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Event Handling in AWS using SNS, SQS, and Lambda

frosnerd profile image Frank Rosner Updated on ・11 min read

This blog post is part of my AWS series:


In reactive, message-driven applications it is crucial to decouple producers and consumers of messages. Combining publish/subscribe (pub/sub) and queueing components we are able to build resilient, scalable and fault-tolerant application architectures. AWS offers a variety of components which implement pub/sub or queueing.

In this post we want to get to know two simple but powerful components for event and message processing on AWS: The Simple Notification Service (SNS) and the Simple Queue Service (SQS).

The goal is to develop an event pipeline which sends a message to a Slack channel whenever someone uploads a picture to an S3 bucket. For demonstration purposes we will also store the events in a queue for asynchronous processing. The architecture involves S3 event notifications, an SNS topic, an SQS queue, and a Lambda function sending a message to the Slack channel. Here is an animation of the final result.


The remainder of this post is structured as follows. First there will be an overview of the architecture. Then - as usual - we will go into details about how to set things up using Terraform step by step. We are closing the post by discussing the main findings.


Let's look at the high level architecture. When a client uploads an image to the configured S3 bucket, an S3 event notification will fire towards SNS, publishing the event inside the respective topic. There will be two subscribers for that topic: An SQS queue and a Lambda function.

The SQS queue stores the event for asynchronous processing, e.g. thumbnail generation or image classification. The Lambda function parses the event and sends a notification message to a Slack channel. Within the scope of this blog post we are not going to discuss the asynchronous processing part. Due to the decoupling of publishing and subscribing with SNS we are free to add more consumers for the events later.

architecture overview

Let's look at the individual components in detail. S3 organizes objects in buckets. Within a bucket you can reference individual objects by key. Uploading a file to S3 can either be done via the AWS Console, the AWS CLI, or directly through the S3 API.

In this post we are going to use the CLI for uploading. Both the Console as well as the CLI work pretty smoothly, as they handle all the low level communication with S3 for you. If you are using the S3 API and your bucket is not publicly writable, you have to manually authenticate your requests using the AWS Signature Version 4.

S3 allows to configure event notifications. Events can be created based on object creation or deletion, as well as notification in case of object loss for objects with reduced redundancy. You can choose to either send the event towards an SNS topic, an SQS queue, or a Lambda function.

In our case we are going to send the events to SNS and then allow interested applications to subscribe. This is referred to as the messaging fanout pattern. Instead of sending events directly to all parties, by using SNS as an intermediate broker we decouple publishing and subscription.

SNS is a simple pub/sub service which organizes around topics. A topic groups together messages of the same type which might be of interest to a set of subscribers. In case of a new message being published to a topic, SNS will notify all subscribers. You can configure delivery policies including configuration of maximum receive rates and retry delays.

The goal is to send a Slack message on object creation within our S3 bucket. We achieve that by subscribing a Lambda function to the SNS topic. On invocation the Lambda function will parse and inspect the event notification, extract relevant information, and forward it to a preconfigured Slack webhook.

We will also subscribe an SQS queue to the topic, storing the events for asynchronous processing by, e.g., another Lambda function or a long running polling service. The next section explains how to implement the architecture.


Development Tool Stack

To develop the solution we are using the following tools:

  • Terraform v0.11.7
  • SBT 1.0.4
  • Scala 2.12.6
  • IntelliJ + Scala Plugin + Terraform Plugin

The source code is available on GitHub. Now let's look into the implementation details of each component.

S3 Bucket

s3 bucket architecture

First we will create the S3 bucket where we can upload pictures to. We need to provide a bucket name and an ACL. The ACL will be public-read this time as we want to enable people to make their images publicly readable but require authentication for uploads. The force-destroy option allows Terraform to destroy the bucket even if it is not empty.

variable "aws_s3_bucket_upload_name" {
  default = "sns-sqs-upload-bucket"

resource "aws_s3_bucket" "upload" {
  bucket = "${var.aws_s3_bucket_upload_name}"
  acl    = "public-read"
  force_destroy = true

SNS Topic

SNS topic architecture

Next let's create the SNS topic. To create an SNS topic we only need to provide a name.

resource "aws_sns_topic" "upload" {
  name = "sns-sqs-upload-topic"

