Building ordered JSON in perl - json

Question

I'm having a bit of a fiddle with JSON and D3, trying to represent some disk usage information in a 'bubble' style.
Based on this initially: http://bl.ocks.org/mbostock/4063269
The JSON is pretty simple - hierarchical data looking a bit like this:
http://bl.ocks.org/mbostock/raw/4063530/flare.json
{
"name": "flare",
"children": [
{
"name": "analytics",
"children": [
{
"name": "cluster",
"children": [
{"name": "AgglomerativeCluster", "size": 3938},
{"name": "CommunityStructure", "size": 3812},
{"name": "HierarchicalCluster", "size": 6714},
{"name": "MergeEdge", "size": 743}
]
}
]
}
]
}
Now, what I'm trying to do is take a quotas report from a NetApp - in XML form.
I have multiple 'per server' XML files looking approximately like:
<quotas>
<quota>
<quota-target>/vol/vol1/qtree1</quota-target>
<volume>vol1</volume>
<disk-used>554444</disk-used>
<disk-limit>2000000</disk-limit>
</quota>
<quota>
<quota-target>/vol/vol1/qtree2</quota-target>
<volume>vol1</volume>
<disk-used>1235655</disk-used>
<disk-limit>2000000</disk-limit>
</quota>
<quota>
<quota-target>/vol/vol2/qtree1</quota-target>
<volume>vol2</volume>
<disk-used>987664</disk-used>
<disk-limit>2000000</disk-limit>
</quota>
</quotas>
What I'm trying to do is assemble some JSON for use with D3 that's hierarchical:
site
server
volume
quota-target
disk-used
I'm doing ok with a foreach loop:
#!/usr/bin/env perl
use strict;
use warnings;
use XML::Twig;
use JSON;
my %sites = (
'site1' => [qw ( servera serverb )],
'site2' => [qw ( s2serverc s2serverd)],
);
my $data;
$data->{'name'} = "quotas";
foreach my $sitename ( keys %sites ) {
my $site = { 'name' => $sitename };
push( #{ $data->{'children'} }, $site );
foreach my $server ( #{ $sites{$sitename} } ) {
my $server = { 'name' => $server };
push( #{ $site->{'children'} }, $server );
$twig->parsefile("$server.quotas.xml");
foreach my $quota ( $twig->get_xpath('//quota') ) {
push(
#{ $server->{'children'} },
{ 'name' => $quota->first_child_text('quota-target'),
'size' => $quota->first_child_text('disk-used')
}
)
}
}
}
open( my $output, ">", "quotas.json" ) or die $!;
print {$output} to_json( $data, { 'pretty' => 1 } );
close($output);
This is broadly working, and producing me pretty pictures.
However I'm having two problems:
Ordering of the JSON changes each run, because I'm using a hash. Whilst not a show stopper - is there a way I can enforce an order in the JSON output? (Not necessarily just 'sorted' alphabetically)
Similarly - I'm looking at how to insert a 'volume' level node which isn't currently present, as I'm creating new anonymous hashes to insert into children at each layer of the foreach loop. This feels a bit clunky, but what I'm thinking is:
Extract a list of volumes with get_xpath('//volume') and uniqueify it.
Either iterate per volume finding subnodes that match (Is there an xpath expression to specify a child value?)
Or create a 'staging' hash of hashes that I then 'merge' into children in the JSON.
Does anyone have any better suggestions?
I can quite easily create a hash of the desired structure e.g.
$stuff{$site}{$server}{$volume}{$qtree} = $size;
But then would have to turn that into the appropriate JSON (which I suppose might be a better approach overall).

Answer 1

is there a way I can enforce an order in the JSON output?
Yeah, use an array instead of an object. JSON objects are unordered.
An object is an unordered set of name/value pairs.
But it seems you're already using arrays for your lists.
Maybe you're want to be able to perform diffs, in which case JSON.pm provides a coarse means of specifying keys in the form of sort_by. Useful if you want to perform diffs.

Answer 2

If you just want to "enforce" some sorting by the hash key, you can use the "canonical" feature, as also said here.