KCL Basics

Table of Content

• Variable assignments
• Common built-in types
• Lists and maps
• Conditional statements
• The : and = operator
• Advanced KCL capabilities

KCL is the choice of configuration language consumed by Kusion. KCL is an open source constraint-based record and functional language. KCL works well with a large number of complex configurations via modern programming language technology and practice, and is committed to provide better modularity, scalability, stability and extensibility.

Variable assignments

There are two ways to initialize a variable in KCL. You can either use the : operator or the = operator. We will discuss the difference between them in this section later.

Here are the two ways to create a variable and initialize it:

foo = "Foo"  # Declare a variable named `foo` and its value is a string literal "Foo"
bar: "Bar"  # Declare a variable named `bar` and its value is a string literal "Bar"

You will be able to override a variable assignment via the = operator. We will discuss this in depth in the : and = operator section.

Common built-in types

KCL supports int, float, bool and string as the built-in types.

Other types are defined in the packages that are imported into the application configuration files. One such example would be the AppConfiguration object (or Container, Probe, Port object, etc) that are defined in the catalog repository.

Lists and maps

Lists are represented using the [] notation. An example of lists:

list0 = [1, 2, 3]
list1 = [4, 5, 6]
joined_list = list0 + list1  # [1, 2, 3, 4, 5, 6]

Maps are represented using the {} notation. An example of maps:

a = {"one" = 1, "two" = 2, "three" = 3}
b = {'one' = 1, 'two' = 2, 'three' = 3}
assert a == b # True
assert len(a) == 3 # True

Conditional statements

You can also use basic control flow statements when writing the configuration file.

An example that sets the value of replicas conditionally based on the value of containers.myapp.resources.cpu:

import catalog.models.schema.v1 as ac
import catalog.models.schema.v1.workload as wl
import catalog.models.schema.v1.workload.container as c

myapp: ac.AppConfiguration {
    workload: wl.Service {
        containers: {
            "myapp": c.Container {
                image: "<no value>"
                resources: {
                    "cpu": "500m"
                    "memory": "512Mi"
                }
            }
        }
        replicas: 1 if containers.myapp.resources.cpu == "500m" else 2
    }
}

For more details on KCL's control flow statements, please refer to the KCL documentation.

The : and = operator

You might have noticed there is a mixed usage of the : and = in the samples above.

info

TLDR: The recommendation is to use : in the common configurations, and = for override in the environment-specific configurations.

In KCL:

• : represents a union-ed value assignment. In the pattern identifier: E or identifier: T E, the value of the expression E with optional type annotation T will be merged and union-ed into the element value.
• = represents a value override. In the pattern identifier = E or identifier = T E, The value of the expression E with optional type annotation T will override the identifier attribute value.

Let's take a look at an example:

# This is one configuration that will be merged.
config: Config {
    data.d1 = 1
}
# This is another configuration that will be merged.
config: Config {
    data.d2 = 2
}

The above is equivalent to the snippet below since the two expressions for config get merged/union-ed into one:

config: Config {
    data.d1 = 1
    data.d2 = 1
}

whereas using the = operators will result in a different outcome:

# This is first configuration.
config = Config {
    data.d1 = 1
}
# This is second configuration that will override the prior one.
config = Config {
    data.d2 = 2
}

The config above results in:

config: Config {
    data.d2 = 2
}

Please note that the : attribute operator represents an idempotent merge operation, and an error will be thrown when the values that need to be merged conflict with each other.

data0 = {id: 1} | {id: 2}  # Error：conflicting values between {'id': 2} and {'id': 1}
data1 = {id: 1} | {id = 2}  # Ok, the value of `data` is {"id": 2}

More about : and = operator can be found in the KCL documentation.

Advanced KCL capabilities

For more advanced KCL capabilities, please visit the KCL website.