CloudWatch Logs in AWS - Step-by-Step Execution

Process Flow - CloudWatch Logs

Application generates logs

↓

Logs sent to CloudWatch Logs

↓

CloudWatch Logs stores logs in Log Groups

↓

User sets Filters and Alarms

↓

CloudWatch triggers Alarms or Insights queries

↓

User views logs, metrics, or receives alerts

Logs flow from an application to CloudWatch Logs, where they are stored, filtered, and monitored with alarms or queries.

Execution Sample

AWS

1. Application writes log entry
2. Log entry sent to CloudWatch Logs
3. Log stored in Log Group
4. Filter applied to detect errors
5. Alarm triggers if error count high

This sequence shows how logs are generated, stored, filtered, and trigger alarms in CloudWatch Logs.

Process Table

Step	Action	Input/Condition	Result/Output
1	Application writes log	Log message: 'Error: Disk full'	Log entry created
2	Send log to CloudWatch Logs	Log entry created	Log entry received by CloudWatch Logs
3	Store log in Log Group	Log entry received	Log stored in specified Log Group
4	Apply filter	Filter pattern: 'Error'	Log matches filter pattern
5	Check alarm condition	Error count > threshold?	Alarm state: ALARM triggered
6	Notify user	Alarm triggered	User receives alert notification
7	User views logs	Logs stored	Logs displayed in CloudWatch Logs console
8	End	No new logs	Waiting for new log entries

💡 Execution stops when no new logs are generated or processed.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
Log Entry	None	'Error: Disk full'	Sent to CloudWatch	Stored in Log Group	Matches filter	Triggers alarm	Stored and monitored
Alarm State	OK	OK	OK	OK	OK	ALARM	ALARM

Key Moments - 3 Insights

Why does the alarm trigger only after the filter matches logs?

Because the alarm monitors filtered logs matching specific patterns (see step 4 and 5 in execution_table). Without matching logs, the alarm stays OK.

What happens if no logs match the filter pattern?

No alarm is triggered and logs are still stored (step 4 shows filter matching; if no match, alarm condition is not met).

Can users view logs before alarms trigger?

Yes, logs are stored and available for viewing anytime after step 3, regardless of alarm state.

Visual Quiz - 3 Questions

Look at the execution_table, what is the alarm state after step 4?

A ALARM

B OK

C INSUFFICIENT_DATA

D UNKNOWN

💡 Hint

Check the 'Alarm State' variable in variable_tracker after step 4.

At which step does the log entry get stored in the Log Group?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Refer to the 'Action' column in execution_table for when storage happens.

If the filter pattern changes to 'Warning', how would step 4's result change?

A Log does not match filter

B Log matches filter

C Alarm triggers immediately

D Log entry is deleted

💡 Hint

Look at step 4 where the filter pattern is 'Error' and the log message contains 'Error'.

Concept Snapshot

CloudWatch Logs collects and stores logs from applications.
Logs are grouped in Log Groups.
Filters detect patterns in logs.
Alarms trigger based on filtered log metrics.
Users can view logs and receive alerts.
This helps monitor and troubleshoot systems.

Full Transcript

CloudWatch Logs is a service that collects log data from applications and stores it in organized groups called Log Groups. When an application writes a log message, it is sent to CloudWatch Logs and stored. Users can set filters to find specific patterns in logs, such as errors. If the number of matching logs exceeds a threshold, an alarm triggers to notify the user. Logs remain accessible for viewing at any time. This process helps users monitor their systems and quickly respond to issues.

Practice

1. What is the primary purpose of a CloudWatch Logs log group?

easy

A. To act as a container for multiple log streams

B. To store individual log events directly

C. To trigger alarms based on log data

D. To archive logs permanently

Solution

1. Step 1: Understand log group role

   A log group is a container that holds multiple log streams, organizing logs by application or resource.

2. Step 2: Differentiate log streams and groups

   Log streams are individual sequences of log events within a log group, not the group itself.

