Creating an AWS Lambda with Cats Effect + Feral and LocalStack

Written by

Nowadays, almost every industry uses serverless architectures because of the various cost-effective and flexible solutions they offer. AWS Lambdas are a significant part of this trend, and by now, they have become one of the most famous serverless alternatives—especially when it comes to file processing.

This article will explore a method for creating an AWS Lambda using Cats Effect, which offers several advantages while avoiding extended startup times. Specifically, we will create a profile processor Lambda that will execute whenever a new file is added to an S3 bucket. This file will contain a list of profiles with essential information for multiple users in a pipe (|) separated format. The Lambda will process each profile and complete three tasks:

  • Register the profile in a database.
  • Archive a file with the profile information in a S3 bucket.
  • Send a notification to the person via email. 
A diagram of the infrastructure to implement AWS Lamba

What is Feral?

Feral is a framework for writing serverless functions in Scala with Cats Effect and deploying them to the cloud. It targets both JVM and JavaScript runtimes.

This framework supports the premise that JavaScript runtime is more convenient for serverless functions, as it doesn't require the longer startup times that JVM does. Also, Feral leverages the evolution of the Typelevel.js ecosystem, and libraries like Cats Effect and FS2 have versions for both JVM and JavaScript runtimes.

Usually, a Lambda function using Feral will have the following form: first, the definition of the input event and the return types: in our case, an S3Event and no return. Then, we load our Lambda’s configuration to be handled as a Cats Effect Resource.

object ProfileProcessor extends IOLambda.Simple[S3Event, INothing] {
   type Init = ProfileProcessorConfig
   override def init: Resource[IO, Init] = Resource.eval(initConfig[IO])


   override def apply(event: S3Event, context: Context[IO], init: Init): IO[None.type] = {
      handleRequest[IO](event, init).as(None)


Feral has definitions for different AWS event types. In our scenario, we will define a type for the event triggered by S3 after a new file is added. This event type looks something like this:

import io.circe.Decoder

final case class S3Event(records: List[S3EventRecord])

object S3Event {
   implicit val decoder: Decoder[S3Event] = Decoder.forProduct1("Records")(S3Event.apply)

final case class S3EventRecord(
                                                               eventVersion: String,
                                                               eventName: String,
                                                               s3: S3Message

object S3EventRecord {
   implicit val decoder: Decoder[S3EventRecord] = Decoder.forProduct3(

Implementing Our Lambda

Our profile processing Lambda will use a shared queue to store all new profiles waiting to be processed and load the profiles contained in the new S3 files into it. Then, it will start a group of working fibers (small independent processes handled by Cats Effect) which will be in charge of processing each profile. This will allow us to keep track of the active workers' countdown, and when no worker is active, the process will be marked as completed.

private def handleRequest[F[_] : Console : Async](event: S3Event, config: ProfileProcessorConfig): F[Unit] = {
   for {
      queue <- Queue.unbounded[F, Profile]
      latch <- CountDownLatch[F](config.totalWorkers)
      _ <- Console[F].println(s"Received event: $event")
      _ <- event.records.traverse(record => readFile(config, queue,, record.s3.`object`.key))
      totalNew <- queue.size
      _ <- Console[F].println(s"$totalNew new profiles received")
      _ <- List.range(0, config.totalWorkers).parTraverse(workerId => processProfile(config, workerId, queue, latch))
      _ <- latch.await
      _ <- Console[F].println(s"Process completed")
   } yield ()

The read-file process parses the profiles from the files and adds them to the shared queue.

for {
   client <- buildS3Client[F](config)
   response <- getS3Object[F](client, bucketName, objectKey)
   byteArray <- responseToByteArray[F](response)
   _ <-
        .evalMap(line => parseProfile[F](line))
        .collect { case Some(profile) => profile }
   _ <- Console[F].println(s"$bucketName/$objectKey content loaded successfully.")
} yield ()

We assign each profile an ID during the parsing to identify it during the rest of the process. We will use a UUID for this.

def parseProfile[G[_] : Applicative](line: String): G[Option[Profile]] = {
   val result = line.split("\\|").map(_.trim).toList match {
      case firstName :: lastName :: email :: address :: phoneNumber :: age :: Nil =>
         val id = UUID.v4().toOption.getOrElse("")
         Some(Profile(id, firstName, lastName, email, address, phoneNumber, age.toInt))

      case _ => None


We should highlight something here: Because we are using JavaScript runtime, we cannot use JVM’s UUID utilities. Instead, we will use the UUID npm package. To do that, we’ll need to import Scala.js, which will compile our Scala code into JavaScript for interoperability with JS libraries and allow us to create facades to communicate with these libraries using Scala code or, in our case, for using UUID:

import scala.scalajs.js
import scala.scalajs.js.annotation.JSImport

@JSImport("uuid", JSImport.Namespace)
object UUID extends js.Object {
   def v4(): js.UndefOr[String] = js.native

