Localstack is an implementation of AWS cloud stack which makes it very convenient if you want to develop and debug your AWS components locally. Also, it enables the creation of integration tests of your Lambda Functions: you just start Localstack (available as Docker containers, so Testcontainers can be used). Such tests can then be run without a necessity to communicate with real AWS (which would make the tests more fragile, slower and add complexity).

But it turns out that Localstack’s Lambda Function execution model (at least when it comes to Java functions) is different from AWS’s, which may lead you to a problematic behavior of your Lambdas.

Localstack Lambda Function Execution Model

Localstack currently offers three lambda function executors:

  • local - executes the function in the same machine in which Localstack is run
  • docker (default) - each function invocation is made in a fresh Docker container; it is removed after the invocation returns
  • docker-reuse - a Docker container is created per function; each invocation is processed inside the corresponding container

Despite their differences, all the executors execute a lambda function in the following way:

  1. Lambda jar is copied/uploaded to the executor (if needed) along with a special ‘runner’ jar
  2. The ‘runner’ jar is run passing it the Lambda jar as a parameter
  3. Lambda function arguments, env and input are passed in as command line arguments
  4. The ‘runner’ jar invokes the Lambda function
  5. When the function invocation terminates (successfully or not), it dumps the function result to STDERR and exits
  6. Localstack waits for the ‘runner’ process to terminate
  7. When it is terminated, Localstack analyzes the exit code (and errors out if it’s not 0) and obtains the function result from the STDERR of the ‘runner’ process as long as the Function STDOUT/STDERR output
  8. Localstack logs the output of the function to its own STDOUT (this does not happen for docker-reuse executor for some reason)

Note two things:

  1. in Localstack, every Lambda function instance is effectively one-off
  2. function stdout/stderr output will not be seen by Localstack until the invocation is finished

The problem

AWS Lambda documentation encourages us to initialize heavy resources once so that, when AWS sends consequtive invocations to the same Lambda Function instance, we could save on initialization.

Imagine that in our function we create some connection that includes a worker (non-demonic) thread. Let’s look what happens when we invoke such a function in Localstack.

A Java process gets terminated when all non-demonic threads terminate. So, on step 5, the ‘runner’ process will never terminate if we don’t terminate the worker thread! And we should not terminate it as it will require its resource reinitialization on next invocation in AWS! So Localstack at step 6 will never see the ‘runner’ process termination (only timeout will kill it), which means that synchronous invocations will be blocked forever.

What makes the situation even more frustrating is that you will not see any Lambda logging output in the Localstack stdout which makes you think that nothing happens, although you can observe side effects (DB writes, for example), which convince you that the function has been executed.

Ok, how about Lambda Function graceful shutdown?

Usually, when you stumble upon something like this, you expect that the component has some means to be cleaned up/shut down. But for AWS Lambda Functions will not work. Remember that the Lambda Functions are meant to be initialized once and then reused? There is no need in shutting them down gracefully, so Lambda interfaces do provide us any means to do it.

The solution

As non-terminated threads cause problems in Localstack only (not in AWS itself), and Function instances are effectively one-off in Localstack, we can just shut down the lambda function after each invocation using an env flag:

public String handleRequest(Map<String, String> event, Context context) {
    try {
        return doHandleRequest(event, context);
    } finally {
        if (Boolean.parseBoolean(System.getenv("SHUTDOWN_AFTER_INVOCATION"))) {
            // stop all the threads, pools, release resources, etc

Having this code and environment variable SHUTDOWN_AFTER_INVOCATION set to true, the Lambda Function will be successfully called (and the control will be returned to the caller right away after it finished its execution) in Localstack.