Add option to set Topology Spread Constraints, rather than just Affinity

charts/backstage/Chart.yaml

@@ -38,4 +38,4 @@ sources:
 # This is the chart version. This version number should be incremented each time you make changes
 # to the chart and its templates, including the app version.
 # Versions are expected to follow Semantic Versioning (https://semver.org/)
-version: 1.10.0
+version: 1.11.0

charts/backstage/README.md

@@ -2,7 +2,7 @@
 # Backstage Helm Chart
 
 [![Artifact Hub](https://img.shields.io/endpoint?url=https://artifacthub.io/badge/repository/backstage)](https://artifacthub.io/packages/search?repo=backstage)
-![Version: 1.10.0](https://img.shields.io/badge/Version-1.10.0-informational?style=flat-square)
+![Version: 1.11.0](https://img.shields.io/badge/Version-1.11.0-informational?style=flat-square)
 ![Type: application](https://img.shields.io/badge/Type-application-informational?style=flat-square)
 
 A Helm chart for deploying a Backstage application
@@ -146,6 +146,7 @@ Kubernetes: `>= 1.19.0-0`
 | backstage.revisionHistoryLimit | Define the [count of deployment revisions](https://kubernetes.io/docs/concepts/workloads/controllers/deployment/#clean-up-policy) to be kept. May be set to 0 in case of GitOps deployment approach. | int | `10` |
 | backstage.startupProbe | Startup Probe Backstage doesn't provide any health endpoints by default. A simple one can be added like this: https://backstage.io/docs/plugins/observability/#health-checks <br /> Ref: https://kubernetes.io/docs/tasks/configure-pod-container/configure-liveness-readiness-startup-probes <!-- E.g. startupProbe:   failureThreshold: 3   httpGet:     path: /healthcheck     port: 7007     scheme: HTTP   initialDelaySeconds: 60   periodSeconds: 10   successThreshold: 1   timeoutSeconds: 2 | object | `{}` |
 | backstage.tolerations | Node tolerations for server scheduling to nodes with taints <br /> Ref: https://kubernetes.io/docs/concepts/scheduling-eviction/taint-and-toleration/ | list | `[]` |
+| backstage.topologySpreadConstraints | Topology Spread Constraints for pod assignment <br /> Ref: https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#pod-topology-spread-constraints | list | `[]` |
 | clusterDomain | Default Kubernetes cluster domain | string | `"cluster.local"` |
 | commonAnnotations | Annotations to add to all deployed objects | object | `{}` |
 | commonLabels | Labels to add to all deployed objects | object | `{}` |

charts/backstage/ci/topologySpreadConstraints-values.yaml

@@ -0,0 +1,10 @@
+backstage:
+  topologySpreadConstraints:
+  - maxSkew: 1
+    topologyKey: kubernetes.io/hostname
+    whenUnsatisfiable: DoNotSchedule
+    labelSelector:
+      matchLabels:
+        app.kubernetes.io/component: backstage
+    matchLabelKeys:
+      - pod-template-hash

charts/backstage/templates/backstage-deployment.yaml

@@ -47,6 +47,10 @@ spec:
       affinity:
         {{- include "common.tplvalues.render" ( dict "value" .Values.backstage.affinity "context" $) | nindent 8 }}
       {{- end }}
+      {{- if .Values.backstage.topologySpreadConstraints }}
+      topologySpreadConstraints:
+        {{- include "common.tplvalues.render" ( dict "value" .Values.backstage.topologySpreadConstraints "context" $) | nindent 8 }}
+      {{- end }}
       {{- if .Values.backstage.nodeSelector }}
       nodeSelector:
         {{- include "common.tplvalues.render" ( dict "value" .Values.backstage.nodeSelector "context" $) | nindent 8 }}

