSI

  2. Content • XML and existing packages • Concept of XML I/O for ROOT • Possible implementations • Problems and questions • Conclusion Using of XML for object store. S.Linev

  3. eXtensible Markup Language (XML) • Tree like structure (not ROOT tree) of text tags • Each tag opened should be closed • Tag can include other tags, contain text, has attributes • In addition: DTD, XSLT, XML schema, namespaces, … <?xml version="1.0"?> <Example> <item1>item text</item1> <item2 id="001"> <subitem>subitem text</subitem> </item2> <item3 ref="001"/> </Example> Example values: item1 “item text“ item2 id="001" subitem “subitem text“ item3 ref="001" Using of XML for object store. S.Linev

  4. Document Type Definition (DTD) • Used to validate structure of XML document • Defines legal building blocks of an XML document like • valid elements names • list of element subitems • list of element attributes • allowed values for attributes <?xml version="1.0"?> <!DOCTYPE Example SYSTEM “def.dtd”> <Example> <item1>item text</item1> <item2 id="001"> <subitem>subitem text</subitem> </item2> <item3 ref="001"/> </Example> <!ELEMENT Example (item1,item2,item3)> <!ELEMENT item1 (#PCDATA)> <!ELEMENT item2 (subitem)> <!ATTLIST item2 id ID #REQUIRED> <!ELEMENT subitem (#PCDATA)> <!ELEMENT item3 (#PCDATA)> <!ATTLIST item3 ref IDREF #REQUIRED> Using of XML for object store. S.Linev

  5. XML packages C/C++ based XML packages: • libxml (Gnome) http://xmlsoft.org • Xerces-C++ (Apache) http://xml.apache.org/xerces-c/ • expat (Mozilla) http://expat.sourceforge.net Benchmarks of XML packages (less is better) http://xmlbench.sourceforge.net Using of XML for object store. S.Linev

  6. Usage of libxml2 library Example of code to create XML file: xmlDocPtr fDoc = xmlNewDoc(0); xmlNodePtr fNode = xmlNewDocNode(fDoc, 0, (const xmlChar*) "Example", 0); xmlDocSetRootElement(fDoc, fNode); xmlNewTextChild(fNode, 0, (const xmlChar*) "item1",(const xmlChar*) "item text"); xmlNodePtr sub2 = xmlAddChild(fNode, xmlNewNode(0, (const xmlChar*) "item2")); xmlNewTextChild(sub2, 0, (const xmlChar*) "subitem", (const xmlChar*) "subitem text"); xmlNewProp(sub2, (const xmlChar*) "id", (const xmlChar*) "001"); xmlNodePtr sub3 = xmlAddChild(fNode, xmlNewNode(0, (const xmlChar*) "item3")); xmlNewProp(sub3, (const xmlChar*) "ref", (const xmlChar*) "001"); xmlSaveFormatFile("Example.xml", fDoc, 1); xmlFreeDoc(fDoc); Using of XML for object store. S.Linev

  7. XML and ROOT • XML as metadata storage place: configuration, parameters and geometry objects • XML files can be viewed and edited (with some restriction) with standard XML tools • Data exchange between different packages But currently: • There is no XML support in ROOT (yet) • Each new class requires its own XML streamer Using of XML for object store. S.Linev

  8. Motivation • ROOT has all class information in TStreamerInfo class with methods to serialize/deserialize objects • Why not implement similar mechanism for XML, not only for binary ROOT format? • Aim – introduce XML I/O in ROOT, where user should not write I/O code himself Using of XML for object store. S.Linev

  9. <TXmlExample ref="id0"> <fValue v="10"/> <fArray> <Double v="1.0"/> <Double v="10.0"/> <Double v="5.0"/> <Double v="7.0"/> <Double v="2.0"/> </fArray> <fStr TString="Hello"/> <fSelfPtr ptr="id0"/> </TXmlExample> array Object representation in XML object id class name class TXmlExample { public: Int_t fValue; Double_t fArray[5]; TString fStr; TXmlExample* fSelfPtr; ClassDef(TXmlExample, 1); }; basic type string (special case) pointer Using of XML for object store. S.Linev

