vercel LAMBDA_RUNTIME Failed to post handler success response. Http response code: "400"
What is vercel LAMBDA_RUNTIME
All the code that you want to execute on vercel server is wrapped in a Lambda function and is executed when client sends the request So when you deploy the code on vercel, your app does not run on a standalone server. But it runs on server less architecture using AWS Lambda.
Some common examples are
- performing database operations e.g. mongodb, graphql queries, postgress sql etc
- making api calls
Some people have reported errors like "[AWS-Lambda] - NetworkingError: Client network socket disconnected before secure TLS connection was established"
Why this error occurs
You will see below type of error if you have not handled the errors in your database functions or api handlers properly. So when something goes wrong, error is thrown and eventually captured by AWS Lambda. That's when you will see below type of error.
vercel LAMBDA_RUNTIME Failed to post handler success response. Http response code: "400" site:stackoverflow.com
How to fix it
To fix this error, you will need to make sure that you are handling the errors in your database or api code properly. You can use try ...catch blocks to do this. In the case of my nextjs 13 app, what was happening is that I was making calls to database in server components. But I did not handle the error - UnhandledPromiseRejection mongodbserverselection error. That's why I was seeing LAMBDA_RUNTIME error.
In case you are trying to work with mongodb in your nextjs app, you will need to follow tutorial at https://www.section.io/engineering-education/build-nextjs-with-mongodb-and-deploy-on-vercel/#connecting-to-mongodb-atlas-cluster
This tutorial explains how to cache and use the mongodb client and database.
You can also think about other solutions like
- https://github.com/surmon-china/mongodb-data-api
- https://www.mongodb.com/use-cases/serverless
- add this to mongodb connectio string - ?serverSelectionTimeoutMS=60000&readPreference=secondaryPreferred
- check mongodb and vercel server error logs when the error occurs
- Atlas mongodb Free tier may cause issues due to shared cluster
Other types of issue you may see on vercel.
- 504 GATEWAY TIMEOUT - This happens if lambda does not finish executing before N seconds. There is limit to how long lambda can execute. So if your api is making third party calls and if that is taking long time to respond, you will see 504 error.
- [ERROR] [1607717147403] LAMBDA_RUNTIME Failed to post handler success response. Http response code: 413 - This happens if you are transferring the data over the limit set in Lambda. Read more at https://docs.aws.amazon.com/lambda/latest/dg/gettingstarted-limits.html
- Lambda Python Error: Runtime exited with error: signal: segmentation fault (core dumped) Runtime.ExitError - This error occurs if maximum memory limit is crossed.