The topic alone is not going to be useful if we do not allow anyone to publish messages. In order to do that we attach a policy to the topic which allows our bucket resource to perform the SNS:Publish action on the topic.

resource "aws_sns_topic_policy" "upload" {
  arn = "${aws_sns_topic.upload.arn}"

  policy = "${data.aws_iam_policy_document.sns_upload.json}"

data "aws_iam_policy_document" "sns_upload" {
  policy_id = "snssqssns"
  statement {
    actions = [
    condition {
      test = "ArnLike"
      variable = "aws:SourceArn"

      values = [
    effect = "Allow"
    principals {
      type = "AWS"
      identifiers = [
    resources = [
    sid = "snssqssnss3upload"

S3 Event Notification

S3 event notification architecture

With our SNS topic and S3 bucket resource defined we can combine them by creating an S3 bucket notification which will publish to the topic. We can control the events we want to be notified about. In our case we are interested in all object creation events. We can also specify optional filters, e.g. only notifications for *.jpeg files in this case.

resource "aws_s3_bucket_notification" "upload" {
  bucket = "${aws_s3_bucket.upload.id}"

  topic {
    topic_arn     = "${aws_sns_topic.upload.arn}"
    events        = ["s3:ObjectCreated:*"]
    filter_suffix = ".jpeg"

SQS Queue

SQS queue architecture

The creation of the SQS queue works in a similar fashion. We have to provide a name for the queue and a policy which allows SNS to send messages to the queue.

resource "aws_sqs_queue" "upload" {
  name = "sns-sqs-upload"
resource "aws_sqs_queue_policy" "test" {
  queue_url = "${aws_sqs_queue.upload.id}"
  policy = "${data.aws_iam_policy_document.sqs_upload.json}"

data "aws_iam_policy_document" "sqs_upload" {
  policy_id = "snssqssqs"
  statement {
    actions = [
    condition {
      test = "ArnEquals"
      variable = "aws:SourceArn"

      values = [
    effect = "Allow"
    principals {
      type = "AWS"
      identifiers = [
    resources = [
    sid = "snssqssqssns"

SQS Subscription

SQS subscription architecture

Next we need to subscribe the queue to the topic. SNS topic subscriptions support multiple protocols: http, https, email, email-json, sms, sqs, application, lambda. In this case we will use the sqs protocol and provide both the topic and the queue endpoint.

resource "aws_sns_topic_subscription" "sqs" {
  topic_arn = "${aws_sns_topic.upload.arn}"
  protocol  = "sqs"
  endpoint  = "${aws_sqs_queue.upload.arn}"

Slack Webhook

slack webhook architecture

Before we can write our Lambda function and subscribe it to the SNS topic as well, we will create the Slack webhook. Working with incoming webhooks in Slack is done in four steps:

  1. Create a Slack app. A Slack app behaves like a technical user within your workspace.
  2. Enable incoming webhooks in your app.
  3. Create a new incoming webhook. You will receive the webhook URL.
  4. Use the webhook URL to send messages via HTTP POST.

After you completed the steps you will see your app and the configured webhook in the Slack app overview page. It might look like this.

slack app

slack webhook

Lambda Function

lambda architecture

Message Format

We can use the webhook URL to create our Lambda function. The function will receive S3 notifications wrapped inside SNS notifications. Both are sent in JSON format, but the S3 notification is stored in the .Records.Sns.Message field as a JSON string and has to be parsed as well. This is an example of an SNS notification wrapper message.

