YAML for Configuration

I was wrong.

See, I love JSON. I love how simple it is. I love how it is easy to represent, parse, and produce in nearly every language I use. I love how it has become the de-facto interchange language of the web.

So naturally, I thought JSON was right for every situation. Because I love JSON, I was so blinded that I thought its strengths made it a natural choice to use for configuration files. But I was wrong.

Thankfully, there's a better option for configuration files. YAML.

Easier to use

I've had quite a bit of experience with using JSON files for configuration. package.json works fine. Some of my own custom services use it. Everything seemed to be, well, OK.

And then I was forced – yes forced – to use YAML for a Symfony project.

Through that process, I realized that it's just much easier to use YAML. Most configuration is simple key-value pairs, and being able to just write key: value is not only simple, but closely resembles most Linux configuration files.


database_host: my.database.example
database_port: 4218
database_username: web_app_user
database_password: IHLhbef1WYpwxg
database_database: awesome_web_app
port: 8080


  "database_host": "my.database.example",
  "database_port": 4218,
  "database_username": "web_app_user",
  "database_password": "IHLhbef1WYpwxg",
  "database_database": "awesome_web_app",
  "port": 8080

It's not huge, but YAML makes setting basic values simpler. Almost no need for quotes (or for escaping quotes). No surrounding brackets to match. No commas to remember. Just add a key, a colon, a value, and you're done.

Fewer hangups

I can't tell you how many times I've done this. Take the JSON example from right up there. And then pretend the port key is no longer required. So remove it from the config file.

  "database_host": "my.database.example",
  "database_port": 4218,
  "database_username": "web_app_user",
  "database_password": "IHLhbef1WYpwxg",
  "database_database": "awesome_web_app",

Restart the service that is being configured and... error.

If you've done a lot in JSON, you already know the problem. I removed the last key, and now there's a dangling comma which makes the file invalid json. Ugh, what a frustrating hangup to run into. So now I have to update the configuration again and restart the service again.

I could just get better at life and not make that mistake again (ha! wishful thinking). Or I could use YAML and not run into that hangup ever again.

And for the record, I have similar issues with matching curly braces.

Of course, YAML has some of its own hangups. But I seem to run into them less frequently.

Comments are allowed

The biggest win when choosing YAML for a configuration language over JSON is comments.

You know, those things that let you describe what a configuration option means or why it is set the way it is.

I mean, let's get this straight. I understand, and even sometimes agree, with why Douglas Crockford forbid comments from JSON. That doesn't make JSON bad. In fact, that's great for a data-interchange language. It just makes JSON a less-good fit for configuration files.

  "standalone": true,
  "secure_port": 8443
# Whether the application serves requests directly, and 
# must enable https and redirect. Set this to false if a 
# TLS terminator is sitting in front of the application. 
standalone: true
# Defaults to 443. 
secure_port: 8443

Comments are also useful for temporarily removing a configuration value, but keeping it around. If we take the same example from above and want to use the default for secure_port, we need to completely remove it from the JSON (and remove the trailing comma from standalone). Then what? If we want it back, we have to remember it? Write it down in some other file? On a sticky note? With YAML, we can just pop a comment in front of secure_port, allowing us to easily restore it later.

  "standalone": false
standalone: false
# secure_port: 8443 

I love JSON. But configuration files are meant for humans. And, as a human, I've found that YAML configuration files are much easier to maintain than JSON.