cfn-signal

Description

The cfn-signal helper script signals CloudFormation to indicate whether Amazon EC2 instances have been successfully created or updated. If you install and configure software applications on instances, you can signal CloudFormation when those software applications are ready.

You use the cfn-signal script in conjunction with a CreationPolicy or an Auto Scaling group with a WaitOnResourceSignals update policy. When CloudFormation creates or updates resources with those policies, it suspends work on the stack until the resource receives the requisite number of signals or until the timeout period is exceeded. For each valid signal that CloudFormation receives, CloudFormation publishes the signals to the stack events so that you track each signal. For a walkthrough that uses a creation policy and cfn-signal, see Deploying applications on Amazon EC2 with AWS CloudFormation.

Note

cfn-signal doesn't require credentials, so you don't need to use the --access-key, --secret-key, --role, or --credential-file options. However, if no credentials are specified, CloudFormation checks for stack membership and limits the scope of the call to the stack that the instance belongs to.

Syntax for resource signaling (recommended)

If you want to signal CloudFormation resources, use the following syntax.

cfn-signal --success|-s signal.to.send \
        --access-key access.key \
        --credential-file|-f credential.file \
        --exit-code|-e exit.code \
        --http-proxy HTTP.proxy \
        --https-proxy HTTPS.proxy \
        --id|-i unique.id \
        --region AWS.region \
        --resource resource.logical.ID \
        --role IAM.role.name \
        --secret-key secret.key \
        --stack stack.name.or.stack.ID \
        --url AWS CloudFormation.endpoint

Syntax for use with wait condition handle

If you want to signal a wait condition handle, use the following syntax.

cfn-signal --success|-s signal.to.send \
        --reason|-r resource.status.reason \
        --data|-d data \
        --id|-i unique.id \
        --exit-code|-e exit.code \
        waitconditionhandle.url

Options

The options that you can use depend on whether you're signaling a creation policy or a wait condition handle. Some options that apply to a creation policy might not apply to a wait condition handle.

Name	Description	Required
--access-key (resource signaling only)	AWS access key for an account with permission to call the CloudFormation SignalResource API. The credential file parameter supersedes this parameter. Type: String	No
-d, --data (wait condition handle only)	Data to send back with the waitConditionHandle. Defaults to blank. Type: String Default: blank	No
-e, --exit-code	The error code from a process that can be used to determine success or failure. If specified, the --success option is ignored. Type: String Examples: -e $? (for Linux), -e %ERRORLEVEL% (for Windows cmd.exe), and -e $lastexitcode (for Windows PowerShell).	No
-f, --credential-file (resource signaling only)	A file that contains both a secret access key and an access key. The credential file parameter supersedes the --role, --access-key, and --secret-key parameters. Type: String	No
--http-proxy	An HTTP proxy (non-SSL). Use the following format: http://user:password@host:port Type: String	No
--https-proxy	An HTTPS proxy. Use the following format: https://user:password@host:port Type: String	No
-i, --id	The unique ID to send. Type: String Default: The ID of the Amazon EC2 instance. If the ID can't be resolved, the machine's Fully Qualified Domain Name (FQDN) is returned.	No
-r, --reason (wait condition handle only)	A status reason for the resource event (currently only used on failure) - defaults to 'Configuration failed' if success is false. Type: String	No
--region (resource signaling only)	The CloudFormation regional endpoint to use. Type: String Default: us-east-1	No
--resource (resource signaling only)	The logical ID of the resource that contains the creations policy you want to signal. Type: String	Yes
--role (resource signaling only)	The name of an IAM role that's associated with the instance. Type: String Condition: The credential file parameter supersedes this parameter.	No
-s, --success	If true, signal SUCCESS, else FAILURE. Type: Boolean Default: true	No
--secret-key (resource signaling only)	AWS secret access key that corresponds to the specified AWS access key. Type: String	No
--stack (resource signaling only)	The stack name or stack ID that contains the resource you want to signal. Type: String	Yes
-u, --url (resource signaling only)	The CloudFormation endpoint to use. Type: String	No
waitconditionhandle.url (wait condition handle only)	A presigned URL that you can use to signal success or failure to an associated WaitCondition Type: String	Yes

Example

Amazon Linux example

A common usage pattern is to use cfn-init and cfn-signal together. The cfn-signal call uses the return status of the call to cfn-init (using the $? shell construct). If the application fails to install, the instance will fail to create and the stack will rollback. For Windows stacks, see Bootstrapping AWS CloudFormation Windows stacks.

JSON

{
    "AWSTemplateFormatVersion": "2010-09-09",
    "Description": "Simple EC2 instance",
    "Resources": {
        "MyInstance": {
            "Type": "AWS::EC2::Instance",
            "Metadata": {
                "AWS::CloudFormation::Init": {
                    "config": {
                        "files": {
                            "/tmp/test.txt": {
                                "content": "Hello world!",
                                "mode": "000755",
                                "owner": "root",
                                "group": "root"
                            }
                        }
                    }
                }
            },
            "Properties": {
                "ImageId": "ami-a4c7edb2",
                "InstanceType": "t2.micro",
                "UserData": {
                    "Fn::Base64": {
                        "Fn::Join": [
                            "",
                            [
                                "#!/bin/bash -x\n",
                                "# Install the files and packages from the metadata\n",
                                "yum install -y aws-cfn-bootstrap",
                                "/opt/aws/bin/cfn-init -v ",
                                "         --stack ",
                                {
                                    "Ref": "AWS::StackName"
                                },
                                "         --resource MyInstance ",
                                "         --region ",
                                {
                                    "Ref": "AWS::Region"
                                },
                                "\n",
                                "# Signal the status from cfn-init\n",
                                "/opt/aws/bin/cfn-signal -e $? ",
                                "         --stack ",
                                {
                                    "Ref": "AWS::StackName"
                                },
                                "         --resource MyInstance ",
                                "         --region ",
                                {
                                    "Ref": "AWS::Region"
                                },
                                "\n"
                            ]
                        ]
                    }
                }
            },
            "CreationPolicy": {
                "ResourceSignal": {
                    "Timeout": "PT5M"
                }
            }
        }
    }
}

YAML

AWSTemplateFormatVersion: 2010-09-09
Description: Simple EC2 instance
Resources:
  MyInstance:
    Type: 'AWS::EC2::Instance'
    Metadata:
      'AWS::CloudFormation::Init':
        config:
          files:
            /tmp/test.txt:
              content: Hello world!
              mode: '000755'
              owner: root
              group: root
    Properties:
      ImageId: ami-a4c7edb2
      InstanceType: t2.micro
      UserData: !Base64 
        'Fn::Join':
          - ''
          - - |
              #!/bin/bash -x
            - |
              # Install the files and packages from the metadata
            - yum install -y aws-cfn-bootstrap
            - '/opt/aws/bin/cfn-init -v '
            - '         --stack '
            - !Ref 'AWS::StackName'
            - '         --resource MyInstance '
            - '         --region '
            - !Ref 'AWS::Region'
            - |+

            - |
              # Signal the status from cfn-init
            - '/opt/aws/bin/cfn-signal -e $? '
            - '         --stack '
            - !Ref 'AWS::StackName'
            - '         --resource MyInstance '
            - '         --region '
            - !Ref 'AWS::Region'
            - |+

    CreationPolicy:
      ResourceSignal:
        Timeout: PT5M

Examples

Several CloudFormation sample templates use cfn-signal, including the following templates.

• LAMP: Single EC2 instance with local MySQL database

• WordPress: Single EC2 instance with local MySQL database