Eventing in AWS


Amazon EventBridge is a service that offers us a serverless event bus with real-time access to changes in our AWS environments, custom applications, or third-party SaaS vendors.

EventBridge was the evolution essentially of the CloudWatch Events service offering which used a single default bus for all events and rules. One of the beautiful things about EventBridge is that it uses the same CloudWatch Events API to allow for a smoother transition with our applications and our system design.

Key Components

Event Bus🚌:

Serverless pipeline that receives and sends events from different sources. These can be default AWS services, custom sources, or they can be third-party sources.

  • Default - Created at account creation, 1 per region. Used formerly by CloudWatch Events.

  • Custom - Created by users. They can be cross-account or cross-region as well if you define appropriate Resource Policy like so:

      "Version": "2012-10-17",
      "Statement": [
          "Sid": "allow_all_accounts_from_organization_to_put_events",
          "Effect": "Allow",
          "Principal": "*",
          "Action": "events:PutEvents",
          "Resource": "arn:aws:events:us-east-1:<ACCOUNT_ID>:event-bus/custom-event-bus",
          "Condition": {
            "StringEquals": {
              "aws:PrincipalOrgID": "ou-12345"
  • SaaS vendors - Third-Party event buses like SalesForce, Datadog and others*



There are 3 main components to an event which we need to lookout for when creating rules to target events:

  • detail - A JSON object that contains information about the event. The service generating the event determines the content of this field.
  • source - The source field is meant to identify what source the event came from. All events that originate from other AWS services begin with aws.
  • detail_type - The detail-type is related to the source AWS service or customer-generated event source. The detail-type identifies, in combination with the source field, what event occurred.

Here’s an example of a EC2 Termination event:

	"version": "0",
	"id": "ec55f137-57e0-43e0-a9fa-df5d188c17d0",
	"detail-type": "EC2 Instance State-change Notification",
	"source": "aws*ec2",
	"account": "123456789101",
	"time": "2022-02-22T18:43:48Z",
	"region": "us-east-1",
	"resources": [
	"detail": {
		"instance-id": "i-1234567890abcdef0",
		"state": "terminated"

Rules are created and associated with Event Buses and these rules are evaluated whenever an event is received. If an event matches a rule, it will trigger that action and send the events to their designated targets. There’s fan-out pattern possible i.e single rule triggering multiple targets.

Create rules the following ways:

  • Event pattern

    • EventBridge supports declarative content filtering using event patterns. Rules use event patterns to select events and send them to targets. An event pattern either matches an event or it doesn’t.

      The following event pattern processes all Amazon EC2 instance-termination events.

        "source": ["aws.ec2"],
        "detail-type": ["EC2 Instance State-change Notification"],
        "detail": {
          "state": ["terminated"]
    • You can write complex event patterns (with prefix matching, suffix matching, numeric matching…1) that only match events under very specific circumstances.

      The following event pattern processes all S3 Object Created events in buckets with names prefixed with example-bucket and files with ".png" suffix.

        "source": ["aws.s3"],
        "detail-type": ["Object Created"],
        "detail": {
          "bucket": {
            "name": [ { "prefix": "example-bucket" } ]
          "object": {
            "key": [ { "suffix": ".png" } ]
  • Schedule

    • Event Bridge rule can run periodically on a schedule (cron and rate).
    • With EventBridge Scheduler, you can schedule one-time or recurrently tens of millions of tasks across many AWS services without provisioning or managing underlying infrastructure.
    • While you can use rules to schedule tasks, Amazon EventBridge Scheduler is better suited for scheduling events at scale. If you need to schedule tasks that exceed the EventBridge rules quotas on the number of rules or invocation throughput, EventBridge Scheduler might fit your needs.
  • There is a limit of 300 rules per event bus.
  • You can only create scheduled rules using the default event bus.
  • Targets are resources (like Lambda) and endpoints(like REST API) that EventBridge can send events to after matching a rule.
  • For endpoints any HTTP method is supported besides CONNECT and TRACE.
  • We can transform2 event inputs, if need be, before events get sent to their destinations.
  • Requests cannot extend past 5 seconds or a timeout will occur.
  • We can configure upto 5 targets for a single EventBridge rule

Archive and Replay

  • Amazon EventBridge can capture past events and archive📮them for testing later on. We no longer need to wait for new events to come in.
  • We specify event patterns to designate which type of events we want to archive
  • AWS encrypts event data by default using an AWS-owned customer managed key (CMK)
  • We dictate the retention period of an archived event. This can be indefinite!
  • We can replay📨 the archived events when need be. Obviosly this is huge for testing and troubleshooting. We can specify the following options when replaying:
    • Source archive
    • Start and end time for the event replay
    • Target event bus (currently limited to source event bus)
    • Replays processed events based on the event time and repeats in 1-minute intervals
    • Maximum of 10 concurrent replays

Schema Registry

Schema🔗 registries are a container that defines the structure of events that are sent to EventBridge.

  • We can create and upload our own schemas, or we can infer/discover them based on events within EventBridge.
  • Both OpenAPI 3 and JSONSchema Draft4 formats are supported.
  • Schemas could be versioned.
  • Code Bindings are downloadable libraries and packages that can be used directly in your code to help develop applications that use events in EventBridge. And we can download them for the language of our choice like Python, Java, Go, Typescript.

Use Cases

There’s a plethora of use cases for EventBridge integration, here’s some examples:

  • Reduce Costs by Deleting Orphaned EBS Volumes

  • Remediating EC2 Auto Scaling Group Modifications with EventBridge

  • Implementing Amazon GuardDuty and Amazon EventBridge

  • Triggering Events with CloudTrail Logs

Event Driven Architectures

Here’s a few important motivations for a Event Driven Architecture and EventBridge is at the cornerstone of this pattern:


Event-driven architectures are push-based, so everything happens on-demand as the event presents itself in the router. This way, you’re not paying💰 for continuous polling to check for an event. This means less network bandwidth consumption, less CPU utilization, less idle fleet capacity, and less SSL/TLS handshakes.


Let the cloud provider handle the scaling for you. No more estimating workloads and scaling policies. And if you’ve been a part of that process, you know it can be a nightmare💤💀


By decoupling⛓️ your services, they are only aware of the event router, not each other. This means that your services are interoperable, but if one service has a failure, the rest will keep running. The event router acts as an elastic buffer that will accommodate surges in workloads.


An event router acts as a centralized location to audit🕵 your application and define policies. These policies can restrict who can publish and subscribe to a router and control which users and resources have permission to access your data.

Further Read

https://aws.amazon.com/blogs/compute/introducing-amazon-eventbridge-scheduler/ https://dev.to/deeheber/eventbridge-emoji-event-patterns-53h4