Ingress resource definition in Kubernetes - Time & Space Complexity

Time Complexity: Ingress resource definition

O(n)

Understanding Time Complexity

We want to understand how the time to process an Ingress resource changes as it grows.

Specifically, how does adding more rules affect the work Kubernetes does?

Scenario Under Consideration

Analyze the time complexity of the following Ingress resource definition.

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: example-ingress
spec:
  rules:
  - host: example.com
    http:
      paths:
      - path: /app1
        pathType: Prefix
        backend:
          service:
            name: app1-service
            port:
              number: 80
      - path: /app2
        pathType: Prefix
        backend:
          service:
            name: app2-service
            port:
              number: 80

This Ingress routes requests to different services based on URL paths.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Kubernetes checks each path rule in the Ingress to find a match for incoming requests.
• How many times: It examines each path rule one by one until it finds a match or finishes all rules.

How Execution Grows With Input

As the number of path rules increases, the time to find the right backend grows roughly in a straight line.

Input Size (n)	Approx. Operations
10	About 10 path checks
100	About 100 path checks
1000	About 1000 path checks

Pattern observation: The work grows directly with the number of path rules.

Final Time Complexity

Time Complexity: O(n)

This means the time to process the Ingress grows linearly with the number of path rules.

Common Mistake

[X] Wrong: "Adding more path rules won't affect performance because Kubernetes handles all paths instantly."

[OK] Correct: Kubernetes must check each path rule to find a match, so more rules mean more checks and more time.

Interview Connect

Understanding how resource size affects processing time helps you design scalable Kubernetes configurations and shows you think about system behavior beyond just writing code.

Self-Check

"What if the Ingress used a more efficient matching method like a prefix tree? How would the time complexity change?"

Practice

1. What is the main purpose of a Kubernetes Ingress resource?

easy

A. To route external HTTP(S) traffic to internal services based on rules

B. To store configuration data for pods

C. To manage persistent storage volumes

D. To schedule pods on specific nodes

Solution

1. Step 1: Understand Ingress role

   An Ingress resource defines rules to route external traffic to services inside the cluster.

2. Step 2: Compare with other resources

   ConfigMaps store config, PersistentVolumes manage storage, and scheduling is done by the scheduler, not Ingress.

3. Final Answer:

   To route external HTTP(S) traffic to internal services based on rules -> Option A

4. Quick Check:

   Ingress routes external traffic [OK]

Hint: Ingress routes external traffic to services inside Kubernetes [OK]

Common Mistakes:

• Confusing Ingress with ConfigMap or PersistentVolume
• Thinking Ingress schedules pods
• Assuming Ingress manages storage

2. Which of the following is the correct minimal YAML snippet to define an Ingress that routes /app path to a service named my-service on port 80?

easy

A. apiVersion: networking.k8s.io/v1\nkind: Service\nmetadata:\n name: example-ingress\nspec:\n selector:\n app: my-service\n ports:\n - protocol: TCP\n port: 80\n targetPort: 80

B. apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: example-ingress\nspec:\n rules:\n - http:\n paths:\n - path: /app\n pathType: Prefix\n backend:\n service:\n name: my-service\n port:\n number: 80

C. apiVersion: v1\nkind: Ingress\nmetadata:\n name: example-ingress\nspec:\n rules:\n - host: example.com\n http:\n paths:\n - path: /app\n backend:\n serviceName: my-service\n servicePort: 80

D. apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: example-ingress\nspec:\n rules:\n - http:\n paths:\n - path: /app\n backend:\n service:\n name: my-service\n port: 8080

Solution

1. Step 1: Check apiVersion and kind

   Correct apiVersion for Ingress is networking.k8s.io/v1 and kind is Ingress.

2. Step 2: Validate backend service and port

   apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: example-ingress\nspec:\n rules:\n - http:\n paths:\n - path: /app\n pathType: Prefix\n backend:\n service:\n name: my-service\n port:\n number: 80 uses correct new syntax with service name and port number 80 matching the question.

3. Final Answer:

   YAML with apiVersion networking.k8s.io/v1, kind Ingress, path /app, service my-service, port 80 -> Option B

4. Quick Check:

   Correct Ingress YAML uses networking.k8s.io/v1 and service port number [OK]

Hint: Use networking.k8s.io/v1 and service.port.number for Ingress backend [OK]

Common Mistakes:

• Using old apiVersion v1 for Ingress
• Using serviceName/servicePort instead of service.name/port.number
• Wrong port number in backend
• Defining Service instead of Ingress

3. Given this Ingress YAML snippet, what host and path will route traffic to web-service?

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: test-ingress
spec:
  rules:
  - host: example.com
    http:
      paths:
      - path: /web
        pathType: Prefix
        backend:
          service:
            name: web-service
            port:
              number: 80

medium

A. Traffic to host example.com and path starting with /web

B. Traffic to any host and path /web

C. Traffic to host example.com and any path

D. Traffic to host example.com and path exactly /web

Solution

1. Step 1: Identify host and path in rules

   The rule specifies host example.com and path /web with pathType Prefix.