Then, the workers will look for the next profile from the shared queue. If the queue is empty, the worker finalizes. Otherwise, the worker registers the profile in the database, archives it in the bucket, and sends the user an email notification before moving on to the next profile.

private def processProfile[F[_] : Console : Async](config: ProfileProcessorConfig, workerId: Int, queue: Queue[F, Profile], latch: CountDownLatch[F]): F[Unit] = {
   for {
      pending <- queue.size
      _ <-
         pending match {
            case 0 =>
               latch.release >> Console[F].println(s"Worker $workerId finalized")
            case _ =>
               for {
                  profile <- queue.take
                  _ <- registerProfile(config, profile)
                  _ <- archiveProfile(config, profile)
                  _ <- sendEmail(config, profile)
                  _ <- processProfile(config, workerId, queue, latch)
               } yield ()
   } yield ()

Each processing stage will have a similar structure to the following one. However, we will not use aws-js-sdk to create the facades; instead, we will use the aws-sdk-scalajs-facade library, which already provides the required facades. Also, the asynchronous API is generally preferred in cases like ours, so we use the putItemFuture function instead of the putItem one.

private def registerProfile[F[_] : Console : Async](config: ProfileProcessorConfig, profile: Profile): F[Unit] = {
   def putDynamoDBItem[G[_] : Async](client: DynamoDB, config: ProfileProcessorConfig, profile: Profile): G[PutItemOutput] = {
      val item: PutItemInputAttributeMap = Dictionary(
         ("id", AttributeValue.S(,
         ("firstName", AttributeValue.S(profile.firstName)),
         ("lastName", AttributeValue.S(profile.lastName)),
         ("email", AttributeValue.S(,
         ("address", AttributeValue.S(profile.address)),
         ("phoneNumber", AttributeValue.S(profile.phoneNumber)),
         ("age", AttributeValue.NFromInt(profile.age)),
         ("timestamp", AttributeValue.NFromDouble(

      Async[G].fromFuture {
         Async[G].pure(client.putItemFuture(PutItemInput(item, config.tableName)))

   for {
      client <- buildDynamoDBClient[F](config)
      _ <- putDynamoDBItem[F](client, config, profile)
      _ <- Console[F].println(s"${} registered successfully.")
   } yield ()

Creating Our Infrastructure on LocalStack with Terraform

Now, we can begin testing our Lambda locally. We will use LocalStack (a cloud service emulator that runs in a single container) and Terraform (an infrastructure-as-code tool) to create our required infrastructure locally.

Our required infrastructure will consist of:

  • An IAM role for Lambda execution with policies to access DynamoDB, S3, and SES services.
  • A Lambda function called profile-processor.
  • S3 buckets: one for deploying the Lambda code, one for new profile files, and one for processed profile files.
  • A DynamoDB profiles table.
  • An email identity for sending email notifications.
A diagram of the infrastructure to implement AWS Lamba

While creating the infrastructure, you might have noticed that we have a dependency unavailable in LocalStack—the uuid-npm package. Because of this, we need to install the JavaScript dependencies before creating our Lambda function's zip so they can be contained within it.

resource "terraform_data" "profile-processor-lambda-dependencies" {
   provisioner "local-exec" {
      command     = "npm install && zip -r ."
      working_dir = "${path.module}/profile-processor-lambda"

resource "aws_s3_object" "profile-processor-lambda-object" {
   bucket =
   key    = ""
   source = "${path.module}/profile-processor-lambda/"

   depends_on = [terraform_data.profile-processor-lambda-dependencies]

An advantage of this approach is that we can specify the parametrization of our Lambda through Terraform variables, like the number of workers in this case.

resource "aws_lambda_function" "profile-processor-lambda" {
   function_name = "profile-processor"

   s3_bucket =
   s3_key    = aws_s3_object.profile-processor-lambda-object.key

   runtime   = "nodejs14.x"
   handler   = "index.ProfileProcessor"
   timeout   = 30

   role = aws_iam_role.lambda-exec-role.arn

   environment {
      variables = {
         REGION = "${var.aws_region}"
         ACCESS_KEY = "${var.aws_access_key}"
         SECRET_KEY = "${var.aws_secret_access_key}"
         DYNAMODB_TABLE = "${}"
         NEW_BUCKET_NAME = "${}"
         ARCHIVED_BUCKET_NAME = "${aws_s3_bucket.archived-profiles-bucket.bucket}"
         SES_SOURCE_ADDRESS = "${}"
         TOTAL_WORKERS = "${var.number_workers}"

Verifying Our Lambda

After executing the Lambda in a LocalStack container in a machine with a file of 1000 profiles and 10 workers, we could verify in the container’s output that our Lambda started about one second after the file was added to the S3 bucket.

