如何将CloudWatch规则正确附加到SNS主题？

Question

I'm trying to start StepFunction from CloudWatch. This works like a charm when event source is a schedule. However, when I try to create event pattern to watch corresponding SNS topic, CloudWatch doesn't sees publications to it. I've tried both default pattern:

{
  "source": [
    "aws.sns"
  ]
}

And using arn:

{
  "resources": [
    "// SNS topic arn here"
   ],
}

In both cases when I publish to the topic nothing happens. What am I doing wrong? Update: unfortunatelly I can't understand what is "Event Pattern" rule for, it doesn't work in other condition - even when event is pushed manually from lambda. The approach to my problem I currently see is to create CloudWatch schedule rule to start every 1-3 seconds, disable it. Then from a lambda I was going to push event to SNS enable this CloudWatch rule to fire. When it fires and start Step Function, inside Step Function disable the rule. It's nonsense but it should work.

Answer 1

Make sure your SNS topic has a policy set that allows events.amazonaws.com to publish to it, such as:

  statement {
    effect  = "Allow"
    actions = ["SNS:Publish"]

    principals {
      type        = "Service"
      identifiers = ["events.amazonaws.com"]
    }

    resources = ["${aws_sns_topic.sns_topic.arn}"]
  }