    "Records": [
            "EventSource": "aws:sns",
            "EventVersion": "1.0",
            "EventSubscriptionArn": "arn:aws:sns:eu-central-1:195499643157:sns-sqs-upload-topic:c7173bbb-8dda-47f6-9f54-a6aa81f65aac",
            "Sns": {
                "Type": "Notification",
                "MessageId": "10a7c00e-af4b-5d93-9459-93a0604d93f5",
                "TopicArn": "arn:aws:sns:eu-central-1:195499643157:sns-sqs-upload-topic",
                "Subject": "Amazon S3 Notification",
                "Message": "<inner_message>",
                "Timestamp": "2018-06-28T11:55:50.578Z",
                "SignatureVersion": "1",
                "Signature": "sTuBzzioojbez0zGFzdk1DLiCmeby0VuSdBvg0yS6xU+dKOk3U8iFUzbS1ZaNI6oZp+LHhehDziaMkTHQ7qcLBebu9uTI++mGcEhlgz+Ns0Dx3mKXyMTZwEcNtwfHEblJPjHXRsuCQ36RuZjByfI0pc0rsISxdJDr9WElen4U0ltmbzUJVpB22x3ELqciEDRipcpVjZo+V2J8GjdCvKu4uFV6RW3cKDOb91jcPc1vUnv/L6Q1gARIUFTbeUYvLbbIAmOe5PiAT2ZYaAmzHKvGOep/RT+OZOA4F6Ro7pjY0ysFpvvaAp8QKp4Ikj40N9lVKtk24pW+/7OsQMUBGOGoQ==",
                "SigningCertUrl": "https://sns.eu-central-1.amazonaws.com/SimpleNotificationService-ac565b8b1a6c5d002d285f9598aa1d9b.pem",
                "UnsubscribeUrl": "https://sns.eu-central-1.amazonaws.com/?Action=Unsubscribe&SubscriptionArn=arn:aws:sns:eu-central-1:195499643157:sns-sqs-upload-topic:c7173bbb-8dda-47f6-9f54-a6aa81f65aac",
                "MessageAttributes": {}

Inside the <inner_message> part you will find the actual S3 notification, which might look like this:

    "Records": [
            "eventVersion": "2.0",
            "eventSource": "aws:s3",
            "awsRegion": "eu-central-1",
            "eventTime": "2018-06-28T11:55:50.528Z",
            "eventName": "ObjectCreated:Put",
            "userIdentity": {
                "principalId": "AWS:AIDAI3EXAMPLEEXAMP"
            "requestParameters": {
                "sourceIPAddress": "xxx.yyy.zzz.qqq"
            "responseElements": {
                "x-amz-request-id": "0A8A0DA78EF73966",
                "x-amz-id-2": "/SD3sDpP1mcDc6pC61573e4DAFSCnYoesZxeETb4MV3PpVgT4ud8sw0dMrnWI9whB3RYhwGo+8A="
            "s3": {
                "s3SchemaVersion": "1.0",
                "configurationId": "tf-s3-topic-20180628113348955100000002",
                "bucket": {
                    "name": "sns-sqs-upload-bucket",
                    "ownerIdentity": {
                        "principalId": "A2OMJ1OL5PYOLU"
                    "arn": "arn:aws:s3:::sns-sqs-upload-bucket"
                "object": {
                    "key": "3427394.jpeg",
                    "size": 25044,
                    "eTag": "a3cf1dabef657a65a63a270e27312ddc",
                    "sequencer": "005B34CCC64D9E046E"

The most interesting part is within the s3 object which holds information about the S3 bucket and the object that has been uploaded. I'm sure that the AWS Java SDK has some classes which represent this information but for this blog post I decided to decode the parts that I am interested in manually using circe.

Source Code

The Lambda function will use the same RequestStreamHandler that we used before. This class is part of the aws-lambda-java-libs and provides only raw input and output streams, leaving the serialization and deserialization to us. Here's the source code:

class Handler extends RequestStreamHandler {
  override def handleRequest(input: InputStream,
                             output: OutputStream,
                             context: Context): Unit = {
    val hookUrl = System.getenv("hook_url")
    val inputJsonString = Source.fromInputStream(input).mkString
    val processingResult = for {
      notification <- decodeNotification(inputJsonString)
      message <- decodeMessage(notification)
    } yield {
      implicit val backend = HttpURLConnectionBackend()
        .body(SlackMessage(messageText(notification, message)).asJson.noSpaces)

    val out = new PrintStream(output)
    processingResult match {
      case Right(response) => out.print(s"Response from hook: ${response.code}")
      case Left(error)     => out.print(s"Failed: $error")

First we are extracting the hook URL from the $hook_url environment variable. Error handling and logging is omitted for readability reasons at this moment. Then we are reading the notification JSON string from the input stream and parsing it in two steps because I was too lazy to provide a custom deserialization format.

If parsing was successful we are sending an HTTP POST request to the hook URL. Slack expects the body to be a JSON having at least a text field. SlackMessage is a case class that captures this. In our case we will construct a message text based on the S3 bucket and object key. To send our message to the channel we would have to use the following body:

  "text": "Someone uploaded ${s3.`object`.key} to ${s3.bucket.name}."

