Lianja

Efficiently Process a Huge XML File

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Demonstrates a technique for processing a huge XML file (can be any size, even many gigabytes).

Note: This example requires Chilkat v9.5.0.80 or greater.

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llSuccess = .F.

// This example shows a way to efficiently process a gigantic XML file -- one that may be too large
// to fit in memory.  
// 
// Two types of XML parsers exist: DOM parsers and SAX parsers.

// A DOM parser is a Document Object Model parser, where the entire XML is loaded into memory
// and the application has the luxury of interacting with the XML in a convenient, random-access
// way.  The Chilkat Xml class is a DOM parser.  Because the entire XML is loaded into memory,
// huge XML files (on the order of gigabytes) are usually not loadable for memory constraints.

// A SAX parser is such that the XML file is parsed as an input stream.  No DOM exists.  
// Using a SAX parser is generally less palatable than using a DOM parser, for many reasons.
// 
// The technique described here is a hybrid.  It streams the XML file as unstructured text
// to extract fragments that are individually treated as separate XML documents loaded into
// the Chilkat Xml parser.
// 
// For example, imagine your XML file is several GBs in size, but has a relatively simple structure, such as:
// 
// <Transactions>
//     <Transaction id="1">
//          ...
//     </Transaction>
//     <Transaction id="2">
//          ...
//     </Transaction>
//     <Transaction id="3">
//          ...
//     </Transaction>
// ...
// </Transactions>

// In the following code, each <Transaction ...> ... </Transaction>
// is extracted and loaded separately into an Xml object, where it can be manipulated
// independently.  The entire XML file is never entirely loaded into memory.

loFac = createobject("CkFileAccess")

llSuccess = loFac.OpenForRead("qa_data/xml/transactions.xml")
if (llSuccess = .F.) then
    ? loFac.LastErrorText
    release loFac
    return
endif

loXml = createobject("CkXml")
loSb = createobject("CkStringBuilder")
llFirstIteration = .T.
lnRetval = 1
lnNumTransactions = 0

// The begin marker is "XML tag aware".  If the begin marker begins with "<"
// and ends with ">", then it is assumed to be an XML tag and it will also match
// substrings where the ">" can be a whitespace char.
lcBeginMarker = "<Transaction>"
lcEndMarker = "</Transaction>"

do while lnRetval = 1
    loSb.Clear()
    // The retval can have the following values:
    // 0: No more fragments exist.
    // 1: Captured the next fragment.  The text from beginMarker to endMarker, including the markers, are returned in sb.
    // -1: Error.
    lnRetval = loFac.ReadNextFragment(llFirstIteration,lcBeginMarker,lcEndMarker,"utf-8",loSb)
    llFirstIteration = .F.

    if (lnRetval = 1) then
        lnNumTransactions = lnNumTransactions + 1
        llSuccess = loXml.LoadSb(loSb,.T.)
        // Your application may now do what it needs with this particular XML fragment...
    endif

enddo

if (lnRetval < 0) then
    ? loFac.LastErrorText
endif

? "numTransactions: " + str(lnNumTransactions)


release loFac
release loXml
release loSb