TCP probe configuration in Kubernetes - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a TCP probe in Kubernetes?

A TCP probe checks if a container is accepting TCP connections on a specified port. It helps Kubernetes know if the container is healthy by trying to open a TCP connection.

beginner

Which Kubernetes probe type uses a TCP socket to check container health?

The TCP Socket probe type uses a TCP connection to check if the container is ready or alive.

intermediate

What fields are required to configure a TCP probe in a Kubernetes pod spec?

You need to specify tcpSocket with a port number. Optionally, you can set initialDelaySeconds, periodSeconds, and timeoutSeconds.

intermediate

How does Kubernetes react if a TCP probe fails?

If the TCP probe fails, Kubernetes considers the container unhealthy. For liveness probes, it may restart the container. For readiness probes, it stops sending traffic to the container.

beginner

Example snippet: How to define a TCP readiness probe on port 8080 in a pod spec?

readinessProbe: tcpSocket: port: 8080 initialDelaySeconds: 5 periodSeconds: 10 timeoutSeconds: 1

What does a TCP probe check in Kubernetes?

A If the container accepts TCP connections on a port

B If the container returns HTTP status 200

C If the container's CPU usage is low

D If the container's disk space is sufficient

A TCP probe tries to open a TCP connection to check container health.

Which field specifies the port number in a TCP probe configuration?

A port

B path

C command

D host

The 'port' field inside 'tcpSocket' defines the port to check.

What happens if a liveness TCP probe fails repeatedly?

A Kubernetes ignores the failure

B Kubernetes deletes the pod permanently

C Kubernetes restarts the container

D Kubernetes scales the pod up

Failing liveness probes cause Kubernetes to restart the container.

Which probe type can use a TCP socket check?

A None of the above

B Liveness and Readiness probes

C Only Readiness probes

D Only Liveness probes

Both liveness and readiness probes can use TCP socket checks.

What is the purpose of 'initialDelaySeconds' in a TCP probe?

A Number of retries before failure

B Timeout for each probe attempt

C Interval between probes

D Wait time before starting probes after container start

'initialDelaySeconds' sets how long to wait before the first probe.

Explain how to configure a TCP readiness probe in a Kubernetes pod and why it is useful.

Think about how Kubernetes decides if a container is ready to receive requests.

Describe what happens when a TCP liveness probe fails and how Kubernetes responds.

Consider how Kubernetes keeps containers healthy automatically.

Practice

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

easy

A. To check the container's memory usage

B. To monitor CPU usage of a container

C. To verify the container's file system integrity

D. To check if a container accepts TCP connections on a specified port

Solution

1. Step 1: Understand TCP probe function

   A TCP probe tests if a container is accepting TCP connections on a specific port.

2. Step 2: Compare with other options

   CPU, memory, and file system checks are not done by TCP probes but by other mechanisms.

3. Final Answer:

   To check if a container accepts TCP connections on a specified port -> Option D

4. Quick Check:

   TCP probe = connection check [OK]

Hint: TCP probe checks port connectivity, not resource usage [OK]

Common Mistakes:

• Confusing TCP probe with CPU or memory monitoring
• Thinking TCP probe checks file system health
• Assuming TCP probe checks HTTP endpoints

2. Which of the following is the correct syntax to define a TCP liveness probe in a pod spec?

easy

A. livenessProbe:
tcpSocket:
path: "/health"
initialDelaySeconds: 5
periodSeconds: 10

B. livenessProbe:
httpGet:
port: 8080
initialDelaySeconds: 5
periodSeconds: 10

C. livenessProbe:
tcpSocket:
port: 8080
initialDelaySeconds: 5
periodSeconds: 10

D. livenessProbe:
exec:
command: ["curl", "localhost:8080"]
initialDelaySeconds: 5
periodSeconds: 10

Solution

1. Step 1: Identify TCP probe syntax

   TCP probes use tcpSocket with a port field, no path.

2. Step 2: Check options for correctness

   livenessProbe: tcpSocket: port: 8080 initialDelaySeconds: 5 periodSeconds: 10 uses tcpSocket with port 8080 and timing fields correctly. Others use HTTP or exec or invalid fields.

