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I aim to shed light on a peculiar issue that developers often encounter when working with Vercel’s serverless Next.js applications.

The crux of the problem lies in the handling of asynchronous operations. In a typical serverless environment on Vercel, one might expect that all asynchronous tasks would be allowed to be completed before the process exits. However, this is not always the case.

In certain scenarios, if a task is queued to run at the end of the event loop, it might start executing but not be allowed to finish. The serverless function on Vercel seems to terminate the process prematurely, leaving the task in limbo. This unexpected behavior can lead to incomplete operations and potential data inconsistencies, posing a significant challenge for developers.

In the article, I will delve deeper into this issue, exploring its implications and discussing potential reasons behind this behavior

review below code

The selected TypeScript code is part of a route handler in a Next.js application. It handles incoming requests, specifically ones related to an event from an app.

Here’s a breakdown of what the code does:

if (resData.event.type === ‘app_mention’)

This line checks if the type of the event in the received data (resData) is ‘app_mention’. This suggests that the application is listening for events where the app is mentioned, possibly in a chat or a comment.

setImmediate(async () => { await writeInDataBase(resData); });

If the event type is ‘app_mention’, the code then schedules the writeInDataBase(resData) function to run as soon as the current event loop cycle finishes. This function is presumably writing the received data into a database. The use of setImmediate here allows the function to run asynchronously without blocking the rest of the code.

return new NextResponse(‘’, { status: 200 });

After scheduling the database write operation, the code immediately responds to the request with a status of 200, indicating a successful HTTP request. The NextResponse object is likely a custom response object, from the Next.js framework.

else { //handle else part here. }:

If the event type is not ‘app_mention’, the code falls into this else block. The comment suggests that additional handling should be implemented here for other types of events.

writeInDataBase not finishing on Vercel is likely due to the nature of serverless functions. Serverless functions on platforms like Vercel and AWS Lambda are designed to be stateless and short-lived, usually timing out after a few seconds or minutes. When the function returns a response (which happens when return new NextResponse('', { status: 200 }); is executed), Vercel considers the function to be done and cleans up or freezes the function, even if there are still pending asynchronous operations.

In this case, setImmediate schedules writeInDataBase to run in the next cycle of the event loop, but Vercel cannot wait for the next cycle if the function has already returned a response. That’s the reason is not finished.

In traditional development environments, we often build and test our applications on monolithic, non-serverless machines. We naturally expect that the behavior we observe locally will be replicated in the production environment post-deployment. However, when deploying on platforms like Vercel, our applications transition into a serverless architecture. This shift can lead to unexpected behavior due to the fundamental differences between monolithic and serverless architectures.

As a result, some operations that work seamlessly on our local machines might not function as expected in the serverless environment. This highlights the importance of understanding the nuances of serverless architectures and the need for thorough testing in an environment that closely mirrors the production setup.


In conclusionbefore deploying the application the one so-called aware developer should test this application on the Vercel local environment and then go ahead with the clearance of your code and logic.

Rudresh Sisode
Rudresh Sisode

Software Engineer

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