Xojo Plugin

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.

Xojo Plugin

Dim success As Boolean
success = 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.

Dim fac As New Chilkat.FileAccess

success = fac.OpenForRead("qa_data/xml/transactions.xml")
If (success = False) Then
    System.DebugLog(fac.LastErrorText)
    Return
End If

Dim xml As New Chilkat.Xml
Dim sb As New Chilkat.StringBuilder
Dim firstIteration As Boolean
firstIteration = True
Dim retval As Int32
retval = 1
Dim numTransactions As Int32
numTransactions = 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.
Dim beginMarker As String
beginMarker = "<Transaction>"
Dim endMarker As String
endMarker = "</Transaction>"

While retval = 1
    sb.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.
    retval = fac.ReadNextFragment(firstIteration,beginMarker,endMarker,"utf-8",sb)
    firstIteration = False

    If (retval = 1) Then
        numTransactions = numTransactions + 1
        success = xml.LoadSb(sb,True)
        // Your application may now do what it needs with this particular XML fragment...
    End If

Wend

If (retval < 0) Then
    System.DebugLog(fac.LastErrorText)
End If

System.DebugLog("numTransactions: " + Str(numTransactions))