Startup probe concept in Kubernetes - Step-by-Step Execution

Process Flow - Startup probe concept

Pod starts

↓

Startup probe runs

↓

Probe success?

No→Retry after delay

↓

Failure threshold reached?

No→Retry

↓

Container marked started

↓

Readiness & Liveness probes start

The startup probe runs repeatedly after pod start until it succeeds or fails too many times. Success means container is ready to run readiness and liveness probes. Failure triggers pod restart.

Execution Sample

Kubernetes

startupProbe:
  exec:
    command: ["cat", "/tmp/healthy"]
  failureThreshold: 3
  periodSeconds: 5

This startup probe runs a command every 5 seconds, retries up to 3 times before failing.

Process Table

Attempt	Time (s)	Command Result	Probe Status	Action
1	0	File not found	Failure	Retry after 5s
2	5	File not found	Failure	Retry after 5s
3	10	File not found	Failure	Failure threshold reached, pod restart
4	15	Pod restarts	N/A	Startup probe restarts

💡 Failure threshold 3 reached at 10s, pod restarts and startup probe restarts

Status Tracker

Variable	Start	After Attempt 1	After Attempt 2	After Attempt 3	After Restart
failureCount	0	1	2	3	0
probeStatus	N/A	Failure	Failure	Failure	N/A
podState	Starting	Starting	Starting	Restarting	Starting

Key Moments - 3 Insights

Why does the pod restart after 3 failed attempts?

Because failureThreshold is 3, after 3 consecutive failures (see execution_table rows 1-3), Kubernetes restarts the pod to try again.

When do readiness and liveness probes start running?

They start only after the startup probe succeeds, which means the container is fully started (not shown in this failure example).

What happens to failureCount after pod restart?

It resets to 0 because the pod and its probes start fresh (see variable_tracker After Restart).

Visual Quiz - 3 Questions

Look at the execution table, what is the failureCount after the second attempt?

A 3

B 1

C 2

D 0

💡 Hint

Check variable_tracker row for failureCount after Attempt 2

At which attempt does the pod restart happen?

A After Attempt 3

B After Attempt 1

C After Attempt 2

D After Attempt 4

💡 Hint

See execution_table Action column for when failure threshold triggers restart

If the failureThreshold was set to 5, what would change in the execution table?

A Pod would restart after 3 failures

B Pod would restart after 5 failures

C Pod would never restart

D Pod would restart immediately

💡 Hint

failureThreshold controls max failures before restart, see execution_table rows and exit_note

Concept Snapshot

Startup probe runs after pod starts to check if container is ready.
It retries command or check every periodSeconds.
If failures reach failureThreshold, pod restarts.
Success means readiness and liveness probes start.
Useful for slow-starting containers.

Full Transcript

The startup probe in Kubernetes runs repeatedly after a pod starts to check if the container is ready. It executes a command or HTTP check every periodSeconds. If the probe fails, it retries until failureThreshold is reached. If failures reach this threshold, Kubernetes restarts the pod to try again. Once the startup probe succeeds, readiness and liveness probes begin to monitor the container's health. This mechanism helps with containers that take longer to start, preventing premature restarts.

Practice

1. What is the main purpose of a startupProbe in Kubernetes?

easy

A. To monitor CPU and memory usage of the pod

B. To restart the pod immediately when it crashes

C. To check if the application has started before other health checks run

D. To manage network traffic routing to the pod

Solution

1. Step 1: Understand the role of startupProbe

   The startupProbe is designed to check if an application has started successfully before Kubernetes runs readiness or liveness probes.

2. Step 2: Differentiate from other probes

   Unlike liveness or readiness probes, startupProbe prevents Kubernetes from killing pods that take longer to start.

3. Final Answer:

   To check if the application has started before other health checks run -> Option C

4. Quick Check:

   startupProbe checks app start status = A [OK]

Hint: Startup probe delays other checks until app starts [OK]

Common Mistakes:

• Confusing startupProbe with livenessProbe
• Thinking startupProbe manages resource usage
• Assuming startupProbe handles traffic routing

2. Which of the following is the correct syntax to define a startupProbe using an HTTP GET request in a pod spec?

easy

A. startupProbe:
exec:
command: ["curl", "-f", "http://localhost:8080/healthz"]
timeoutSeconds: 5

B. startupProbe:
httpGet:
path: /healthz
port: 8080
failureThreshold: 30
periodSeconds: 10

C. startupProbe:
tcpSocket:
port: 8080
initialDelaySeconds: 5

D. startupProbe:
httpGet:
path: /status
port: 80
successThreshold: 3

Solution

1. Step 1: Identify correct HTTP GET syntax for startupProbe

   The startupProbe uses httpGet with path and port fields, plus timing settings like failureThreshold and periodSeconds.

