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ytt, before a Data Value can be used in a template, it must be declared. This is typically done via Data Values Schema.
This guide shows how to write such schema.
(For a broader overview of Data Values, see Using Data Values).
Starting a Schema Document ¶
One writes Data Values Schema in a YAML document annotated with
#@data/values-schema --- #! Schema contents
Files containing Schema documents are included via the
$ ytt ... -f schema.yml ...
The contents of such a document are the Data Values being declared.
Declaring Data Values ¶
Each item in the schema document declares a Data Value.
A Data Value declaration has three (3) parts: a name, a default value, and a type.
#@data/values-schema --- system_domain: ""
declares the Data Value, with the name
system_domain to be of type string with the default value of an empty string (i.e.
Implying Types ¶
ytt Schema types of Data Values are inferred from the values given. So, when one writes schema, they are implying the type of each Data Value through the default value they give.
#@data/values-schema --- system_domain: "" load_balancer: enabled: true static_ip: "" app_domains: - "" databases: - name: "" adapter: postgresql host: "" port: 5432 user: admin secretRef: name: ""
effectively declares the following data values:
system_domain— a string
load_balancer— a map containing two items:
enabled— a boolean
static_ip— a string
app_domains— an array of strings (and only strings)
databases— an array where each element is a map. Each map has exactly six items:
name— a string
adapter— a string
host— a string
port— an integer
user— a string
secretRef— a map having exactly one item:
name— a string
(see Data Values Schema Reference: Types for details.)
Setting Default Values ¶
ytt Schema, the default value for a Data Value is almost always simply the value specified.
From the example, above, the corresponding Data Values have the following defaults:
system_domainis empty string (i.e.
load_balancer.static_ipinitializes to empty string.
For details on how to set individual default values, see Data Values Schema Reference: Default Values.
Special Case: Arrays
There is one exception: arrays. As described in Data Values Schema Reference: Defaults for Arrays, the default value for arrays, by default, is an empty list (i.e.
). That said, when an item is added to the array, that item’s value is defaulted as defined in the schema.
In the example, above, the definition of
databases is an array. Each item in that array is a map with six keys including
database starts out as an empty list. Then, as each item added to it, they will be defaulted with the values given in the schema.
Continuing with our example schema, above, if a Data Values overlay gives an actual value for the array:
#@data/values --- databases: - name: core host: localhost
That item will be filled-in with defaults:
name: core adapter: postgresql host: localhost port: 5432 user: admin secretRef: name: ""
In order to override the default of the array, itself, within schema see Setting a Default Value for Arrays.
Specific Use-Cases ¶
A few less common, but real-world scenarios:
- setting a default value for arrays
- marking a Data Value as optional
- allowing multiple types of maps or arrays
- declaring “pass-through” Data Values
Setting a Default Value for an Array ¶
As explained in Data Values Schema Reference: Defaults for Arrays, unlike all other types, the default value for an array is an empty list (i.e.
In some cases, it is useful to provide a non-empty default value for an array. To do so, one uses the
For example, with this schema:
#@data/values-schema --- #@schema/default ["apps.cf-apps.io", "mobile.cf-apps.io"] app_domains: - ""
The default value for
app_domains will be
See also: @schema/default.
Marking a Data Value as Optional ¶
Sometimes, it can be useful to define a section of Data Values as optional. This typically means that templates that rely on those values conditionally include output that use the contained value.
For example, the following template:
#@ load("@ytt:data", "data") --- ... spec: #@ if data.values.load_balancer: loadBalancerIP: #@ data.values.load_balancer.static_ip #@ end ...
will only include
spec.loadBalancerIP if a value is provided for the
load_balancer Data Value.
One notes this in Schema using the
#@data/values-schema --- #@schema/nullable load_balancer: static_ip: ""
which indicates that
null, by default. However, if a value is provided for
load_balancer, it must be the
static_ip and have a value that is a string.
For more details see Data Values Schema Reference:
Allowing Multiple Types of Maps or Arrays ¶
In rare cases, a given Data Value needs allow more than one type.
ytt Schema does not explicitly support specifying more than one type for a Data Value.
In the meantime, one can mark such Data Values as having
#@schema/type any=True int_or_string: ""
int_or_stringis, by default, an empty string
- it can accept an integer or a string … or any other type,
- and the value of
int_or_stringand its children is not checked by schema.
If it is critical to ensure that the type of
int_or_string to be only an integer or string, one can include a validating library that does so explicitly:
load("@ytt:assert", "assert") load("@ytt:data", "data") if type(data.value.int_or_string) not in ("int", "string"): assert.fail("'int_or_string' must be either an integer or a string.") end
Declaring “Pass-through” Data Values ¶
In certain cases, one designs a Data Value to carry a chunk of YAML whose exact type or shape is unimportant to templates. In these situations, it is undesirable for that chunk of YAML to be type-checked.
One can effectively disable schema type checking and defaulting by marking the Data Value as of type “any”:
#@data/values-schema --- honeycomb: enabled: false api_key: so124me14v4al1i5da5p5i180key #@schema/type any=True optional_config: null
additional_config can contain any valid YAML.
#@data/values --- honeycomb: optional_config: default_series: id: 1001 description: Administrative Actions
In a template, the Data Value can be referenced and its contents will be inserted:
#@ load("@ytt:data", "data") --- #@ if/end data.values.honeycomb.enabled: honeycomb: config: api_key: #@ data.values.honeycomb.api_key #@ if/end data.values.honeycomb.optional_config: optional: #@ data.values.honeycomb.optional_config
Next Steps ¶
Once you’ve declared your Data Values, they can be referenced in
ytt templates as described in Using Data Values > Referencing Data Values
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