Tag Archives: Kubernetes Labels

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Kubernetes Scheduling: nodeSelector vs nodeAffinity

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When deploying workloads in Kubernetes, controlling where your pods land is crucial. Two primary mechanisms facilitate this: nodeSelector and nodeAffinity. While they might seem similar at first glance, they serve different purposes and offer varying degrees of flexibility.

The Basics: nodeSelector

The nodeSelector is the simplest way to constrain pods to specific nodes. It matches pods to nodes based on key-value pairs. For instance:

spec:
  nodeSelector:
    disktype: ssd

This configuration ensures that the pod is scheduled only on nodes labeled with disktype=ssd.

However, nodeSelector has its limitations. It doesn’t support complex queries or multiple values for a single key. If you attempt to specify multiple values for the same key, like so:

nodeSelector:
  topology.kubernetes.io/zone: us-east-1a
  topology.kubernetes.io/zone: us-east-1b

Only the last key-value pair is considered, effectively ignoring the previous ones. This behavior stems from the fact that YAML maps require unique keys, and Kubernetes doesn’t merge these entries.

Enter nodeAffinity

For more granular control, nodeAffinity comes into play. It allows you to define rules using operators like In, NotIn, Exists, and DoesNotExist. This flexibility enables you to match pods to nodes based on a range of criteria.

Suppose you want to schedule a pod on nodes in either us-east-1a or us-east-1b. Here’s how you’d achieve that with nodeAffinity: Continue reading Kubernetes Scheduling: nodeSelector vs nodeAffinity →

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Highlight Kubernetes Labels in your Terminal with AWK

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A quick tip and bit of code: if you’re outputting a lot of Kubernetes metadata using the --show-labels command, it can feel like looking for a needle in a haystack. The snippet below colorizes key label outputs to make them stand out.

The Code Snippet

When working with Kubernetes, it can be helpful to visually scan for certain node labels—such as service.cilium.io/node=... or custom readiness flags like ingress-ready=true. Using a simple awk script, we can colorize these labels directly in our terminal output. This script uses ANSI escape codes to wrap matched text in color and awk’s gsub() function to apply substitutions line by line. It’s a lightweight and effective way to highlight key data points in otherwise dense CLI output.

kubectl get ciliumnodes --show-labels | awk '
BEGIN {
  color_start = "\033[1;36m"; # cyan
  color_end = "\033[0m";
}
{
  gsub(/service\.cilium\.io\/node=[^, ]+/, color_start "&" color_end);
  gsub(/ingress-ready=true/, color_start "&" color_end);
  print
}'

Screenshot Example


Screenshot showing the use of an awk command to color-highlight the ingress-ready=true label in red within kubectl get ciliumnodes --show-labels output in a Kubernetes terminal session.

Breakdown of the Code

We pipe the output of the kubectl command to awk. The BEGIN block sets up the ANSI color codes used for matching patterns.

  • \033[1;36m is an ANSI escape code that starts cyan-colored text.
  • \033[0m resets the text color back to normal.

gsub(...)

These two lines apply substitutions to each input line:

  • gsub() is a global substitution function that replaces all matches in the line.
    • service\.cilium\.io\/node=[^, ]+ matches a full key-value pair like service.cilium.io/node=mynode
    • [^, ]+ grabs the node value until the next comma or space
    • ingress-ready=true matches the exact label string
    • & refers to the entire matched string, which we wrap in color codes

print

This prints the modified line after substitutions are applied.

Customize the Highlight Color

You can change \033[1;36m to another color code:

  • Red: \033[1;31m
  • Green: \033[1;32m
  • Yellow: \033[1;33m
  • Blue: \033[1;34m
  • Magenta: \033[1;35m

A Final Note on sub() vs gsub()

  • sub() replaces only the first occurrence of the regex in the line
  • gsub() replaces all occurrences of the regex in the line

Regards


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Dean Lewis