Bolt-js: Bot replies twice when Lambda cold starts

[jc1518]

Description

I have setup a Slack Bolt on AWS Lambda by following this instruction - https://slack.dev/bolt-js/deployments/aws-lambda#set-up-aws-lambda

I want the Bot to reply 'hi' if someone says 'help'. Sample code:

app.message('help', async ({ message, say }) => {
  await say(`hi`);
});

I noticed that whenever the Lambda is cold, the Bot always says hi twice. But when Lambda is warm, the Bot only replies once. What is the possible solution to prevent the Bot from answering it twice when Lambda cold starts. Thanks!

What type of issue is this? (place an x in one of the [ ])

• [ ] bug
• [ ] enhancement (feature request)
• [x] question
• [ ] documentation related
• [ ] example code related
• [ ] testing related
• [ ] discussion

Requirements (place an x in each of the [ ])

• [x] I've read and understood the Contributing guidelines and have done my best effort to follow them.
• [x] I've read and agree to the Code of Conduct.
• [x] I've searched for any related issues and avoided creating a duplicate issue.

---

Bug Report

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Reproducible in:

package version:

        "@slack/bolt": "^3.2.0",
        "@vendia/serverless-express": "^4.3.3",

node version:
v12.14.1

Environment:
AWS Lambda

[mwbrooks]

Hey @jc1518 👋🏻

Thanks for the details, especially including the code sample and package versions!

The AWS Lambda/Slack API cold start issue can be frustrating because Slack requires a 3 second response time.

Just to make sure, have you already added the processBeforeResponse flag to your ExpressReceiver?

// Initialize your custom receiver
const expressReceiver = new ExpressReceiver({
  signingSecret: process.env.SLACK_SIGNING_SECRET,
  // The `processBeforeResponse` option is required for all FaaS environments.
  // It allows Bolt methods (e.g. `app.message`) to handle a Slack request
  // before the Bolt framework responds to the request (e.g. `ack()`). This is
  // important because FaaS immediately terminate handlers after the response.
  processBeforeResponse: true
});

[jc1518]

Hi @mwbrooks ,

Thanks for the quick response. Yes, I have the processBeforeResponse: true settings. I am thinking this issue could be caused by the Lambda concurrent execution? When Lambda cold starts, it may trigger more than one execution.

[jc1518]

I changed the limit of the concurrent execution to 1, and the Bot only replies once now when Lambda cold starts.

[seratch]

@jc1518 You can check the initialization time in CloudWatch Logs. How long does it take for cold-starts?

Bolt itself is a relatively small package. As long as a function built with Bolt does not have any other dependencies, cold-starts cannot be an issue. Thus, I'm guessing some other packages may take a bit long for their initialization.

I don't have the past examples with Node but let me share two examples in other runtimes:

• The root cause of https://github.com/slackapi/bolt-python/issues/155 was AWS's SSM module initialization
• In the Java SDK, we've observed the initial API call with S3Client can take longer

In your case, the cause of slow bootstrapping time can be a difference module or code. I'd suggest checking the actual spent time and trying to figure the major causes out step by step.

[jc1518]

Hi @seratch,

Yes, it has other package as well which may contribute to the slowness. I am thinking to split the Lambda function into multiple smaller ones to keep the main one light. All the main one does is to ack to Slack and forward the message to EventBridge. Other lambdas functions that takes long time to process will be triggered by the events from the main one. Does it sound right to you? Thanks.

[seratch]

@jc1518 Yes, your idea sounds a great approach to me 👍

[seratch]

@jc1518 Thanks for your 👍 on my comment. It seems we're done with this topic. Let me close this issue now.