Resolvers¶
Sceptre implements resolvers, which can be used to resolve a value of a
CloudFormation parameter or sceptre_user_data value at runtime. This is
most commonly used to chain the outputs of one Stack to the inputs of another.
If required, users can create their own resolvers, as described in the section on Custom Resolvers.
Syntax:
parameters:
<parameter_name>: !<resolver_name> <resolver_value>
sceptre_user_data:
<name>: !<resolver_name> <resolver_value>
Available Resolvers¶
environment_variable¶
Fetches the value from an environment variable.
Syntax:
parameter|sceptre_user_data:
<name>: !environment_variable ENVIRONMENT_VARIABLE_NAME
Example:
parameters:
database_password: !environment_variable DATABASE_PASSWORD
file¶
A Sceptre resolver to get file contents. The returned value can be passed into a parameter as a string, json, or yaml object.
Refer to sceptre-file-resolver for documentation.
stack_output¶
Fetches the value of an output from a different Stack controlled by Sceptre.
Syntax:
parameters | sceptre_user_data:
<name>: !stack_output <stack_name>.yaml::<output_name>
Example:
parameters:
VpcIdParameter: !stack_output shared/vpc.yaml::VpcIdOutput
Sceptre infers that the Stack to fetch the output value from is a dependency, and builds that Stack before the current one.
This resolver will add a dependency for the Stack in which needs the output from.
stack_output_external¶
Fetches the value of an output from a different Stack in the same account and region. You can specify a optional AWS profile to connect to a different account/region.
If the Stack whose output is being fetched is in the same StackGroup, the basename of that Stack can be used.
Syntax:
parameters/sceptre_user_data:
<name>: !stack_output_external <full_stack_name>::<output_name> <optional-aws-profile-name>
Example:
parameters:
VpcIdParameter: !stack_output_external prj-network-vpc::VpcIdOutput prod
Custom Resolvers¶
Users can define their own resolvers which are used by Sceptre to resolve the value of a parameter before it is passed to the CloudFormation template.
A resolver is a Python class which inherits from abstract base class
Resolver found in the sceptre.resolvers module.
Resolvers are require to implement a resolve() function that takes no
parameters and to call the base class initializer on initialisation.
Resolvers may have access to argument, stack_config,
stack_group_config and connection_manager as an attribute of self.
For example self.stack_config.
Sceptre uses the sceptre.resolvers entry point to locate resolver classes.
Your custom resolver can be written anywhere and is installed as Python
package.
In case you are not familiar with python packaging, this is great place to start.
Example¶
The following python module template can be copied and used:
custom_resolver
├── custom_resolver.py
└── setup.py
The following python module template can be copied and used:
custom_resolver.py¶
from sceptre.resolvers import Resolver
class CustomResolver(Resolver):
"""
The following instance attributes are inherited from the parent class Resolver.
Parameters
----------
argument: str
The argument of the resolver.
stack: sceptre.stack.Stack
The associated stack of the resolver.
"""
def __init__(self, *args, **kwargs):
super(CustomResolver, self).__init__(*args, **kwargs)
def resolve(self):
"""
resolve is the method called by Sceptre. It should carry out the work
intended by this resolver. It should return a string to become the
final value.
To use instance attribute self.<attribute_name>.
Examples
--------
self.argument
self.stack
Returns
-------
str
Resolved value
"""
return self.argument
setup.py¶
from setuptools import setup
setup(
name='<custom_resolver_package_name>',
py_modules=['<custom_resolver_module_name>'],
entry_points={
'sceptre.resolvers': [
'<custom_resolver_command_name> = <custom_resolver_module_namef>:CustomResolver',
],
}
)
Then install using python setup.py install or pip install . commands.
This resolver can be used in a Stack config file with the following syntax:
template:
path: <...>
type: <...>
parameters:
param1: !<custom_resolver_command_name> <value> <optional-aws-profile>
resolver arguments¶
Resolver arguments can be a simple string or a complex data structure.
template:
path: <...>
type: <...>
parameters:
Param1: !ssm "/dev/DbPassword"
Param2: !ssm {"name": "/dev/DbPassword"}
Param3: !ssm
name: "/dev/DbPassword"