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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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Local $bSuccess = False

; 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.

$oFac = ObjCreate("Chilkat.FileAccess")

$bSuccess = $oFac.OpenForRead("qa_data/xml/transactions.xml")
If ($bSuccess = False) Then
    ConsoleWrite($oFac.LastErrorText & @CRLF)
    Exit
EndIf

$oXml = ObjCreate("Chilkat.Xml")
$oSb = ObjCreate("Chilkat.StringBuilder")
Local $bFirstIteration = True
Local $iRetval = 1
Local $iNumTransactions = 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.
Local $sBeginMarker = "<Transaction>"
Local $sEndMarker = "</Transaction>"

While $iRetval = 1
    $oSb.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.
    $iRetval = $oFac.ReadNextFragment($bFirstIteration,$sBeginMarker,$sEndMarker,"utf-8",$oSb)
    $bFirstIteration = False

    If ($iRetval = 1) Then
        $iNumTransactions = $iNumTransactions + 1
        $bSuccess = $oXml.LoadSb($oSb,True)
        ; Your application may now do what it needs with this particular XML fragment...
    EndIf

Wend

If ($iRetval < 0) Then
    ConsoleWrite($oFac.LastErrorText & @CRLF)
EndIf

ConsoleWrite("numTransactions: " & $iNumTransactions & @CRLF)