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One benefit of a public cloud like AWS is the ability for developers to automate tasks. AWS CloudFormation, which defines an application stack's resources in a declarative template, lets developers define and deploy infrastructure as code. Initially, developers needed to use JSON -- an expressive, human-readable data format that some find confusing -- to format CloudFormation templates. The difficulty attributed to using JSON was a drawback to the automation process.
Comparing YAML and JSON formatted templates
Which template format to use is a matter of personal preference; teams that collaborate on templates likely want to standardize on a single format, but there is no right or wrong answer. Admins should choose the format that they're most comfortable with. However, developers who are new to both the JSON and YAML formats should compare the two to determine which would work best. Figure 1 shows a simple CloudFormation template, authored in JSON, to deploy a single Elastic Compute Cloud (EC2) instance.
The template defines a single resource, an EC2 instance called WebServer. A few of the required properties are set for the instance, which include the Amazon Machine Image (AMI), the instance type and the key name to use for remote access via secure socket shell protocol.
Figure 2 shows the same template formatted in YAML.
In Figure 2, the YAML-based template is more concise compared to its JSON counterpart, with nearly 100 fewer characters. This may not seem like a lot, but CloudFormation templates include several independent resources. CloudFormation templates that use the YAML format will reduce the amount of code.
Support for code comments
A big issue with building CloudFormation templates in JSON format is the lack of support for comments. Some CloudFormation templates are several thousand lines long and, in those cases, comments can be a lifesaver. YAML-based CloudFormation templates can include inline comments, as shown in Figure 3.
This code snippet defines a single EC2 instance in a YAML template. In this example, two comments were added. On the second line, an inline comment was added after declaring the server name of WebServer; this comment explains the intent of the item. The only requirement is to prefix comments with the hash (#) character.
Figure 3 also includes a comment that explains which AMI is used for the image ID -- ImageId property -- of the EC2 instance. Developers can use these techniques to add as many comments as needed throughout CloudFormation templates.
Make function calls with YAML
Another benefit for developers who use YAML to create CloudFormation templates is the use of abbreviated syntax to make calls to CloudFormation intrinsic functions, as shown in Figure 4.
The template in Figure 4 has a section for input parameters. In this case, the template accepts one input parameter called KeyName, which defines the name of the EC2 key pair for the new instance. Within the properties of the WebServer resource, the CloudFormation Ref function references the KeyName parameter. This allows an operator to set the KeyName value at runtime, as opposed to hard-coding the value into the template.
When the template launches, the Ref function is called and the value input through the KeyName parameter is used in the KeyName property of the EC2 instance. A tag-based syntax is used to call the Ref function, which requires a developer to prefix the function name with an exclamation point (!). Without this shortcut, function calls are more verbose and require more code.
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