3. Final Answer:

   livenessProbe with tcpSocket and port field -> Option C

4. Quick Check:

   TCP probe syntax = tcpSocket + port [OK]

Hint: TCP probes use tcpSocket with port, no path [OK]

Common Mistakes:

• Using httpGet or exec instead of tcpSocket for TCP probes
• Adding a path field to tcpSocket (invalid)
• Omitting port number in tcpSocket

3. Given this pod spec snippet, what will happen if the container does not accept TCP connections on port 3306?

livenessProbe:
  tcpSocket:
    port: 3306
  initialDelaySeconds: 10
  periodSeconds: 5

medium

A. Kubernetes will ignore the failure and keep the container running

B. Kubernetes will restart the container after the probe fails

C. The container will be marked ready immediately

D. The pod will be deleted permanently

Solution

1. Step 1: Understand liveness probe behavior

   Liveness probes check if a container is alive; failure triggers a restart.

2. Step 2: Apply to TCP probe failure

   If TCP connection on port 3306 fails, Kubernetes restarts the container to fix it.

3. Final Answer:

   Kubernetes will restart the container after the probe fails -> Option B

4. Quick Check:

   Liveness probe failure = container restart [OK]

Hint: Liveness probe failure triggers container restart [OK]

Common Mistakes:

• Confusing liveness with readiness probe effects
• Thinking pod is deleted permanently on failure
• Assuming failure is ignored

4. You configured a TCP readiness probe but Kubernetes reports probe failures immediately after pod start. Which is the most likely cause?

medium

A. initialDelaySeconds is missing or too low, so probe runs before container is ready

B. tcpSocket port is set to 0, which is invalid

C. periodSeconds is set too high, causing delayed checks

D. readinessProbe cannot use tcpSocket, only httpGet

Solution

1. Step 1: Analyze probe timing parameters

   Without initialDelaySeconds, probe starts immediately, possibly before container is ready.

2. Step 2: Evaluate other options

   Port 0 is invalid but would cause different errors; periodSeconds affects frequency, not immediate failure; tcpSocket is valid for readiness probes.

3. Final Answer:

   initialDelaySeconds is missing or too low, so probe runs before container is ready -> Option A

4. Quick Check:

   Missing initialDelaySeconds = early probe failure [OK]

Hint: Set initialDelaySeconds to delay probe start [OK]

Common Mistakes:

• Assuming tcpSocket is invalid for readiness probes
• Ignoring initialDelaySeconds importance
• Confusing periodSeconds with initialDelaySeconds

5. You want to configure a TCP readiness probe for a database container on port 5432. The container takes about 20 seconds to start accepting connections. Which configuration is best to avoid false failures?

hard

A. readinessProbe:
tcpSocket:
port: 5432
initialDelaySeconds: 25
periodSeconds: 10
failureThreshold: 3

B. readinessProbe:
tcpSocket:
port: 5432
initialDelaySeconds: 5
periodSeconds: 5
failureThreshold: 1

C. readinessProbe:
tcpSocket:
port: 5432
initialDelaySeconds: 0
periodSeconds: 10
failureThreshold: 5

D. readinessProbe:
tcpSocket:
port: 5432
initialDelaySeconds: 10
periodSeconds: 20
failureThreshold: 1

Solution

1. Step 1: Match initialDelaySeconds to container startup time

   Container needs ~20 seconds to accept connections, so initialDelaySeconds should be at least 20 to avoid early failures.

2. Step 2: Use periodSeconds and failureThreshold to allow retries

   Period of 10 seconds and failureThreshold of 3 means Kubernetes retries for up to 30 seconds before marking not ready, avoiding false negatives.

3. Final Answer:

   readinessProbe with initialDelaySeconds 25, periodSeconds 10, failureThreshold 3 -> Option A

4. Quick Check:

   Delay + retries prevent false failures [OK]

Hint: Set initialDelaySeconds > startup time, use retries [OK]

Common Mistakes:

• Setting initialDelaySeconds too low causing false failures
• Using failureThreshold of 1 with short delays
• Ignoring container startup time in probe config