  10. First implementation • New class with two functions similar to TStreamerInfo::WriteBuffer() and TStreamerInfo::ReadBuffer()were implemented to serialize/deserialize objects to/from XML structures • Libxml2 library was used • Requires no any ROOT modifications Using of XML for object store. S.Linev

  11. Problems • Only relatively “simple” objects can be stored • Custom streamers are not supported • As a result, ROOT classes like histograms (TH1), containers (TObjArray) and many other can not be supported Using of XML for object store. S.Linev

  12. TBuffer class modification • Make six methods of TBuffer virtual: void WriteObject(const void *actualObjStart, TClass *actualClass); void* ReadObjectAny(const TClass* cast); Int_t CheckByteCount(UInt_t startpos, UInt_t bcnt, const TClass *clss); void SetByteCount(UInt_t cntpos, Bool_t packInVersion = kFALSE); Version_t ReadVersion(UInt_t *start = 0, UInt_t *bcnt = 0); UInt_t WriteVersion(const TClass *cl, Bool_t useBcnt = kFALSE); • Redefine these methods in new TXmlBuffer class to perform XML specific actions • To support “TFile-like” key organization, new TXmlFile and TXmlKey classes have been created Using of XML for object store. S.Linev

  13. TObjArray arr; arr.Add(new TNamed("name1", "title1")); arr.Add(new TNamed("name2", "title2")); arr.Add(new TNamed("name3", "title3")); TXmlFile file("test.xml","1xxox"); file.Write(&arr, "array"); XmlKey with name and setup info TObjArray tag with version Part of TObjArray streamer (writing): … fName.Streamer(b); nobjects = GetAbsLast()+1; b << nobjects; b << fLowerBound; … Now only version, later full class info Example with TObjArray <?xml version="1.0"?> <root> <XmlKey name="array" setup="1xxox"> <TObjArray version="3"> <XmlBlock size="9"> 0000 00 00 0300 00 00 00 </XmlBlock> <XmlObject> <TNamed> <fName TString="name1"/> <fTitle TString="title1"/> </TNamed> </XmlObject> <XmlObject> <TNamed> <fName TString="name2"/> <fTitle TString="title2"/> </TNamed> </XmlObject> <XmlObject> <TNamed> <fName TString="name3"/> <fTitle TString="title3"/> </TNamed> </XmlObject> </TObjArray> <XmlClasses> <TNamed version="1"/> </XmlClasses> </XmlKey> </root> <?xml version="1.0"?> <root> <XmlKey name="array" setup="1xxox"> <TObjArray version="3"> <UChar>0</UChar> <Int>3</Int> <Int>0</Int> <XmlObject> <TNamed> <fName TString="name1"/> <fTitle TString="title1"/> </TNamed> </XmlObject> <XmlObject> <TNamed> <fName TString="name2"/> <fTitle TString="title2"/> </TNamed> </XmlObject> <XmlObject> <TNamed> <fName TString="name3"/> <fTitle TString="title3"/> </TNamed> </XmlObject> </TObjArray> <XmlClasses> <TNamed version="1"/> </XmlClasses> </XmlKey> </root> Using of XML for object store. S.Linev

  14. Consequence of TBuffer modification • Most of ROOT classes can be stored • Users classes with custom streamers can be supported • Works, if reading and writing parts of custom streamer have similar sequence of I/O actions (normal situation) • Some classes like TTree & TClonesArray are not tested and may be not required to be stored in XML format • At worse case 10% lost of I/O performance Still not fully acceptable because: • this is just “hacking” of ROOT code • TXmlFile and TXmlKey repeats a lot of functionality of similar TFile and TKey classes Using of XML for object store. S.Linev

  15. Further investigations • Producing of DTD files for validation purposes • Using of XML namespaces to avoid names intersection • Extension of TFile and TKey logic on XML files (via abstract interfaces) • C++ code generator for XML I/O to access ROOT objects outside a ROOT environment • Support of different XML packages Using of XML for object store. S.Linev

  16. Conclusion • There is no general XML I/O in ROOT • Very limited solution possible without ROOT changing • With slight TBuffer modifications acceptable XML support in ROOT is possible • Further investigations required Using of XML for object store. S.Linev

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