Is it safe to call an async function during lambda nodejs init? - aws-lambda

Question

I would like to know whether it is safe to make HTTP(S) requests during the Init phase of a NodeJS Lambda function. In particular, I would like to make calls to AWS SSM GetParameter using #aws-sdk/client-ssm or AWS KMS Decrypt using #aws-sdk/client-kms to load secrets that will be used within the handler.
I have found one example online of someone creating a Promise outside of the handler and then awaiting it within the handler (Async Initialisation of a Lambda Handler), but I haven’t seen this approach endorsed in the official Lambda sample applications. None of the official examples do any work outside of the handler.
According to AWS Lambda execution environment: Lambda execution environment lifecycle, “Lambda freezes the execution environment when the runtime and each extension have completed and there are no pending events.” AWS Lambda Runtime API: Next invocation elaborates on the http://${AWS_LAMBDA_RUNTIME_API}/2018-06-01/runtime/invocation/next endpoint, “Do not set a timeout on the GET call. Between when Lambda bootstraps the runtime and when the runtime has an event to return, the runtime process may be frozen for several seconds.” I take this to mean that Lambda will signal the process with SIGSTOP at the time of the next call if Provisioned Concurrency is enabled as well as between requests.
In addition, when I look at lambcli’s lambda sources (docker run -it --rm lambci/lambda:build-nodejs12.x cat /var/runtime/Runtime.js), I see that scheduleIteration calls setImmediate(() => this.handleOnce()…) which calls this.client.nextInvocation so I don’t see a way to delay the nextInvocation call.
Questions:
In the Lambda nodejs runtime, is it possible to perform a HTTP request and await its response entirely within the Function init phase?
If you make a request outside of the handler, will the server time out the connection, resulting in Connection Closed errors when the handler awaits the response?
Is there a better recommended way to perform one-time initialization of secrets?

Answer 1

While I'm not able to fully answer all of your questions, I found this blog post that describes a possible solution: https://barker.tech/aws-lambda-nodejs-async-init-88b5c24af567
So in the end, the answers would be:
Yes, it is possible - see linked blog post.
Yes, it will time out. At least, this is what I experienced an issue when I tried to establish a MongoDB-connection outside the function handler with Provisioned Concurrency configured.
I can't really help you with that one...

Answer 2

AWS recently added top level await support in node14 and newer lambdas: https://aws.amazon.com/blogs/compute/using-node-js-es-modules-and-top-level-await-in-aws-lambda/ . Using this you can simply make the init phase wait by using top level await like so:
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms))
console.log("start init");
await sleep(1000);
console.log("end init");
export const handler = async (event) => {
return {
statusCode: 200,
body: JSON.stringify('Hello from Lambda!'),
};
};
This works great if you are using ES modules. If for some reason you are stuck using commonjs (e.g. because your tooling like jest or ts-node doesn't yet fully support ES modules) then you can make your commonjs module look like an es module by making it export a Promise that waits on your initialization rather than exporting an object. Like so:
const sleep = ms => new Promise(resolve => setTimeout(resolve, ms))
const main = async () => {
console.log("start init");
await sleep(1000);
console.log("end init");
const handler = async (event) => {
return {
statusCode: 200,
body: JSON.stringify('Hello from Lambda!'),
};
};
return { handler };
};
# note we aren't exporting main here, but rather the result
# of calling main() which is a promise resolving to {handler}:
module.exports = main();