[aws-events] concatenating event fields should produce an error

[rirze]

Creating an input transformer for a Cloudwatch Rule results in questionable syntax in the Cloudformation output causing the stack to unsuccessfully deploy.

Reproduction Steps

from aws_cdk import core
from aws_cdk import aws_events
from aws_cdk import aws_events_targets
from aws_cdk import aws_sns


class Scheduling(core.Construct):
    def __init__(self, scope: core.Construct, id: str):
        super().__init__(scope, id)

        topic = aws_sns.Topic(self, 'dummy_sns_topic')

        aws_events.Rule(self, 'test_rule',
                        description='test_rule',
                        schedule=aws_events.Schedule.rate(core.Duration.minutes(5)),
                        targets=[
                            aws_events_targets.SnsTopic(topic, message=aws_events.RuleTargetInput.from_object(
                                {
                                    'name': 'testing_at_' + aws_events.EventField.from_path('$.time')
                                }
                            ))
                        ])


class CdkTestRuleStack(core.Stack):

    def __init__(self, scope: core.Construct, id: str, **kwargs) -> None:
        super().__init__(scope, id, **kwargs)

        Scheduling(self, 'test_rule')

This results in CFN output where I've trimmed some of the irrelevant parts:

Resources:
  testruleB258B0E0:
    Type: AWS::Events::Rule
    Properties:
      Description: test_rule
      ScheduleExpression: rate(5 minutes)
      State: ENABLED
      Targets:
        - Arn:
            Ref: testruledummysnstopic5D840FE8
          Id: Target0
          InputTransformer:
            InputPathsMap:
              time: $.time
            InputTemplate: '{"name":"testing_at_<<${time>}'
    Metadata:
      aws:cdk:path: cdk-test-rule/test_rule/test_rule/Resource

I'm expecting InputTemplate: '{"name":"testing_at_<time>}'. My question is, am I building this correctly or is this a bug?

Error Log

11:46:48 AM | CREATE_FAILED        | AWS::Events::Rule     | testruleB258B0E0
Invalid InputTemplate for target Target0 : [Source: (String)"{"name":"testing_at_<<${time>}"; line: 1, column: 61]. (
Service: AmazonCloudWatchEvents; Status Code: 400; Error Code: ValidationException; Request ID: b09abd15-ff6b-4006-93
63-55b950ebf41d)

        new Rule (/tmp/jsii-kernel-R4yAYC/node_modules/@aws-cdk/aws-events/lib/rule.js:25:26)
        \_ ~/.local/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.js:7836:49
        \_ Kernel._wrapSandboxCode (~/.local/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.
js:8296
:20)
        \_ Kernel._create (~/.local/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.js:7836:2
6)
        \_ Kernel.create (~/.local/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.js:7583:21
)
        \_ KernelHost.processRequest (~/.local/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtim
e.js:73
71:28)
        \_ KernelHost.run (~/.local/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.js:7311:1
4)
        \_ Immediate._onImmediate (~/.local/lib/python3.7/site-packages/jsii/_embedded/jsii/jsii-runtime.j
s:7314:
37)
        \_ processImmediate (internal/timers.js:456:21)


 ❌  cdk-test-rule failed: Error: The stack named cdk-test-rule failed creation, it may need to be manually deleted fr
om the AWS console: ROLLBACK_COMPLETE
    at Object.waitForStack (~/.npm-packages/lib/node_modules/aws-cdk/lib/api/util/cloudformation.ts:264:11
)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)
    at Object.deployStack (~/.npm-packages/lib/node_modules/aws-cdk/lib/api/deploy-stack.ts:267:26)
    at CdkToolkit.deploy (~/.npm-packages/lib/node_modules/aws-cdk/lib/cdk-toolkit.ts:181:24)
    at main (~/.npm-packages/lib/node_modules/aws-cdk/bin/cdk.ts:264:16)
    at initCommandLine (~/.npm-packages/lib/node_modules/aws-cdk/bin/cdk.ts:185:9)
The stack named cdk-test-rule failed creation, it may need to be manually deleted from the AWS console: ROLLBACK_COMPLETE

Environment

• CLI Version : 1.51.0 (build 8c2d53c)
• Framework Version: 1.51.0
• Node.js Version: v13.14.0
• OS : Ubuntu 20.04 (WSL)
• Language (Version): Python 3.7

Other

---

This is 🐛 Bug Report

[michaelwiles]

Yeah... this kind of thing you cannot do in step functions.

You're basically asking event bridge to do is to concatenate strings in json and this cannot be done.

see common issues section

It's got nothing to do with cdk - it's an event bridge issue.

Try this:

targets=[
    aws_events_targets.SnsTopic(topic, message=aws_events.RuleTargetInput.from_object(
    {
        'name': aws_events.EventField.from_path('$.time')
    }))
]

[rirze]

Sure, I've figured that much out in the meantime and agree that it cannot be done.

With that in mind, should there be a in-library measure against this behavior? Should the CDK allow this to compile when it obviously results in strange wrapping tokens?

I think the CDK ought to detect this and raise an appropriate warning.

[thantos]

Event Bridge has been updated to support string formatting in a json object.

https://docs.aws.amazon.com/eventbridge/latest/APIReference/API_InputTransformer.html

"InputTransformer":
{
   "InputPathsMap": {"instance": "$.detail.instance","status": "$.detail.status"},
   "InputTemplate": "<instance> is in state <status>"
}

"InputTransformer": 
{
   "InputPathsMap": {"instance": "$.detail.instance","status": "$.detail.status"},
   "InputTemplate": "<instance> is in state \"<status>\""
}

We would expect something like this to work now

RuleTargetInput.fromObject({
   data: `some text ${EventField.fromPath('$')}`
})

[github-actions]

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