3. Final Answer:

   To act as a container for multiple log streams -> Option A

4. Quick Check:

   Log group = container for streams [OK]

Hint: Log groups hold streams; streams hold logs [OK]

Common Mistakes:

• Confusing log groups with log streams
• Thinking logs are stored directly in groups
• Assuming groups trigger alarms directly

2. Which of the following is the correct AWS CLI command to create a new CloudWatch Logs log group named MyAppLogs?

easy

A. aws cloudwatch create-log-group --name MyAppLogs

B. aws logs create-log-group --log-group-name MyAppLogs

C. aws logs new-log-group --group-name MyAppLogs

D. aws logs add-log-group --name MyAppLogs

Solution

1. Step 1: Identify correct AWS CLI service and command

   The correct service is 'logs' and the command to create a log group is 'create-log-group'.

2. Step 2: Check command syntax

   The parameter for the log group name is '--log-group-name', matching aws logs create-log-group --log-group-name MyAppLogs exactly.

3. Final Answer:

   aws logs create-log-group --log-group-name MyAppLogs -> Option B

4. Quick Check:

   Correct CLI syntax = aws logs create-log-group --log-group-name MyAppLogs [OK]

Hint: Use 'aws logs create-log-group' with --log-group-name [OK]

Common Mistakes:

• Using wrong service name like 'cloudwatch' instead of 'logs'
• Incorrect command verbs like 'new-log-group'
• Wrong parameter names like '--name'

3. Given the following AWS CLI command, what will be the retention period set for the log group AppLogs?

aws logs put-retention-policy --log-group-name AppLogs --retention-in-days 14

medium

A. Logs will be kept for 7 days

B. Logs will be kept indefinitely

C. Logs will be kept for 14 days

D. Logs will be deleted immediately

Solution

1. Step 1: Understand the put-retention-policy command

   This command sets how many days logs are retained before deletion.

2. Step 2: Interpret the retention-in-days value

   The parameter '--retention-in-days 14' means logs are kept for 14 days.

3. Final Answer:

   Logs will be kept for 14 days -> Option C

4. Quick Check:

   Retention days = 14 [OK]

Hint: Retention days = value in --retention-in-days [OK]

Common Mistakes:

• Assuming default retention instead of set value
• Confusing retention period with log group creation
• Thinking logs are kept indefinitely by default

4. You run this command but get an error:

aws logs put-retention-policy --log-group-name NonExistentGroup --retention-in-days 30

What is the most likely cause?

medium

A. The log group 'NonExistentGroup' does not exist

B. Retention period 30 days is not allowed

C. You must specify a log stream name instead

D. The AWS CLI command is misspelled

Solution

1. Step 1: Check existence of log group

   The command requires the log group to exist before setting retention.

2. Step 2: Validate retention period and command syntax

   Retention of 30 days is valid and the command syntax is correct.

3. Final Answer:

   The log group 'NonExistentGroup' does not exist -> Option A

4. Quick Check:

   Retention needs existing log group [OK]

Hint: Retention policy requires existing log group [OK]

Common Mistakes:

• Assuming retention period is invalid
• Confusing log stream with log group
• Thinking command syntax is wrong

5. You want to organize logs from multiple microservices in CloudWatch Logs. Which approach best follows AWS best practices?

hard

A. Store all logs in a single log stream for simplicity

B. Create one log stream for all microservices in a single log group

C. Create one log group per instance and one log stream per microservice

D. Create one log group per microservice and multiple log streams per instance

Solution

1. Step 1: Understand log group and stream organization

   Best practice is to use log groups to separate different applications or microservices.

2. Step 2: Assign log streams per instance or source

   Within each log group, use multiple log streams to separate logs by instance or container.

3. Step 3: Evaluate options

   Create one log group per microservice and multiple log streams per instance matches best practice: one log group per microservice, multiple streams per instance.

4. Final Answer:

   Create one log group per microservice and multiple log streams per instance -> Option D

5. Quick Check:

   Group per microservice, streams per instance [OK]

Hint: Group by microservice, stream by instance [OK]

Common Mistakes:

• Mixing microservices in one log group
• Using one stream for all logs causing confusion
• Creating groups per instance instead of per service