Explore

Guided Scenarios

Scenario 1 – Intermittent 500 Errors (NullPointer)

Scenario (remind them):

Users report: “Sometimes login works, sometimes it fails.”

Log snippet:

2025-11-07 10:32:45 ERROR AuthService - Failed to validate user

Caused by: java.lang.NullPointerException

at com.app.auth.UserValidator.validate(UserValidator.java:54)

at com.app.auth.AuthService.login(AuthService.java:37)

On screen/board:

Show the log and ask:

Ask the group:

“Where will you first look – top or bottom of the trace?”

“Which is the most important line here?”

Expected: NullPointerException + the line UserValidator.java:54

“Is this App / DB / Infra?”

Expected: App

“What could be null?”

Inputs from front-end? Returned DB object? Dependency not injected?

You model:

“Step 1: Identify exception type → NullPointer”

“Step 2: Identify location → Class + line number”

“Step 3: Classify → App issue”

“Step 4: Next actions: Check that line in code, see which object is null, maybe cross-check sample request payload in logs.”

Scenario 2 – CrashLoopBackOff (Config File Missing)

Scenario:

After deployment, service keeps restarting.

Log:

2025-11-06 12:10:10 ERROR org.springframework.boot.SpringApplication - Application run failed

Caused by: java.lang.IllegalStateException: Failed to load ApplicationContext

Caused by: java.io.FileNotFoundException: /opt/configs/app-config.yaml

Ask:

“What is the real root cause line?”

Expected: FileNotFoundException: /opt/configs/app-config.yaml

“Is this App, DB, or Infra/deployment?”

Expected: App/Infra boundary (config/deployment)

“Why does K8s keep restarting the pod?”

App exits with error → K8s restarts → loop.

“What would you do now?”

kubectl exec into pod, check file path, ConfigMap/volume mount.

You can quickly sketch the mental flow on board:

“Big exception → ApplicationContext fail → Caused by FileNotFound → Check filesystem & config.”

Scenario 3 – OutOfMemory

2025-11-06 12:42:31 ERROR [main] - Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

at com.app.invoice.Generator.createReport(Generator.java:112)

at com.app.invoice.Service.generate(Service.java:45)

Task (write answers):

What is the root cause line?

Is this App / DB / Infra?

List two first actions you will take.

(Expected for debrief: – Root cause: OutOfMemoryError: Java heap space – Classification: App/JVM config – Actions: check heap usage with VisualVM/JConsole; review report size / memory heavy operations; consider increasing -Xmx.)

How to Run These 20 Minutes

5 mins – Give instructions and distribute scenarios.

10 mins – Groups discuss and fill answers.

5 mins – Quick debrief: pick 1 scenario, ask 1–2 groups to share their reasoning.

Encourage them to say the chain out loud:

“Exception type → Line → Domain → First action.”

That repetition builds the habit.

Log Triage Drill

Now do a fast round with short log snippets to build speed.

Format:

You show a snippet on screen.

Ask: “App, DB, or Infra?” and “What’s your first check?”

Let people shout answers / raise hands.

Mini-Case 1 – DB Credentials

Caused by: java.sql.SQLException: ORA-01017: invalid username/password; logon denied

Classification: DB/config

First check: credentials in application.properties, DB user validity.

Mini-Case 2 – SQLSyntaxErrorException

Caused by: java.sql.SQLSyntaxErrorException: ORA-00942: table or view does not exist

Classification: DB/schema

First check: table existence in DB, schema name, migrations.

⁠

Mini-Case 3 – HTTP 404

WARN org.springframework.web.servlet.PageNotFound - No mapping found for HTTP request with URI [/getUser]

Classification: App (routing)

First check: controller mapping, path, HTTP method.

Mini-Case 4 – Port Conflict

java.net.BindException: Address already in use: bind

Classification: Infra

First check: lsof -i :<port> or netstat.

Mini-Case 5 – Network to DB

Caused by: java.sql.SQLRecoverableException: IO Error: The Network Adapter could not establish the connection

Classification: Infra/Network → DB

First check: ping DB host, check DB up, firewall, connection string.

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