The Lambda function does not really have to return anything but we are going to return a readable string message indicating whether the hook responded or if parsing of the SNS message failed.

Now we only have to package the Lambda handler in a fat jar file using the sbt-assembly plugin again. After running sbt assembly the artifact can be uploaded to AWS Lambda using Terraform.


Before we can create the Lambda function we have to create an IAM role for the execution. Then we can create the Lambda function itself and also setup the permissions for the SNS notification to be able to invoke our Lambda function. First the IAM role:

resource "aws_iam_role" "lambda_exec" {
  name = "sns-sqs-slack-lambda"

  assume_role_policy = <<EOF
  "Version": "2012-10-17",
  "Statement": [
      "Action": "sts:AssumeRole",
      "Principal": {
        "Service": "lambda.amazonaws.com"
      "Effect": "Allow",
      "Sid": ""

To make the Terraform code a bit more reusable we will introduce two variables: The artifact version and the Slack webhook URL. I will pass the webhook URL through my local terraform.tfvars file.

variable "slack_lambda_version" {
  type = "string"
  default = "0.1-SNAPSHOT"

locals {
  slack_lambda_artifact = "../slack/target/scala-2.12/sns-sqs-chat-assembly-${var.slack_lambda_version}.jar"

variable "slack_hook_url" {
  type = "string"

Now we can define the Lambda function resource. This time we will not choose an S3 artifact but upload our assembled archive directly. It is useful to also specify the source_code_hash in order to trigger updates if the file has changed although the name did not. You might think about including the commit SHA and whether the repository was clean inside the filename to have more transparency though.

We will also assign 1 GB of RAM just because AWS Lambda assigns CPU corresponding to memory and JVM class loading takes a lot of CPU for the initial request so we need that computing power 🔥. At this moment we are also passing the Slack URL as an environment variable.

resource "aws_lambda_function" "slack" {
  function_name = "sns-sqs-upload-slack"
  filename = "${local.slack_lambda_artifact}"
  source_code_hash = "${base64sha256(file(local.slack_lambda_artifact))}"
  handler = "de.frosner.aws.slack.Handler"
  runtime = "java8"
  role = "${aws_iam_role.lambda_exec.arn}"
  memory_size = 1024
  timeout = 5

  environment {
    variables {
      hook_url = "${var.slack_hook_url}"

Finally we have to create a permission which allows SNS messages to trigger the Lambda function.

resource "aws_lambda_permission" "sns" {
  statement_id  = "AllowExecutionFromSNS"
  action        = "lambda:InvokeFunction"
  function_name = "${aws_lambda_function.slack.function_name}"
  principal     = "sns.amazonaws.com"
  source_arn = "${aws_sns_topic.upload.arn}"

Lambda Subscription

lambda sns subscription architecture

The only link that is missing to complete our pipeline is the subscription of the Lambda function to the SNS topic. This is basically the same as the SQS subscription but with the lambda protocol this time.

resource "aws_sns_topic_subscription" "lambda" {
  topic_arn = "${aws_sns_topic.upload.arn}"
  protocol  = "lambda"
  endpoint  = "${aws_lambda_function.slack.arn}"


Now we can run terraform apply. Make sure you executed sbt assembly before to have the artifact available for Terraform to upload.

terraform deployment


Finally, we are done! That was quite some work. We could have simplified the architecture by sending S3 notifications to our Lambda function directly. But I wanted to demonstrate the fanout pattern, which is also why we introduced the SQS queue which is not used at the moment.

We have seen how it is possible to implement an event processing pipeline with potentially multiple consumers and producers using fully managed building blocks like SNS, SQS, and Lambda. SNS provides pub/sub functionality to decouple producers and consumers, while SQS gives us the ability to process events asynchronously.

Did you ever use SNS or SQS? What is your experience with Amazon MQ or Kinesis and in which cases do you think SQS is suitable? Let me know what you think in the comments.

Posted on by:

frosnerd profile

Frank Rosner


My professional interests are cloud and big data technologies, machine learning, and software development. I like to read source code and research papers to understand how stuff works.


markdown guide

From reading AWS documentation understand you can trigger a Lambda from a SNS. If the Lambda fails AWS will retry it twice some time later. You can configure the SNS/Lambda to send the failed messages (after the 3 attempts) to a dead-letter queue. Why/When should I put a SQS between the SNS and the Lambda?