2. Step 2: Compare options for correctness

   startupProbe: httpGet: path: /healthz port: 8080 failureThreshold: 30 periodSeconds: 10 correctly uses httpGet with path /healthz, port 8080, and timing fields. Other options use exec or tcpSocket or wrong ports/fields.

3. Final Answer:

   startupProbe: httpGet: path: /healthz port: 8080 failureThreshold: 30 periodSeconds: 10 -> Option B

4. Quick Check:

   startupProbe HTTP GET with path and port = A [OK]

Hint: HTTP GET startupProbe needs path, port, failureThreshold [OK]

Common Mistakes:

• Using exec probe syntax instead of httpGet
• Missing failureThreshold or periodSeconds
• Incorrect port or path values

3. Given this pod spec snippet, what will happen if the startupProbe fails repeatedly during startup?

startupProbe:
  httpGet:
    path: /start
    port: 8080
  failureThreshold: 5
  periodSeconds: 2
livenessProbe:
  httpGet:
    path: /live
    port: 8080
  initialDelaySeconds: 10
  periodSeconds: 5

medium

A. Kubernetes will never restart the pod because startupProbe is ignored

B. Kubernetes will restart the pod after 10 seconds of livenessProbe failures

C. Kubernetes will restart the pod after 10 seconds of readinessProbe failures

D. Kubernetes will restart the pod after 10 seconds of startupProbe failures

Solution

1. Step 1: Calculate startupProbe failure timeout

   failureThreshold 5 and periodSeconds 2 means Kubernetes waits 5*2=10 seconds of failed startupProbe checks before restarting.

2. Step 2: Understand probe precedence during startup

   During startup, startupProbe controls pod health. LivenessProbe starts after startupProbe succeeds, so restart depends on startupProbe failures first.

3. Final Answer:

   Kubernetes will restart the pod after 10 seconds of startupProbe failures -> Option D

4. Quick Check:

   startupProbe failureThreshold * periodSeconds = restart delay [OK]

Hint: Restart delay = failureThreshold x periodSeconds of startupProbe [OK]

Common Mistakes:

• Confusing startupProbe failure timing with livenessProbe
• Assuming readinessProbe triggers restart
• Ignoring startupProbe during pod startup

4. You configured a startupProbe but your pod keeps restarting immediately. Which fix is most likely correct?

medium

A. Increase failureThreshold and periodSeconds to allow more startup time

B. Remove the startupProbe to rely only on livenessProbe

C. Change the startupProbe to a readinessProbe

D. Set initialDelaySeconds to 0 in the livenessProbe

Solution

1. Step 1: Identify cause of immediate restarts

   If the startupProbe fails too quickly, Kubernetes restarts the pod before it finishes starting.

2. Step 2: Adjust startupProbe timing

   Increasing failureThreshold and periodSeconds gives the app more time to start before Kubernetes kills it.

3. Final Answer:

   Increase failureThreshold and periodSeconds to allow more startup time -> Option A

4. Quick Check:

   Longer startupProbe timeout prevents premature restarts [OK]

Hint: Longer failureThreshold and periodSeconds prevent early restarts [OK]

Common Mistakes:

• Removing startupProbe loses slow start protection
• Confusing startupProbe with readinessProbe
• Setting initialDelaySeconds on livenessProbe doesn't fix startup timing

5. You have a slow-starting app that takes about 3 minutes to initialize. How should you configure the startupProbe to avoid premature pod restarts?

hard

A. Set failureThreshold to 18 and periodSeconds to 10

B. Set failureThreshold to 3 and periodSeconds to 30

C. Set failureThreshold to 1 and periodSeconds to 100

D. Disable startupProbe and rely on livenessProbe with initialDelaySeconds 180

Solution

1. Step 1: Calculate total startupProbe timeout needed

   App needs 3 minutes (180 seconds) to start. Total timeout = failureThreshold x periodSeconds.

2. Step 2: Choose values matching 180 seconds

   Set failureThreshold to 18 and periodSeconds to 10: 18 x 10 = 180 seconds, allowing full startup time before restart.

3. Step 3: Evaluate other options

   Set failureThreshold to 1 and periodSeconds to 100: failureThreshold to 1 and periodSeconds to 100 = 100 seconds (too short). Set failureThreshold to 3 and periodSeconds to 30: 3 x 30 = 90 seconds (too short). Disable startupProbe and rely on livenessProbe with initialDelaySeconds 180 disables startupProbe, risking premature restarts from livenessProbe.

4. Final Answer:

   Set failureThreshold to 18 and periodSeconds to 10 -> Option A

5. Quick Check:

   failureThreshold x periodSeconds = startup timeout [OK]

Hint: Multiply failureThreshold by periodSeconds to match startup time [OK]

Common Mistakes:

• Configuring insufficient total timeout (failureThreshold x periodSeconds < 180s)
• Disabling startupProbe causing premature restarts
• Setting only initialDelaySeconds without startupProbe