Sunday, June 6, 2021

XML, JSON, YAML - Python data structures and visualization for infrastructure engineers

At some point, we can't "do it all" with one block of code. 

As developers, we need to store persistent data for a variety of reasons:

  • We want it for later execution (or to compare it to another result)
  • We're sick of storing variables in code. This matters a lot more in compiled languages than runtime ones
  • We want the results to end up in some form of a deliverable report

Let's cover a computer science concept being used here - semaphores). Edsger Dijkstra coined this term from Greek sema(sign) and phero(bearer) (you may remember him from OSPF) to solve Inter-Process Communications(IPC) issues.

To provide a reductionist example, process A and process B need to communicate somehow, but shouldn't access each other's memory or, in the '60s, it wasn't available. To solve this problem, the developer needs to develop a method of storing variables in a manner that is both efficient and can be consistently interpreted.

Dijkstra's example, in this case, was binary, and required a schema to interpret the same data - but was not specifically limited to single binary blocks. This specific need actually influenced one of the three data types we're comparing here - consequently the oldest.

But which one do I use? TL;DR?

Spoiler alert - anyone working with automation MUST learn all three to be truly effective. My general guidance would be:

  • This is a personal preference, but I would highly recommend YAML for human inputs. It's extremely easy to write, and while I generally prefer JSON it's much easier to first write a document into YAML and then convert it. If you take user input or just want to get a big JSON document started, I'd do it this way.
    • YAML User input drivers can also parse JSON, making this an extremely flexible approach.
  • JSON is good for storing machine inputs/outputs. Because all typing is pretty explicit with JSON, json.dumps(dict, indent=4) is pretty handy for previewing what your code thinks structured data looks like. Technically this is possible with YAML, but conventions on, say, a string literal can be squishy.
    • YAML with name: True could be interpreted as:
      • JSON of "name": true, indicating a Boolean value
      • JSON of "name": "True", indicating a String
    • Sure, this is oversimplified, and YAML can be explicitly typed, but generally, YAML is awesome for its speed low initial friction. If an engineer knows YAML really well (and writes their own classes for it) going all-YAML here is completely possible - but to me that's just too much work.
    • If you use it in the way I recommend, just learn to interpret JSON and use Python's JSON library natively, and remember json.dumps(dict, indent=4) for outputs. You'll pick it up in about half an hour and just passively get better over time.
  • Use XML if that's what the other code is using, Python's Element and ElementTree constructs are more nuanced than dictionaries, so a package like defusedXML is probably the best way to get started. There are a lot of binary invocations/security issues with XML, so using basic XML libraries by themselves is ill-advised. xmltodict is pretty handy if you just want to convert it into another format.

Note: JSON and XML both support Schema Validation, an important aspect of semaphores. YAML doesn't have a native function like this, but I have used Python's Cerberus modules to do the same thing here.

YAML

YAML was initially released in 2001 and has gained recent popularity with projects like Ansible. YAML 1.2 was released in 2009 and is publicly maintained by the community, so it won't have industry bias (but also won't change as quickly). YAML writes a lot like Python, consuming a ton of whitespace and being particular about tags. Users either love or hate it - I typically only use it for human inputs and objects that are frequently peer-reviewed.

NOTE: one big upside to YAML with people processes is comment support. YAML supports comments, but JSON does not.

YAML is pretty easy to start using in Python. I'm a big fan of the ruamel.YAML library, which adds on some interesting capabilities when parsing human inputs. I've found a nifty way to parse using try/except blocks - making a parser that is supremely agnostic, ingesting JSON or YAML, as a string or a file!

---
message:
  items:
    item:
      "@tag": Blue
      "#text": Hello, World!
#!/usr/bin/python3

import json

from ruamel.yaml import YAML
from ruamel.yaml import scanner

# Load Definition Classes
yaml_input = YAML(typ='safe')
yaml_dict = {}

# Input can take a file first, but will fall back to YAML processing of a string
try:
    yaml_dict = yaml_input.load(open('example.yml', 'r'))
except FileNotFoundError:
    print('Not found as file, trying as a string...')
    yaml_dict = yaml_input.load('example.yml')
finally:
    print(json.dumps(yaml_dict, indent=4))

JSON

JSON was first implemented in 2006 and is currently maintained by the IETF. Currently, Python 3 will visually represent dicts using JSON as well - making things pretty intuitive. In my experience, writing JSON is pretty annoying because it's picky.

{
    "message": {
        "items": {
            "item": {
                "@tag": "Blue",
                "#text": "Hello, World!"
            }
        }
    }
}
#!/usr/bin/python3

import json

with open('example.json', 'r') as file:
    print(json.dumps(json.loads(file.read())))

Typically, I'll just use json.dumps(dict, indent=4) on a live dict when I'm done with it - dumping it to a file. JSON is a well-defined standard and software support for it is excellent.

Due to its IETF bias, JSON's future seems to focus on streaming/logging required for infrastructure management. JSON-serialized Syslog is a neat application here, as you can write it to a file as a single line, but also explode for readability, infuriating grep users everywhere.

XML

XML is the oldest data language typically used for automation/data ingestion, and it really shows. XML was originally established by the W3C in 1998 and is used for many document types like Microsoft Office.

XML's document and W3C bias read very strongly. Older Java-oriented platforms like Jenkins CI heavily leverage XML for semaphores, document reporting, and configuration management. Strict validation (MUST be well-formed) required for compiled languages to synergize well with the capabilities provided. XML also heavily uses HTML-style escaping and tagging approaches, making it familiar to some web developers.

XML has plenty of downsides. Crashing on invalid input is generally considered excessive or "Steve-Ballmer"-esque, making the language favorable for mission-critical applications where misinterpretation of data MUST not be processed, and miserable everywhere else. For human inputs, it's pretty wordy which impacts readability quite a bit.

Schemas

XML has two tiers of schema - Document Type Definition (DTD) and XML Schema. DTD is very similar to HTML DTDs and provides a method of validating that the language is correctly used. XML Schema definitions (XSD) provide typing and structures for validation and is a more commonly used tool.

Python Example

XML Leverages the Element and ElementTree constructs in Python instead of dicts. This is due to XML being capable of so much more, but it's still pretty easy to use:

XML Document:

<?xml version="1.0" encoding="ISO-8859-1" ?>
<message>
    <items>
        <item tag="Blue">Hello, World!</item>
    </items>
</message>
#!/usr/bin/python3

from defusedxml.ElementTree import parse
import xmltodict
import json

document = parse('example.xml').getroot()

print(document[0][0].text + " " + json.dumps(document[0][0].attrib))

file = open('example.xml', "r").read()
print(json.dumps(xmltodict.parse(file), indent=4))

After using both methods, I generally prefer using xmltodict for data processing - it lets me use a common language, Python lists and dicts to process all data regardless of source, allowing me to focus more on the payload. We're really fortunate to have this fantastic F/OSS community enabling that!

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