3 minutes read

POSTED Mar, 2020 dot IN Debugging

Debugging Node.js AWS Lambda Functions using Thundra Debugger

Hamit Burak Emre

Written by Hamit Burak Emre

Ex-Software Engineer @Thundra


While developing your serverless functions, debugging may become cumbersome because you are now dealing with a black box compute service that provides log lines after the execution is finished. You might get lost in the logs. Redeploying and trying again with different parameters over and over can be a tedious process. AWS SAM CLI’s local debugging capabilities are really helpful at this point. However, simulating the AWS Lambda environment in your local might not be what you want always. Sometimes we need to debug our lambda functions in its native environment with all of the permissions, VPC’s, triggers and the stuff. Only in this way, you can “re-play” the exact behavior of your AWS Lambda functions. 

As a solution to this problem, Thundra released the Thundra Online Debugger which is the most native debugging experience ever for AWS Lambda! You will be able to debug Lambda functions as they run in their native cloud environment, with all the traditional debugging capabilities like tracking changes in local variables, updating them on the fly and putting breakpoints.

Using Thundra Debugger with Node.js AWS Lambda Functions

To demonstrate how you can use the Thundra Online Debugger with Node.js AWS Lambda functions, we will use a fully serverless blog site application. The architecture of this serverless blog site application is fairly simple. Basically a static website served through S3, contains the frontend logic of our blog site application. All the operations made through the application; like creating a new blog post, viewing a blog post or deleting a blog post invokes the corresponding Lambda function through the API Gateway.


We will first create a blog post in our blog site application, then we will enable the Thundra’s Online Debugging feature for one of the Lambda functions in our system which is blog-site-dev-deleteBlogPost. This lambda function is responsible for the deletion of a blog post in our system. To invoke this Lambda function we will delete the blog post that we are going to add and will debug this invocation in our local IDE which is VSCode for this particular example.

image3blog-site-dev-deleteBlogPost function in AWS Console

Firstly, let's add a new blog post in Add Blog tab of our blog site application:


Then, we can enable the Online Debugging feature for the blog-site-dev-deleteBlogPost by adding the thundra_agent_lambda_debugger_auth_token environment variable. This will tell the Thundra agent inside our lambda function to start Thundra Debugger using the authentication token specified in the environment variable.


adding our authentication token using the environment variables

Now, we can turn back to our blog site application to delete the blog post that we have added above to invoke the blog-site-dev-deleteBlogPost Lambda function and debug the invocation in the VSCode. To delete the blog post, clicking the trash button in the action column below.


After having invoked the blog-site-dev-deleteBlogPost, we can now open the VSCode and open our blog site application project and start the Thundra Debugger.


When the connection is made, our Lambda function blog-site-dev-deleteBlogPost will hit the breakpoint that we put as in the below:


From now on, you can debug this invocation just like debugging a local Node.js application. You can put breakpoints on the fly, change the values of the local variables, add expressions to watch and so on.  To learn more about the Thundra’s new Online Debugging feature you can check out how to get started in the Thundra documentation.

How it works at the background

To make debugging possible on the AWS Lambda environment, Thundra provides a proxy-based solution that sets up a secure bridge between the AWS Lambda environment and your debugger on IDE with a web socket communication. In this way, you can debug your applications like you could debug any code that you host on your computer with native debug action. We are calling this bridge as “broker” which is basically a proxy for the communication between your AWS Lambda function and your IDE. 

Another important point is that Thundra only lets one invocation to start a debugging session. This way while you are debugging an invocation of your Lambda function, other invocations will start and finish normally instead of waiting for a debugger to connect. You should also be aware that you might need to increase your function’s timeout value to prevent your invocation from a timeout while you are debugging.



As a developer working with serverless functions, I was really having hard time understanding the behavior of Lambda functions from time to time with logs. Writing a new log statement, redeploying function and checking the logs again was very time-consuming while debugging AWS Lambda functions. I’m proud to be part of such innovation with Thundra making possible the debugging AWS Lambda functions as if they were running locally. I took part in the development of our VSCode plugin for Node.js functions but you can also check the Python on VSCode, Java on IntelliJ IDEA as well. If you aren’t using any of these IDEs, that’s also fine because we are providing a portable client that makes it possible to use any IDE for debugging AWS Lambda functions. You can start your native debugging experience by signing up for Thundra for free directly or from AWS Marketplace. Start following us on Twitter to catch up with our latest announcements.