Adding new child nodes to an existing XML file with Ruby & Nokogiri - ruby

Question

I have this server project in Ruby, and I would like to keep tracks of events and user sessions in a XML file. I'm totally new to this, and after days of research, I'm hitting a wall.
Here's my current sample code, assuming there's already a file named "test.xml" that contains a root node called
$ cat test.xml
<server></server>
and the code :
require 'nokogiri'
require 'securerandom'
logintime = Time.now
sessionid = SecureRandom.hex(10)
file = File.open("test.xml",'a+')
doc = Nokogiri::XML.parse file
session_node = Nokogiri::XML::Node.new("session",doc)
session_node['id'] = sessionid
logintime_node = Nokogiri::XML::Node.new("logintime",doc)
logintime_node.content = logintime
session_node << logintime_node
doc.root << session_node
file.print doc.to_xml
file.close
and here's the test.xml file after 4 runs
<server></server>
<?xml version="1.0"?>
<server>
<session id="5ef27ade2afaf5c2162f">
<logintime>2015-07-07 17:27:20 +0200</logintime>
</session>
</server>
<?xml version="1.0"?>
<server>
<session id="637595bd0857c8af1cc0">
<logintime>2015-07-07 17:27:36 +0200</logintime>
</session>
</server>
<?xml version="1.0"?>
<?xml version="1.0"?>
<server>
<session id="41e6082c4db7d1dc8692">
<logintime>2015-07-07 17:27:37 +0200</logintime>
</session>
</server>
<?xml version="1.0"?>
<?xml version="1.0"?>
<server>
<session id="1cad6c3d38d4fb96632b">
<logintime>2015-07-07 17:27:38 +0200</logintime>
</session>
</server>
<?xml version="1.0"?>
And the desired output should be something like this :
<?xml version="1.0"?>
<server>
<session id="5ef27ade2afaf5c2162f">
<logintime>2015-07-07 17:27:20 +0200</logintime>
</session>
<session id="637595bd0857c8af1cc0">
<logintime>2015-07-07 17:27:36 +0200</logintime>
</session>
<session id="41e6082c4db7d1dc8692">
<logintime>2015-07-07 17:27:37 +0200</logintime>
</session>
<session id="1cad6c3d38d4fb96632b">
<logintime>2015-07-07 17:27:38 +0200</logintime>
</session>
</server>
And I really don't know why should I do to obtain that result.
First, if there's no existing file containing the root node, the script run only once, then complains that there's already a root node when I try to run it a second time :
/System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/lib/ruby/gems/2.0.0/gems/nokogiri-1.5.6/lib/nokogiri/xml/document.rb:232:in `add_child': Document already has a root node (RuntimeError)
from /Users/xxx/nokogiri.rb:13:in `<top (required)>'
from -e:1:in `load'
from -e:1:in `<main>'
So... I'm kinda lost here. Any ideas ?

Answer 1

The problem is you're opening your file in append mode with File.open('test.xml', 'a+') and then writing the entire XML doc to it with file.print doc.to_xml. That's why you end up with the entire document written several times into the file.
If you read and write the file independently, the XML doc will replace the file the way you want. If you need to handle the file not existing yet, you can also check for it and initialize the data with your <server> root tag.
require 'nokogiri'
require 'securerandom'
logintime = Time.now
sessionid = SecureRandom.hex(10)
# Read or initialize the data
if File.exist?('test.xml')
data = File.read("test.xml")
else
data = '<server></server>'
end
doc = Nokogiri::XML.parse data
session_node = Nokogiri::XML::Node.new("session",doc)
session_node['id'] = sessionid
logintime_node = Nokogiri::XML::Node.new("logintime",doc)
logintime_node.content = logintime
session_node << logintime_node
doc.root << session_node
# Write the document to disk
File.open('test.xml', 'w') do |file|
file.print doc.to_xml
end
I wouldn't recommend logging sessions this way for long. At any significant user load, writing the file will become very expensive. Also, if you have multiple servers running, they'll all be clobbering the file out from under one another. When you get to that point, you should at least convert your storage to a database, or even better use something like an ELK Stack that's built for this.