localstack_main  | 2024-02-21T17:42:47.280  INFO --- [   asgi_gw_2] localstack.request.http    : OPTIONS /new-profiles-bucket/generated-profiles.csv => 200
localstack_main  | 2024-02-21T17:42:47.299  INFO --- [   asgi_gw_0]     : AWS s3.PutObject => 200
localstack_main  | 2024-02-21T17:42:47.425  INFO --- [   asgi_gw_2] localstack.request.http    : OPTIONS /new-profiles-bucket => 200
localstack_main  | 2024-02-21T17:42:47.456  INFO --- [   asgi_gw_0]     : AWS s3.ListObjectsV2 => 200
localstack_main  | 2024-02-21T17:42:48.820  INFO --- [   asgi_gw_2] localstack.request.http    : POST /_localstack_lambda/f88fd5c026adb16d502c9971330f78ae/status/f88fd5c026adb16d502c9971330f78ae/ready => 202

After reviewing the Lambda execution log, we can also confirm the process completion and that it lasted about 22 seconds.

2024-02-21 17:43:11 [2024/02/21/[$LATEST]f88fd5c026adb16d502c9971330f78ae]
Process completed
2024-02-21 17:43:11
END RequestId: ef8b0abc-ceed-4e44-9e62-e35fd3703134
2024-02-21 17:43:11
REPORT RequestId: ef8b0abc-ceed-4e44-9e62-e35fd3703134 Duration: 22320.84 ms

Then, we can execute the same Lambda implementation, but this time for the JVM Runtime with the same 1000 profile files and the same 10 workers. Taking a look at the logs, we can verify that there is a longer delay before the Lambda starts to process events (about 10 seconds).

localstack_main | 2024-02-22T22:52:45.717  INFO --- [   asgi_gw_0] localstack.request.http    : OPTIONS /new-profiles-bucket/generated-profiles.csv => 200
localstack_main | 2024-02-22T22:52:45.733  INFO --- [   asgi_gw_2]     : AWS s3.PutObject => 200
localstack_main | 2024-02-22T22:52:45.816  INFO --- [   asgi_gw_5] localstack.request.http    : OPTIONS /new-profiles-bucket => 200
localstack_main | 2024-02-22T22:52:45.828  INFO --- [   asgi_gw_4]     : AWS s3.ListObjectsV2 => 200
localstack_main | 2024-02-22T22:52:56.180  INFO --- [   asgi_gw_4] localstack.request.http    : POST /_localstack_lambda/724c16bad9ad750d4a3766022dddbce6/status/724c16bad9ad750d4a3766022dddbce6/ready => 202

Finally, the Lambda execution log also confirms that the process reached completion and lasted about 30 seconds (a little bit more, too).

2024-02-22 17:53:27
Process completed
2024-02-22 17:53:27
END RequestId: d8f48a85-9fbe-4deb-a884-dd10e230f976
2024-02-22 17:53:27
REPORT RequestId: d8f48a85-9fbe-4deb-a884-dd10e230f976 Duration: 30827.65 ms

Wrapping up

The Feral framework is great for creating AWS Lambdas that are unaffected by long startup times. It also lets us take advantage of Cats Effect and FS2 functionalities. The way it handles concurrency is also pretty elegant.

Although this approach could be a bit challenging at the beginning due to the switching between Scala and JavaScript worlds (facades, library availability), the benefits of Scala’s strong type system (catching errors in compilation, easier refactoring) and the benefits of effects systems in the JavaScript runtime definitely make up for it. 

The complete code of these examples can be found on GitHub.

Bonus - Generating Test Data

We can create a test file of fake profiles using the following scala-cli script. Here we use java-faker for the generators of each profile field.

//> using scala "3.3.1"
//> using dep "co.fs2::fs2-core::3.9.4"
//> using dep "co.fs2::fs2-io::3.9.4"
//> using dep "com.github.javafaker:javafaker:1.0.2"
//> using dep "org.typelevel::cats-effect::3.5.3"

import cats.effect._
import com.github.javafaker.Faker
import fs2._
import{Files, Path}

import java.util.Locale

case class Profile(firstName: String, lastName: String, email: String, address: String, phoneNumber: String, age: Int)

object ProfilesGenerator extends IOApp.Simple:
  val totalProfiles = 1000
  val filePath = "generated-profiles.csv"

  def generateProfile: IO[Profile] =
    val faker = new Faker(Locale.US)
    val profile = Profile(
        firstName =,
        lastName =,
        email = faker.internet().emailAddress(),
        address = faker.address().fullAddress(),
        phoneNumber = faker.phoneNumber().phoneNumber(),
        age = faker.number().numberBetween(18, 100)

  def formatProfile(profile: Profile): IO[String] = 

  def generateFile(x: Int): IO[Unit] =
    Stream.range(0, x)
      .evalMap(_ => generateProfile)

  def run = generateFile(totalProfiles) >> IO.println(s"File $filePath generated.")

Learn more

Frequently Asked Questions