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Conditions allow a template developer to create many scenarios using the same template. cfn-lint does not use parameter values to determine the validaty of a template so cfn-lint will try to validate all the possible scenarios that are created by using conditions.
# Using conditions
## Defining conditions
Conditions are defined in a section of your CloudFormation template. At the lowest level all conditions will equate to a set of `Fn::Equals` that compares two values. Advanced condition scenarios can be created using `Fn::And`, `Fn::Or`, `Fn::Not`. Conditions can be nested using `Fn::Condition` function.
```yaml
Parameters:
Environment:
Type: String
Conditions:
IsProduction: !Equals [!Ref Environment, "prod"]
IsDevelopment: !Equals [!Ref Environment, "dev"]
IsProductionAndUsEast1:
!And
- !Condition IsProduction
- !Equals [!Ref AWS::Region, "us-east-1"]
```
## Using conditions
Conditions can be used in two locations. 1/ Is the at the resource or output level defined by the attribute `Condition` 2/ Using the function `Fn::If` under a resources `Properties` property. `Fn::If` can be used at any layer as long as it is the only key in an object.
```yaml
Resources:
HTTPSCertificate:
Type: AWS::CertificateManager::Certificate
Condition: HaveTargets
Properties:
DomainName: 'example.com'
ValidationMethod: DNS
CloudFrontAlias:
Type: AWS::Route53::RecordSet
Condition: CreateDNSRecords
Properties:
HostedZoneId: !Ref ServiceHostedZoneId
Name: 'example.com'
Type: A
AliasTarget:
DNSName: !GetAtt Distribution1.DomainName
HostedZoneId: Z2FDTNDATAQYW2
Distribution1:
Type: AWS::CloudFront::Distribution
Condition: HaveTargets
Properties:
DistributionConfig:
Enabled: true
ViewerCertificate:
AcmCertificateArn: !Ref HTTPSCertificate
SslSupportMethod: sni-only
MinimumProtocolVersion: TLSv1.2_2019
DefaultCacheBehavior:
TargetOriginId: Service1
Outputs:
DomainName:
Value: !If [HaveTargets, !GetAtt Distribution1.DomainName, ""]
```
# Handling conditions
## SymPy
[SymPy](https://docs.sympy.org/) is a python package that allows you to build formulas and calculate if a scenario is a legitimate scenario. For instance cfn-lint will build a solver for conditions `IsProduction` and `IsDevelopment` that will calculate the following possibilites `True`/`False`, `False`/`True`, or `False`/`False`. To build a solver cfn-lint will build formulas accordingly `Fn::And` is converted to `And`, `Fn::Or` is converted to `Or`, and `Fn::Not` is converted to `Not`. It will understand `Fn::Equals` that use the same parameter and create a `Not(And(...))` formula that will make sure that the equals in the `And` are no never `True` together.
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