How to get SNS topic ARN inside lambda handler and set permissions to wite to it? - serverless-framework

Question

I have two lambda functions defined in serverless.yml: graphql and convertTextToSpeech. The former (in one of the GraphQL endpoints) should write to SNS topic to execute the latter one. Here is my serverless.yml file:
service: hello-world
provider:
name: aws
runtime: nodejs6.10
plugins:
- serverless-offline
functions:
graphql:
handler: dist/app.handler
events:
- http:
path: graphql
method: post
cors: true
convertTextToSpeach:
handler: dist/tasks/convertTextToSpeach.handler
events:
- sns:
topicName: convertTextToSpeach
displayName: Convert text to speach
And GraphQL endpoint writing to SNS:
// ...
const sns = new AWS.SNS()
const params = {
Message: 'Test',
Subject: 'Test SNS from lambda',
TopicArn: 'arn:aws:sns:us-east-1:101972216059:convertTextToSpeach'
}
await sns.publish(params).promise()
// ...
There are two issues here:
Topic ARN (which is required to write to a topic) is hardcoded it. How I can get this in my code "dynamically"? Is it provided somehow by serverless framework?
Even when topic arn is hardcoded lambda functions does not have permissions to wrote to that topic. How I can define such permissions in serverless.yml file?

Answer 1

1) You can resolve the topic dynamically.
This can be done through CloudFormation Intrinsic Functions, which are available within the serverless template (see the added environment section).
functions:
graphql:
handler: handler.hello
environment:
topicARN:
Ref: SNSTopicConvertTextToSpeach
events:
- http:
path: graphql
method: post
cors: true
convertTextToSpeach:
handler: handler.hello
events:
- sns:
topicName: convertTextToSpeach
displayName: Convert text to speach
In this case, the actual topic reference name (generated by the serverless framework) is SNSTopicConvertTextToSpeach. The generation of those names is explained in the serverless docs.
2) Now that the ARN of the topic is mapped into an environment variable, you can access it within the GraphQL lambda through the process variable (process.env.topicARN).
// ...
const sns = new AWS.SNS()
const params = {
Message: 'Test',
Subject: 'Test SNS from lambda',
TopicArn: process.env.topicARN
}
await sns.publish(params).promise()
// ...