How to terminate AWS Lambda functions

The AWS Lambda, Programming Model document states:

Your Lambda function code must be written in a stateless style, and have no affinity with the underlying compute infrastructure.

The AWS Lambda, Best Practices document recommends:

Take advantage of container re-use to improve the performance of your function.

Those contradictory statements make sense at scale.

When an AWS Lambda function completes (which means it terminates without unhandled exceptions, either with an error (also referred to as a failure) or a result (also referred to as a success)), the AWS Lambda service will automatically freeze its container and probably reuse it for a new invocation.

When an AWS Lambda function takes longer than the specified maximum execution time, the AWS Lambda service times it out by throwing an exception (unhandled by definition) and then manages it accordingly to some policies.

Given that AWS Lambda functions for a NodeJS runtime are NodeJS applications that, in general, add callbacks to the event loop, the AWS Lambda service will, by default, make sure that there is no pending work before considering the AWS Lambda function fully completed, independently from the fact that its callback was already called or not. To call the callback is, in fact, optional.

Unfortunately, if the AWS Lambda function somehow blocks the event loop (for example, a database connection which is being periodically polled), the AWS Lambda service won’t ever find the event loop empty and the AWS Lambda function will time out.

NodeJS V8 has an experimental Async Hooks module to help debug these situations, while V6 (used by AWS Lambda) has these two undocumented functions:

  • process._getActiveHandles() gets you handles that are still alive
  • process._getActiveRequests() gets you info about active libuv requests.

Conveniently, the AWS Lambda service calls the exports.handler function with a second argument set to the container context. In turn the context has a callbackWaitsForEmptyEventLoop property which can be set to false to signal to the AWS Lambda service that the callback call (in a way) marks the end of the AWS Lambda function.

In reality:

AWS Lambda will freeze the process, any state data and the events in the NodeJS event loop (any remaining events in the event loop [will be] processed when the Lambda function is called next and if AWS Lambda chooses to use the frozen process).

let expensiveResult;

exports.handler = (event, context, callback) => {

    process(event, callback);

// ---

function process(event, done) {
    try {
        if (! expensiveResult) {
            // set expensiveResult
        // use expensiveResult
        // then
        done(null, result);
    } catch (e) {

function endWhenTheCallbackIsCalledOrTheTimeoutIsReached(context) {
    context.callbackWaitsForEmptyEventLoop = false;

function endWhenTheEventLoopIsEmptyOrTheTimeoutIsReached(context) {
    context.callbackWaitsForEmptyEventLoop = true;


How to securely save your secrets for free

This solution is based on Google Chrome, Gmail, and the Mailvelope extension for Chrome, which are all free.

First-time procedure

From the second time on, you only need Step 4 for storing and Step 6 for retrieving.

  1. Register a new Gmail account or use one of yours. Be it ``.
  2. Install Mailvelope.
  3. Generate a key pair for ``.
  4. Compose a message for ``.
    1. Use a Subject like `Awesome Service` for your Awesome Service account.
    2. Click the Mailvelope icon that appears when you hover the body.
    3. You’ll see that a dialog appears with `` in green.
    4. Enter account details, like your password.
    5. `Encrypt`
    6. You’ll see that the body of the message is now encrypted.
    7. Send the message.
  5. Given that `` is just an alias of `` you got the message in your inbox.
  6. Open it and confirm that it’s still encrypted.
    1. Click the Mailvelope icon that appears when you hover the body.
    2. Enter the password you specified in Step 3.
    3. You’ll see that the message is now decrypted.
    4. Close the message.

Why is this secure?

  • The network connection between your browser and Gmail servers is encrypted by default, so only you and Google can read your messages. With this encryption, not even Google will be able to read them.
  • When you send an email message protected by someone’s public encryption key, such a message can only be read by that someone using their matching private decryption key.
  • Anyone who got access to your browser without your consent and tried to steal one of these secrets would have to enter your password to read them.

Why is this practical?

  • It’s secure and free.
  • You can add a filter to label these messages so they appear collected.
  • You can search the subject or any other metadata you added to the bodies.
  • You can access your secrets from any Gmail device, anywhere in the world.
  • You can also keep all related secrets in a text file and send it as an encrypted attachment to ``.