(Ruby) Efficiently Process a Huge XML File

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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require 'chilkat'

# 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.

fac = Chilkat::CkFileAccess.new()

success = fac.OpenForRead("qa_data/xml/transactions.xml")
if (success == false)
    print fac.lastErrorText() + "\n";
    exit
end

xml = Chilkat::CkXml.new()
sb = Chilkat::CkStringBuilder.new()
firstIteration = true
retval = 1
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.
beginMarker = "<Transaction>"
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)
        numTransactions = numTransactions + 1
        success = xml.LoadSb(sb,true)
        # Your application may now do what it needs with this particular XML fragment...
    end

end

if (retval < 0)
    print fac.lastErrorText() + "\n";
end

print "numTransactions: " + numTransactions.to_s() + "\n";