When putting SQS between SNS and Lambda you don't need a dead letter queue (DLQ). In my opinion it depends on what you are doing with events in the DLQ. If those events are treated differently then you can use a DLQ. But if you are feeding them back into the input queue because you want to keep trying to process them until they succeed, just put SQS in front in the first place.

Does that make sense?


Hi Frank,

my thinking is that if a message failed to be processed 3 times (with delays between retries), must be something wrong with the message and needs to be verified kind of manually or will be going around the SQS forever. Another case can be that an external component (3rd party service, database...) is down but then do we want to immediately put the message back in the SQS and try to reprocess again? Better to move to the DLQ and wait little bit longer.

Thanks for your input.

The message processing might fail because there is something wrong with the message, yes. It might also fail however because a downstream system is not available and the Lambda fails because of this. Then it's a useful retry mechanism.

In the end it's an architecture decision and there are no right solutions, only trade-offs. But you're asking the right questions!


I read decoupling is very important in serverless architectures, only do minimal work in synchronous Lambda calls etc.

And I heard AWS works on SQS Lambda triggers, which will ease a lot of pain in that regard :)


I think putting a queue between S3 and Lambda has yet another benefit.
In case, your lambda failed on processing the event from S3 directly connected to it, the event would be lost after second retry. Putting a queue in-between helps to persist the message for longer time.
An alternative would be to add a DLQ to Lambda itself (docs.aws.amazon.com/lambda/latest/...).
Not sure which one is better.


I totally agree with you. I also find it hard to stay on top of all the different limits and behaviours. How many retries you will get depending on the invocation type and resource... Definitely something to think about in advance.


Hey Frank,

I have a similar use case wherein I need a program in EC2 to subscribe to SNS Topic to get text files/data placed in S3 bucket.This program also needs to parse the text file and create a .CSV record in DynamoDB and also save a copy in S3 (destination folder). can you guide me on that?



I assume you already setup a bucket notification that publishes messages to an SNS topic. If you want to use SNS to receive the notifications (because you want to fan-out) then you can either subscribe via HTTP or SQS.

In case you are using HTTP, the EC2 instance needs to expose an HTTP endpoint. If you want a more resilient and secure setup then you can subscribe with SQS and make the EC2 instance poll from SQS.

In case you don't have multiple subscribers on your SNS topic you may also directly notify SQS from S3 and skip SNS inbetween.

Another option is to replace your EC2 instance with a Lambda function and then either directly notify the Lambda or make the Lambda poll the SQS queue (see dev.to/frosnerd/understanding-the-...).

Without knowing more of your context it is hard to recommend something concrete but if possible I'd go for the following setup if I were you: S3 -> SQS -> Lambda. That's easy to manage and gives a reasonable amount of resilience. Another, simpler option would be S3 -> Lambda but then you might lose events in case your Lambda function is broken as S3 will only retry a couple of times as it invokes the Lambda synchronously.

I hope that helps!



Thanks a lot for the details Frank. to give you more context, below digram might help with the requirement:


Great insight Frank. Thanks.

I need to provide a solution through Lambda to notifies an SNS topic when a file is received outside of a specific time in S3. Files are received in S3 at a specific time (4 am - 5 am EST). Do you have any custom solution or code I can use in my Lambda function to send out an SNS notification when the files are received after 5 am?




I see two options:

1) Configure an S3 event notification towards a Lambda function and in that Lambda check the time of the PUT event. If the time is not compliant, make the Lambda publish to your SNS topic.

2) Configure a scheduled event in CloudWatch that periodically invokes your Lambda (e.g. every hour or every day) and then make the Lambda check for any files that have been uploaded at a non-compliant time.

Option 1 is more real-time but might generate a lot of noise and cost in the case of many files. Option 2 has some lag but you can use it to build a summary about all the files that have been uploaded quite easily.

What do you think?


Exactly what I need. I'm going to use Python instead of Java, and Microsoft Teams instead of Slack.

BTW: you may use lambda layers to maintain your dependencies and code separately


AWS documentation is missing the aws_lambda_permission, I noticed it and that actually brought me here. Thanks for confirming it. Awesome post overall, thanks a lot!!


Frank, thanks for the very nice and useful post:-)