TopologySelectorTerm

TopologySelectorTerm is a data structure used in Kubernetes to express a set of topology-based criteria for selecting scheduling domains, such as zones or regions. It participates in scheduling decisions that constrain where pods can be placed by evaluating topology labels on domains.

A TopologySelectorTerm contains one or more label selector requirements, held in a field typically called matchLabelExpressions. Each element is a LabelSelectorRequirement, which includes a key, an operator, and a list of values. The operator can be In, NotIn, Exists, or DoesNotExist. The semantics are that a topology domain satisfies a TopologySelectorTerm if all of its label selector requirements are met (logical AND within a term). The overall constraint across multiple terms is an OR: a domain satisfies the constraint if it matches at least one of the terms.

In practice, TopologySelectorTerm is used within scheduling constructs to influence distribution across topology domains. It is conceptually similar to NodeSelectorTerm used in node affinity, but applies to topology labels rather than node labels. This enables scenarios such as distributing pods across failure domains or ensuring co-location with respect to topology keys.

Example: a term that matches zones where the topology.kubernetes.io/zone label is In with values zone-a and zone-b would select those zones as acceptable targets. If multiple terms exist, a zone only needs to satisfy one term to be considered suitable.

See also: NodeAffinity, NodeSelectorTerm, LabelSelectorRequirement, PodTopologySpread, TopologyKey.