Labels and Selectors: Equality-Based vs Set-Based Selection

-

Identify and Group Resources Meaningfully

Kubernetes uses labels to describe resources and selectors to find them.
Selectors come in two main forms:

  • Equality-based selectors
  • Set-based selectors

Understanding when to use each is essential for scalable Kubernetes designs.

Equality-Based Selectors

Equality-based selectors match objects with exact label key–value pairs.

Supported operators:

  • = or ==
  • != (CLI only)

Example

selector:
  matchLabels:
    app: color-api
    tier: backend

This selector matches only Pods that have both labels.

Set-Based Selectors

Set-based selectors allow matching based on sets of values or label existence.

Supported operators:

  • In
  • NotIn
  • Exists
  • DoesNotExist

Example: Multiple Values

selector:
  matchExpressions:
    - key: tier
      operator: In
      values:
        - frontend
        - backend

Example: Excluding Canary Releases

selector:
  matchExpressions:
    - key: release
      operator: NotIn
      values:
        - canary

This approach is safer and future-proof.

matchLabels vs matchExpressions

FeaturematchLabelsmatchExpressions
Simple syntaxYesNo
Multiple valuesNoYes
ExclusionsNoYes
Exists checksNoYes

Key Takeaways

  • Use matchLabels for simple, stable selectors
  • Use matchExpressions for complex or evolving logic
  • Prefer NotIn over whitelisting values

Labels define what a resource is.
Selectors define what you want.

Comments

Post a Comment

Popular posts from this blog

Highlights from the 2025 Stack Overflow Developer Survey

-
Image
Introduction The  15th Annual Stack Overflow Developer Survey  is now out. With over  49,000 responses  from  177 countries , the 2025 findings offer a rich view into how developers work—and how they feel—amid rising AI adoption ( survey.stackoverflow.co ,  survey.stackoverflow.co ). Dev & AI Trends at a Glance Widespread AI Adoption Coupled with Waning Trust A whopping  84%  of developers now use—or plan to use—AI tools in their workflows, up from  76%  last year ( survey.stackoverflow.co ). Yet trust in AI is falling dramatically:  46%  say they do  not trust  AI output, compared to just  31%  in 2024 ( itpro.com ). Positive sentiment toward AI tools dropped from over 70% to roughly  60%  this year ( survey.stackoverflow.co ). The top frustration? Developers cite “ AI solutions that are almost right, but not quite ” as a pain point—leading to increased time spent debugging ( survey.stackoverf...
Read more

Mastering Caddy Logging: A Complete Guide to Access, Error, and Structured Logs

-
Image
  Mastering Logging in Caddy Caddy is a modern, lightweight, and secure web server that’s rapidly gaining popularity thanks to its simplicity, automatic HTTPS, and powerful configuration options. One of the most important aspects of managing a Caddy-powered application is  logging . Proper logging ensures you have visibility into your system, can debug issues efficiently, and monitor performance in real-time. In this article, we’ll break down how logging works in Caddy, why it matters, and how you can configure it for your own projects. What is Logging in Caddy? Logging in Caddy refers to the process of capturing details about requests, errors, and internal server behavior. With logs, you can: Track incoming requests and responses Debug configuration or runtime issues Monitor server performance and health Meet compliance and audit requirements Caddy uses a structured logging system, making logs easier to parse and integrate with external monitoring tools like  Better Stac...
Read more

psql: error: connection to server at "localhost" (127.0.0.1), port 5433 failed: ERROR: failed to authenticate with backend using SCRAM DETAIL: valid password not found

-
Image
ERROR psql: error: connection to server at "localhost" (127.0.0.1), port 5433 failed: ERROR: failed to authenticate with backend using SCRAM DETAIL: valid password not found Since Pgpool-II is a PostgreSQL proxy that works between clients and PostgreSQL servers, the authentication comprises two steps: Authentication between client and Pgpool-II Authentication between Pgpool-II and PostgreSQL servers Starting with Pgpool-II 4.0, Pgpool-II supports scram-sha-256 authentication. scram-sha-256 authentication method is strongly recommended because it is the most secure password-based authentication method. Solution 1 If PostgreSQL servers require  MD5  or  SCRAM  authentication for some user’s authentication but the password for that user is not present in  pool_passwd , then enabling  allow_clear_text_frontend_auth  will allow the  Pgpool-II  to use clear-text-password authentication with user to get the password in plain text form from the user ...
Read more