Adding constraints to blueprint properties and parameters

You can add constraints to blueprint properties and parameters to limit the values for those properties and parameters.

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You can add the following constraints to blueprint parameters. Each parameter can have any number of constraints.

The allowed_values constraint limits the value to a list of predefined values.
For example, the following parameter has a list of three valid values.
```
availability_zone:
  type: string
  description: Name of availability zone in which to create the instance
  constraints:
    - allowed_values:
      - East
      - West
      - North
```
When you specify valid values for a parameter in this way, the parameter appears as a drop-down list in the provisioning window, as shown in the following figure:
The length constraint limits the character length of the value.
For example, the following parameter must be between 6 and 8 characters in length.
```
image_name:
  type: string
  constraints:
    - length: { min: 6, max: 8 }
```
When you provision an environment from the blueprint and specify a value outside the range, a warning appears, as shown in the following figure:
The allowed_pattern constraint limits the value to a regular expression. For example, the following property is limited to begin with a capital letter, followed by at least one letter or number:
```
image_name:
  type: string
  constraints:
    - allowed_pattern: "[A-Z]+[a-zA-Z0-9]"
```
The range constraint limits a numeric value to a range. For example, the following property is limited to a value between 2 and 64:
```
memory_size:
  type: number
  default: 8
  constraints:
    - range: 
      min: 2
      max: 64
```

You can add these constraints directly to parameters, not to properties. To apply a constraint to a property, add the property to a resource as usual and use the get_param function to retrieve a parameter value. Then, apply the constraint to the parameter.

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