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.

Practice

1. What is the primary purpose of AWS CloudWatch Events (EventBridge)?

easy

A. To create virtual servers in the cloud

B. To store large amounts of data for analysis

C. To manage user permissions and access

D. To automatically respond to events by triggering actions

Solution

1. Step 1: Understand EventBridge role

   EventBridge watches for events happening in AWS or custom apps and triggers actions automatically.

2. Step 2: Compare options

   Virtual servers relate to EC2, data storage to S3, user permissions to IAM; only responding to events matches EventBridge.

3. Final Answer:

   To automatically respond to events by triggering actions -> Option D

4. Quick Check:

   EventBridge triggers actions on events = C [OK]

Hint: EventBridge links events to actions automatically [OK]

Common Mistakes:

• Confusing EventBridge with storage or compute services
• Thinking it manages user permissions
• Assuming it creates servers

2. Which of the following is the correct JSON snippet to create a CloudWatch Events rule that triggers every 5 minutes?

easy

A. {"ScheduleExpression": "rate(5 minute)"}

B. {"ScheduleExpression": "cron(0/5 * * * ? *)"}

C. {"ScheduleExpression": "rate(5 min)"}

D. {"ScheduleExpression": "cron(5 * * * ? *)"}

Solution

1. Step 1: Understand schedule syntax

   EventBridge supports rate and cron expressions. For 5-minute intervals, cron syntax is cron(0/5 * * * ? *).

2. Step 2: Check each option

   {"ScheduleExpression": "rate(5 minute)"} uses rate(5 minute) which is invalid syntax (must use 'minutes' plural). {"ScheduleExpression": "cron(0/5 * * * ? *)"} uses correct cron syntax for every 5 minutes. {"ScheduleExpression": "rate(5 min)"} uses 'min' which is invalid. {"ScheduleExpression": "cron(5 * * * ? *)"} triggers at minute 5 every hour, not every 5 minutes.

3. Final Answer:

   {"ScheduleExpression": "cron(0/5 * * * ? *)"} -> Option B

4. Quick Check:

   Cron for every 5 minutes = D [OK]

Hint: Use cron(0/5 * * * ? *) for every 5 minutes [OK]

Common Mistakes:

• Using 'min' instead of 'minutes' in rate expressions
• Confusing cron syntax for intervals
• Using rate with incorrect pluralization

3. Given this EventBridge rule JSON snippet, what will happen when an EC2 instance state changes to 'stopped'?

{
  "Source": ["aws.ec2"],
  "DetailType": ["EC2 Instance State-change Notification"],
  "Detail": {"state": ["stopped"]}
}

medium

A. The rule triggers only when an EC2 instance stops

B. The rule triggers for any EC2 state change

C. The rule triggers only when an EC2 instance starts

D. The rule never triggers

Solution

1. Step 1: Analyze event pattern

   The pattern filters events from source 'aws.ec2' with detail type 'EC2 Instance State-change Notification' and detail state 'stopped'.

2. Step 2: Understand filtering effect

   Only events where the instance state changes to 'stopped' match this pattern, so the rule triggers only then.

3. Final Answer:

   The rule triggers only when an EC2 instance stops -> Option A

4. Quick Check:

   Detail state filter 'stopped' triggers only on stop = A [OK]

Hint: Detail filters limit triggers to specific event states [OK]

Common Mistakes:

• Assuming all EC2 state changes trigger the rule
• Confusing 'stopped' with 'started' state
• Ignoring the detail filter in the event pattern

4. You created an EventBridge rule to trigger a Lambda function on S3 object creation, but the Lambda never runs. What is the most likely cause?

medium

A. The rule's event pattern does not match S3 object creation events

B. The Lambda function has no code

C. S3 does not support EventBridge integration

D. The Lambda function is in a different AWS region

Solution

1. Step 1: Check EventBridge and S3 integration

   S3 supports EventBridge events for object creation, so integration is possible.

2. Step 2: Verify event pattern correctness

   If the event pattern does not correctly specify S3 object creation events, the rule won't trigger the Lambda.

3. Step 3: Consider Lambda region

   EventBridge and Lambda must be in the same region; cross-region triggers do not work.

4. Final Answer:

   The rule's event pattern does not match S3 object creation events -> Option A

5. Quick Check:

   Incorrect event pattern blocks trigger = B [OK]

Hint: Check event pattern matches actual event source and type [OK]

Common Mistakes:

• Assuming Lambda runs without correct event pattern
• Ignoring region mismatch issues
• Believing S3 can't send events to EventBridge

5. You want to create an EventBridge rule that triggers a Lambda function only when an EC2 instance is launched in a specific VPC. Which approach correctly achieves this?

hard

A. Use an event pattern filtering on 'source' as 'aws.ec2' and 'detail-type' as 'EC2 Instance State-change Notification' with 'detail.vpc-id' matching the VPC ID

B. Use a scheduled rule to check EC2 instances every minute and invoke Lambda if any are in the VPC

C. Use an event pattern filtering on 'source' as 'aws.ec2' and 'detail-type' as 'EC2 Instance Launch Successful' with 'detail.vpc-id' matching the VPC ID

D. Use an event pattern filtering on 'source' as 'aws.ec2' and 'detail-type' as 'EC2 Instance State-change Notification' with 'detail.state' as 'running'

Solution

1. Step 1: Identify correct event type for instance launch

   The event 'EC2 Instance Launch Successful' signals a new instance launch, not state changes.

2. Step 2: Filter by VPC ID in event detail

   Filtering on 'detail.vpc-id' ensures only instances in the specific VPC trigger the rule.

3. Step 3: Evaluate other options

   State-change notification with VPC ID does not indicate launch specifically. Scheduled polling is inefficient. State-change to running filters state but not launch event.

4. Final Answer:

   Use an event pattern filtering on 'source' as 'aws.ec2' and 'detail-type' as 'EC2 Instance Launch Successful' with 'detail.vpc-id' matching the VPC ID -> Option C

5. Quick Check:

   Launch event + VPC filter = A [OK]

Hint: Filter on launch event and VPC ID in event detail [OK]

Common Mistakes:

• Using state-change events instead of launch events
• Trying scheduled polling instead of event-driven
• Ignoring VPC ID filter in event pattern