CloudWatch Events (EventBridge) in AWS - Step-by-Step Execution

Process Flow - CloudWatch Events (EventBridge)

Event Occurs

↓

EventBridge Receives Event

↓

Match Event to Rule?

No→Discard Event

Yes↓

Send Event to Target(s)

↓

Target Executes Action

Events happen and are sent to EventBridge. EventBridge checks rules to find matches. If matched, it sends the event to targets which then act on it.

Execution Sample

AWS

rule = {
  "EventPattern": {"source": ["aws.ec2"]},
  "Targets": [{"Id": "1", "Arn": "arn:aws:lambda:..."}]
}

# EventBridge receives event from EC2 instance
# Rule matches event source
# Target Lambda function invoked

This setup listens for EC2 events and triggers a Lambda function when such an event occurs.

Process Table

Step	Event Received	Rule Match Check	Match Result	Action Taken
1	{"source":"aws.ec2","detail-type":"EC2 Instance State-change Notification"}	Check if source is 'aws.ec2'	Yes	Send event to Lambda target
2	{"source":"aws.s3","detail-type":"Object Created"}	Check if source is 'aws.ec2'	No	Discard event
3	{"source":"aws.ec2","detail-type":"EC2 Instance Launch"}	Check if source is 'aws.ec2'	Yes	Send event to Lambda target
4	No new event	N/A	N/A	Stop processing

💡 No new event to process, execution stops

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
event	None	{"source":"aws.ec2","detail-type":"EC2 Instance State-change Notification"}	{"source":"aws.s3","detail-type":"Object Created"}	{"source":"aws.ec2","detail-type":"EC2 Instance Launch"}	None
rule_match	False	True	False	True	N/A
action	None	Invoke Lambda	Discard	Invoke Lambda	Stop

Key Moments - 2 Insights

Why does the event with source 'aws.s3' get discarded?

Because the rule only matches events where the source is 'aws.ec2' as shown in execution_table row 2 where the match result is No and action is Discard.

What happens when an event matches the rule?

When the event matches the rule (source is 'aws.ec2'), EventBridge sends the event to the target, which in this example is a Lambda function, as shown in rows 1 and 3.

Visual Quiz - 3 Questions

Look at the execution table, what is the action taken at step 2?

A Invoke Lambda function

B Discard event

C Send event to S3

D No action

💡 Hint

Check the 'Action Taken' column in row 2 of the execution_table

At which step does the event source 'aws.ec2' trigger the Lambda target?

A Step 2

B Step 4

C Step 1 and Step 3

D Step 3 only

💡 Hint

Look at the 'Event Received' and 'Action Taken' columns for steps with source 'aws.ec2'

If the rule was changed to match source 'aws.s3', what would happen at step 1?

A Event would be discarded

B Event would trigger Lambda

C No change, event discarded

D EventBridge would error

💡 Hint

Refer to how rule matching works in execution_table and variable_tracker

Concept Snapshot

CloudWatch Events (EventBridge) listens for events from AWS services.
Rules define which events to catch based on patterns like source.
When an event matches a rule, EventBridge sends it to targets (Lambda, SQS, etc).
If no match, event is discarded.
This enables automated responses to AWS events.

Full Transcript

CloudWatch Events, also called EventBridge, works by receiving events from AWS services or custom sources. It checks each event against rules that specify patterns to match, such as the event source. If the event matches a rule, EventBridge forwards it to one or more targets, like Lambda functions, which then perform actions. If the event does not match any rule, it is discarded. This process allows automatic reactions to changes or actions in your AWS environment without manual intervention.