Labels are sets of key/pair values that attach to objects. They're designed to provide meaningful, identifying information about objects. Common usages are to indicate the name of the micro-service, the tier, the environment, and the software version. Users can define meaningful labels that could be used with selectors later. The syntax of labels in an object spec is as follows:

labels:
  $key1: $value1
  $key2: $value2

Along with labels, label selectors are used to filter sets of objects. Separated by commas, multiple requirements will be joined by the AND logical operator. There are two ways to filter:

• Equality-based requirements
• Set-based requirements

Equality-based requirements support the following operators: =, ==, and !=. Take the following diagram as an example: if the selector is chapter=2,version!=0.1, the result will be object C. If the requirement is version=0.1, the result will be object A and object B:

If we write the requirement in the supported object spec, it'll be as follows:

selector:
  $key1: $value1

Set-based requirement supports in, notin, and exists (for key only). For example, if the requirement is chapter in (3, 4),version, then object A will be returned. If the requirement is version notin (0.2), !author_info, the result will be object A and object B. The following example shows an object spec that uses set-based requirements:

selector:
  matchLabels:
    $key1: $value1
  matchExpressions:
  - {key: $key2, operator: In, values: [$value1, $value2]}

The requirements of matchLabels and matchExpressions are combined together. This means that the filtered objects need to be true for both requirements.