2. Step 2: Understand pathType Prefix meaning

   Prefix means any path starting with /web will match, not just exactly /web.

3. Final Answer:

   Traffic to host example.com and path starting with /web -> Option A

4. Quick Check:

   Host and prefix path routing = Traffic to host example.com and path starting with /web [OK]

Hint: Prefix path matches all paths starting with given string [OK]

Common Mistakes:

• Assuming path matches exactly instead of prefix
• Ignoring host field and assuming any host
• Confusing pathType Prefix with Exact

4. You applied this Ingress YAML but traffic is not routing correctly:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  name: bad-ingress
spec:
  rules:
  - http:
      paths:
      - path: /app
        pathType: Prefix
        backend:
          serviceName: app-service
          servicePort: 80

What is the likely cause of the problem?

medium

A. Missing host field in rules

B. Incorrect apiVersion for Ingress

C. Using deprecated backend syntax serviceName and servicePort instead of service.name and service.port.number

D. Missing metadata name field

Solution

1. Step 1: Check backend syntax in Ingress spec

   The YAML uses old syntax with serviceName and servicePort which is deprecated in networking.k8s.io/v1.

2. Step 2: Confirm correct syntax

   Correct syntax uses service.name and service.port.number fields under backend.service.

3. Final Answer:

   Using deprecated backend syntax serviceName and servicePort instead of service.name and service.port.number -> Option C

4. Quick Check:

   Use service.name and service.port.number in Ingress backend [OK]

Hint: Use service.name and service.port.number for backend in networking.k8s.io/v1 [OK]

Common Mistakes:

• Using old backend syntax causing Ingress controller to ignore rules
• Assuming missing host always breaks routing
• Confusing apiVersion correctness
• Forgetting metadata name

5. You want to expose two services, service-a and service-b, on the same IP but different paths /a and /b. Which Ingress YAML snippet correctly defines this?

hard

A. apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: multi-service-ingress\nspec:\n rules:\n - host: example.com\n http:\n paths:\n - path: /a\n pathType: Prefix\n backend:\n service:\n name: service-a\n port:\n number: 80

B. apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: multi-service-ingress\nspec:\n rules:\n - host: example.com\n http:\n paths:\n - path: /a\n backend:\n serviceName: service-a\n servicePort: 80\n - path: /b\n backend:\n serviceName: service-b\n servicePort: 80

C. apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: multi-service-ingress\nspec:\n rules:\n - http:\n paths:\n - path: /a\n pathType: Exact\n backend:\n serviceName: service-a\n servicePort: 80\n - path: /b\n pathType: Exact\n backend:\n serviceName: service-b\n servicePort: 80

D. apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: multi-service-ingress\nspec:\n rules:\n - http:\n paths:\n - path: /a\n pathType: Prefix\n backend:\n service:\n name: service-a\n port:\n number: 80\n - path: /b\n pathType: Prefix\n backend:\n service:\n name: service-b\n port:\n number: 80

Solution

1. Step 1: Check apiVersion and kind

   All options use correct apiVersion and kind for Ingress.

2. Step 2: Validate backend syntax and multiple paths

   apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: multi-service-ingress\nspec:\n rules:\n - http:\n paths:\n - path: /a\n pathType: Prefix\n backend:\n service:\n name: service-a\n port:\n number: 80\n - path: /b\n pathType: Prefix\n backend:\n service:\n name: service-b\n port:\n number: 80 correctly uses service.name and service.port.number with two paths under one rule without host, matching the requirement.

3. Step 3: Compare pathType usage

   Prefix pathType allows routing all requests starting with /a or /b, which is typical for path-based routing.

4. Final Answer:

   Ingress with two paths /a and /b routing to service-a and service-b using correct backend syntax -> Option D

5. Quick Check:

   Multiple paths with service.name and port.number = apiVersion: networking.k8s.io/v1\nkind: Ingress\nmetadata:\n name: multi-service-ingress\nspec:\n rules:\n - http:\n paths:\n - path: /a\n pathType: Prefix\n backend:\n service:\n name: service-a\n port:\n number: 80\n - path: /b\n pathType: Prefix\n backend:\n service:\n name: service-b\n port:\n number: 80 [OK]

Hint: Use one rule with multiple paths and service.name/port.number [OK]

Common Mistakes:

• Using deprecated backend syntax serviceName/servicePort
• Missing one of the paths or services
• Using pathType Exact which may limit routing
• Defining only one path instead of two