Everything You Need to Know About YAML
Syntax, data types, styling, utilities, and more
According to Wikipedia YAML (/ˈjæməl/ and YAH-ml) is a human-readable data-serialization language. It is commonly used for configuration files and in applications where data is being stored or transmitted.
YAML targets many of the same communications applications as Extensible Markup Language (XML) but has a minimal syntax that intentionally differs from SGML. It uses both Python-style indentations to indicate nesting and a more compact format that uses [...] for lists and {...} for maps thus JSON files are valid YAML.
YAML is important to learn since it is been used in a lot of configuration and orchestration tools.
Table of ContentsWhat is YAML?
JSON Vs YAML
Tools Using YAML
YAML Syntax
Data Types
Strings
Timestamp
Comments
Sequences/Lists
Dictionary
Anchors and Alias
Complex Keys
Folded Style
Literal Style
Kubernetes YAML
Docker Compose YAML
Ansible YAML
AWS Cloud Formation YAML
Ansible YAML
YAML Utilities
Conclusion
References
What is YAML?
- Initially, YAML stood for Yet Another Markup Language.
- Later changed to YAML Ain’t Markup Language.
- YAML is a human-readable data serialization standard that can be used in conjunction with all programming languages and is often used to write configuration files.
- YAML is made up of bits and pieces of other languages. It has features derived from Perl, C, HTML, and other languages.
- YAML is a superset of JSON.
- YAML is similar to JSON or XML.
- YAML doesn’t contain any commands or instructions.
- YAML can be converted to JSON or vice versa.
- One of the most common uses for YAML is to create configuration files. YAML is also used by the automation tool Ansible to create automation processes, and for Kubernetes resources and deployments.
JSON vs YAML
YAML is easy and in a readable format. check out the difference between JSON and YAML in the link.
For example, check the differences between the YAML and JSON file. YAML is a more user-friendly and readable format.
- YAML is a superset of JSON.
- YAML has the ability to reference other items within a YAML file using “anchors.” Thus it can handle relational information as one might find in a MySQL database.
- YAML is more robust about embedding other serialization formats such as JSON or XML within a YAML file.
Tools Using YAML:
Some of the popular tools using YAML are
- Docker
- Ansible
- Kubernetes
- Jenkins
- AWS cloud formation
- Azure Devops
YAML Syntax
- All YAML files can optionally begin with
---and end with.... This is part of the YAML format and indicates the start and end of a document. - A file can contain multiple documents. The documents are separated by — begin with
---and end with... - YAML uses indentation like python programming. Spaces are allowed and tabs are not allowed.
- YAML file extensions are .yaml or .yml.
- YAML is case-sensitive. name: Martin <> Name: Martin <> name: martin
- YAML uses key: value pairs
In the above code snippet:
- name: Martin is the key-value pair.
- There should be space after the ‘:’ like
name: Martin. - Scalar is the single value assigned to the key.
- Martin is the scalar.
Data Types
YAML data types are
- Scalar
- List
- Dictionary
The scalar types are
- Strings
- Integer
- Floating
- Booleans
Strings
- In YAML you don’t need to give the string in quotes or double-quotes.
- “Yes” and “No” should be enclosed in strings. Otherwise, they will be interpreted as boolean values.
- If you use special characters then use single or double-quotes. For example email:
yaml@gmail.com→ email: “yaml@gmail.com”.
!!timestamp
According to YAML documentation -A timestamp value represents a single point in time. This can be serialized using a subset of the ISO8601 format and the formats proposed by the W3C note on DateTime.
If the time zone is omitted, the timestamp is assumed to be specified in UTC. The time part may be omitted altogether, resulting in a date format. In such a case, the time part is assumed to be 00:00:00Z (start of day, UTC).
- Also, YAML allows to explicitly declare the data type like below using !!
[DataTypeName]
Comment
- In YAML comments can be made by using ‘#’ sign. This is similar to python.
- Comment can start anywhere in the line.
- Multiple lines can be commented on by using the # sign in each line.
- JSON doesn’t allow comments.
Sequences/Lists
- Sequences are like lists in python.
- The sequence starts with a ‘-’ and a space.
- The sequence can be defined using a block style or flow style.
- The block style is
- The flow style is like writing it in Python list-style enclosed in square brackets.
Dictionary
- A dictionary is a collection of key-value pairs.
- A dictionary is represented in a simple
key: valueform (the colon must be followed by a space). - List members are denoted by a leading hyphen (
-). Please refer to the example dictionary.
The same dictionary in JSON is below:
The same dictionary in XML:
Now you can see how simple and in a readable format is YAML when compared to JSON and XML.
Anchors and Alias:
- Anchors are identified by an
&character, and aliases by an*character. - Anchors are like identifying an item with an anchor in a YAML document and then referring to that item with an alias later in the same document. Let's seen an example to understand.
- There’s no space between the
&/*characters and the following alias name. - For example, check the below example
This example looks simple.
Let's take the below docker-compose file example:
- In the above case, we declare an anchor called
&web. Here the&webrefers to the whole service:
Now we refer to the anchor&web with alias *web and then just change the port mapping 81:80 and 82:80.The overrides << with the characters before the Alias to add more values or existing values.
If you don’t use the anchor and alias then your code looks long and repetitive like below:
Complex Keys
A complex key is created by first inserting a question mark followed by a space, followed by the language-specific tag and the final value of the key.
? - Tesla
- Honda
: [Electric, Economy]Folded Style:
Use > to keep the line breaks preserved like below
Literal Style:
Use | if you want the line breaks.
Kubernetes YAML
The Kubernetes resources are created in a declarative way, thus making use of YAML files. Kubernetes resources, such as pods, services, and deployments are created by using the YAML files.
Let's see the deployment YAML file:
- A Deployment name
nginx-deploymentis created, indicated by the.metadata.namefield. The Deployment creates three replicated Pods, indicated by the.spec.replicasfield. For more information on Kubernetes please refer to this link.
Docker Compose YAML
According to docker documentation,docker-compose is a tool for defining and running multi-container Docker applications. With Compose, you use a YAML file to configure your application’s services. Then, with a single command, you create and start all the services from your configuration.
Let's see a docker-compose YAML file:
Please refer for more information on docker-compose YAML.
AWS Cloudformation
According to AWS documentation, AWS CloudFormation is a service that gives developers and businesses an easy way to create a collection of related AWS and third-party resources, and provision and manage them in an orderly and predictable fashion.
AWS cloud formation uses YAML or JSON.
Let's see an example AWS cloudformation YAML file:
Ansible YAML
Ansible playbooks are written in YAML. An Ansible playbook is a YAML sequence, which itself consists of mappings and sequences. Please refer ansible documentation for more info.
YAML Utilities
- YAML Lint: Check your YAML syntax.
- YAML Lint-Python package: Yamllint does not only check for syntax validity, but for weirdnesses like key repetition and cosmetic problems such as lines length, trailing spaces, indentation, etc.
- YAML Validator: Another one to validate your syntax.
- JSON-To-YAML: Converts Json to Yaml or vice versa.
- YAML Parser: Another tool to convert JSON to YAML.
- All YAML: All in one place.
Conclusion
Again YAML is simple and easy to use. Also, it is been used in all the cloud technologies from docker-compose, Kubernetes, Ansible, Jenkins, and cloud formation. So it is good to know about YAML.
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