Understanding AWS EventBridge: The Automation Service Explained
Jay Tillu
Arjun, a freshly minted cloud engineer, was deep into AWS training. His goal? Pass the AWS Certified Solutions Architect – Associate exam and become a master of building smart, event-driven architectures.
But Arjun had a problem.
He needed to:
Run a Lambda every night at 2 AM.
Get notified when the root account was used (huge security risk).
Automatically react when a build failed in AWS CodeBuild.
Trigger EC2 actions when specific events happened.
At first, he thought of writing custom scripts and setting up crons on an EC2 instance. But it felt… old-school. 🤨
That’s when he stumbled upon Amazon EventBridge.
🧠 The Core Concept: What Is Amazon EventBridge?
Imagine if all your AWS services could talk to each other and react to events in real-time—without you writing a bunch of glue code.
That's EventBridge a smart middleman that listens to everything happening in AWS—and when something important happens, it instantly takes action.
EventBridge allows services (like EC2, S3, CodeBuild, etc.), third-party apps (like Datadog, Zendesk), or even your own apps to send events, and take actions in response—like triggering a Lambda, sending an SNS alert, or starting an ECS task.
It is a managed, pay-as-you-go service that lets your apps and AWS services talk through events—with zero infrastructure headaches.
You can:
Run a function when a file is uploaded
Get notified when root user logs in
Schedule tasks (like a cron job)
Trigger workflows when a build fails
And you do all of this without setting up servers. Just connect the dots.
🔁 Arjun’s First Use-Case: Scheduled Lambda
Arjun needed to run a script every day at 2 AM. Normally, he’d think cron.
But with EventBridge, he simply created a rule like this:
And that rule triggered his Lambda, without needing a server or worrying about uptime. 🔥
He smiled. “Okay, this is cool.”
🚨 Root User Sign-In Alert? Handled!
Security was always on Arjun’s mind.
He created a rule in EventBridge to listen for the RootLogin event from CloudTrail.
When that happened, EventBridge fired a message to SNS, which sent Arjun an email:
📬 "ALERT: Someone used the root account!"
No cron. No polling. Just pure reaction.
🛠️ How Amazon EventBridge Works
🧃 1. Event Sources – “Who’s Speaking?”
Imagine every AWS service is like a character in a movie.
They’re all doing stuff:
EC2 starts a server
S3 uploads a file
CloudTrail logs a login
Or even an external app like Auth0, Zendesk, or Shopify does something
These actions are called events.
So the event source is the origin of that action.
They shout:
“Hey! Something happened!” 👋
And Arjun, being the wise engineer, wants to listen to those important events only.
🚍 2. Event Bus – “The Messenger That Delivers Events”
The Event Bus is like a delivery van that carries these events to whoever is interested.
There are 3 types of buses:
🟢 Default Event Bus
Handles all events from AWS services
Automatically set up
Example: EC2 stops → Event sent to the default event bus
🟣 Partner Event Bus
For 3rd-party SaaS tools (like Auth0, Datadog)
They can publish events directly to your AWS account
🟡 Custom Event Bus
For your own applications
You can write custom events like
orderPlaced,userDeletedetc., and send them to a custom bus
So think of each Event Bus as a channel of communication, depending on who is talking.
🔎 3. Rules – “What Are You Interested In?”
Now Arjun doesn’t want to listen to every event. That’s noisy.
So he sets up rules, like:
"Only tell me when an EC2 instance is terminated"
"Only when the root user signs in"
"Only when an object is added toproduction-bucket"
A rule is like a filter.
It looks at the event and says:
“Do I care?”
❌ No → ignore.
✅ Yes → pass it to the target.
🎯 4. Targets – “What Should Happen Now?”
This is the fun part.
Once a rule matches, it can trigger an action. These are called targets.
Here’s what Arjun can choose to do:
🚀 Lambda Function – Run code instantly
📨 SNS Topic – Notify a team via email, SMS
📬 SQS Queue – Push into a message queue for another service
⚙️ SSM Automation – Trigger automated tasks (like patching)
🛠️ CodeBuild – Start building and deploying code
📦 ECS Task – Run a container
🧭 Step Function – Kick off a multi-step workflow
And it’s all serverless – Arjun doesn’t set up any infrastructure. AWS handles everything behind the scenes.
📈 It Scales Like Magic (aka Serverless)
There are no servers to manage.
No EC2. No autoscaling groups. Nothing.
It handles 100 events or 1 million the same way
No maintenance
Pay only per event processed
That’s why it’s called a serverless event bus — it’s like a central nervous system that just works.
🔁 Event Replay & Archive
One day, Arjun’s app broke because of a bug in his Lambda function. Arjun was smart — he had enabled event archiving in EventBridge.
What does that mean?
EventBridge stores every event it receives
Like a digital notebook that remembers every action — in JSON format
This is helpful when something goes wrong, and you want a second chance
After fixing the bug, he used Replay to resend the same old event — just like hitting a rewind button.
✅ Problem solved.
❌ No lost data.
Even in hundreds or thousands of events, you can filter the one you want to replay based on:
Event time
Event type (e.g. only S3 upload events)
Specific resources (like a certain S3 bucket)
🧩 Schema Registry — Making Sense of the Event Data
Arjun was getting lots of events — like S3 uploads, EC2 actions, and more.
But each event was like a different-shaped puzzle piece — confusing to work with!
That’s where Schema Registry comes in:
It watches the events that go through EventBridge
It figures out their structure (what fields they have, like
bucketName,timestamp, etc.)It gives Arjun a ready-made guide (called a "schema") showing what the data looks like
Even better — Arjun can download code templates that already understand this data.
So instead of guessing or checking JSON files manually, Arjun just says:
“Hey AWS, give me the blueprint and the code.”
And boom — he's coding faster and safer. It saves time, avoids errors, and makes coding with events way easier, especially when there are lots of event types coming in.
🔒 Cross-Account Sharing — One Dashboard for All Teams
Arjun’s company had many AWS accounts.
Using permissions (resource policies), he allowed other accounts to send their events to one central account.
Now, Arjun had one clear view of everything happening across all teams — all in one place.
No more jumping between accounts!
🧠 Arjun’s Takeaway
“EventBridge lets me build smart reactions to anything happening in my system — whether it's an AWS service, a 3rd-party app, or my own code — without managing any of the plumbing.”
It's like turning your AWS account into a fully automated robot brain:
It listens
It filters
It reacts
It scales
All without servers.
More AWS SAA Articles
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AWS Load Balancers: How Deregistration Delay Ensures Seamless Shutdowns
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Written by
Jay Tillu
Jay Tillu
Hello! I'm Jay Tillu, an Information Security Engineer at Simple2Call. I have expertise in security frameworks and compliance, including NIST, ISO 27001, and ISO 27701. My specialities include Vulnerability Management, Threat Analysis, and Incident Response. I have also earned certifications in Google Cybersecurity and Microsoft Azure. I’m always eager to connect and discuss cybersecurity—let's get in touch!