charts/backstage/values.schema.json

@@ -5999,6 +5999,102 @@
                     },
                     "title": "Node tolerations for server scheduling to nodes with taints",
                     "type": "array"
+                },
+                "topologySpreadConstraints": {
+                    "default": [],
+                    "description": "Ref: https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#pod-topology-spread-constraints",
+                    "items": {
+                        "description": "TopologySpreadConstraint specifies how to spread matching pods among the given topology.",
+                        "properties": {
+                            "labelSelector": {
+                                "description": "A label selector is a label query over a set of resources. The result of matchLabels and matchExpressions are ANDed. An empty label selector matches all objects. A null label selector matches no objects.",
+                                "properties": {
+                                    "matchExpressions": {
+                                        "description": "matchExpressions is a list of label selector requirements. The requirements are ANDed.",
+                                        "items": {
+                                            "description": "A label selector requirement is a selector that contains values, a key, and an operator that relates the key and values.",
+                                            "properties": {
+                                                "key": {
+                                                    "description": "key is the label key that the selector applies to.",
+                                                    "type": "string"
+                                                },
+                                                "operator": {
+                                                    "description": "operator represents a key's relationship to a set of values. Valid operators are In, NotIn, Exists and DoesNotExist.",
+                                                    "type": "string"
+                                                },
+                                                "values": {
+                                                    "description": "values is an array of string values. If the operator is In or NotIn, the values array must be non-empty. If the operator is Exists or DoesNotExist, the values array must be empty. This array is replaced during a strategic merge patch.",
+                                                    "items": {
+                                                        "type": "string"
+                                                    },
+                                                    "type": "array",
+                                                    "x-kubernetes-list-type": "atomic"
+                                                }
+                                            },
+                                            "required": [
+                                                "key",
+                                                "operator"
+                                            ],
+                                            "type": "object"
+                                        },
+                                        "type": "array",
+                                        "x-kubernetes-list-type": "atomic"
+                                    },
+                                    "matchLabels": {
+                                        "additionalProperties": {
+                                            "type": "string"
+                                        },
+                                        "description": "matchLabels is a map of {key,value} pairs. A single {key,value} in the matchLabels map is equivalent to an element of matchExpressions, whose key field is \"key\", the operator is \"In\", and the values array contains only \"value\". The requirements are ANDed.",
+                                        "type": "object"
+                                    }
+                                },
+                                "type": "object",
+                                "x-kubernetes-map-type": "atomic"
+                            },
+                            "matchLabelKeys": {
+                                "description": "MatchLabelKeys is a set of pod label keys to select the pods over which spreading will be calculated. The keys are used to lookup values from the incoming pod labels, those key-value labels are ANDed with labelSelector to select the group of existing pods over which spreading will be calculated for the incoming pod. The same key is forbidden to exist in both MatchLabelKeys and LabelSelector. MatchLabelKeys cannot be set when LabelSelector isn't set. Keys that don't exist in the incoming pod labels will be ignored. A null or empty list means only match against labelSelector.\n\nThis is a beta field and requires the MatchLabelKeysInPodTopologySpread feature gate to be enabled (enabled by default).",
+                                "items": {
+                                    "type": "string"
+                                },
+                                "type": "array",
+                                "x-kubernetes-list-type": "atomic"
+                            },
+                            "maxSkew": {
+                                "description": "MaxSkew describes the degree to which pods may be unevenly distributed. When `whenUnsatisfiable=DoNotSchedule`, it is the maximum permitted difference between the number of matching pods in the target topology and the global minimum. The global minimum is the minimum number of matching pods in an eligible domain or zero if the number of eligible domains is less than MinDomains. For example, in a 3-zone cluster, MaxSkew is set to 1, and pods with the same labelSelector spread as 2/2/1: In this case, the global minimum is 1. | zone1 | zone2 | zone3 | |  P P  |  P P  |   P   | - if MaxSkew is 1, incoming pod can only be scheduled to zone3 to become 2/2/2; scheduling it onto zone1(zone2) would make the ActualSkew(3-1) on zone1(zone2) violate MaxSkew(1). - if MaxSkew is 2, incoming pod can be scheduled onto any zone. When `whenUnsatisfiable=ScheduleAnyway`, it is used to give higher precedence to topologies that satisfy it. It's a required field. Default value is 1 and 0 is not allowed.",
+                                "format": "int32",
+                                "type": "integer"
+                            },
+                            "minDomains": {
+                                "description": "MinDomains indicates a minimum number of eligible domains. When the number of eligible domains with matching topology keys is less than minDomains, Pod Topology Spread treats \"global minimum\" as 0, and then the calculation of Skew is performed. And when the number of eligible domains with matching topology keys equals or greater than minDomains, this value has no effect on scheduling. As a result, when the number of eligible domains is less than minDomains, scheduler won't schedule more than maxSkew Pods to those domains. If value is nil, the constraint behaves as if MinDomains is equal to 1. Valid values are integers greater than 0. When value is not nil, WhenUnsatisfiable must be DoNotSchedule.\n\nFor example, in a 3-zone cluster, MaxSkew is set to 2, MinDomains is set to 5 and pods with the same labelSelector spread as 2/2/2: | zone1 | zone2 | zone3 | |  P P  |  P P  |  P P  | The number of domains is less than 5(MinDomains), so \"global minimum\" is treated as 0. In this situation, new pod with the same labelSelector cannot be scheduled, because computed skew will be 3(3 - 0) if new Pod is scheduled to any of the three zones, it will violate MaxSkew.",
+                                "format": "int32",
+                                "type": "integer"
+                            },
+                            "nodeAffinityPolicy": {
+                                "description": "NodeAffinityPolicy indicates how we will treat Pod's nodeAffinity/nodeSelector when calculating pod topology spread skew. Options are: - Honor: only nodes matching nodeAffinity/nodeSelector are included in the calculations. - Ignore: nodeAffinity/nodeSelector are ignored. All nodes are included in the calculations.\n\nIf this value is nil, the behavior is equivalent to the Honor policy. This is a beta-level feature default enabled by the NodeInclusionPolicyInPodTopologySpread feature flag.",
+                                "type": "string"
+                            },
+                            "nodeTaintsPolicy": {
+                                "description": "NodeTaintsPolicy indicates how we will treat node taints when calculating pod topology spread skew. Options are: - Honor: nodes without taints, along with tainted nodes for which the incoming pod has a toleration, are included. - Ignore: node taints are ignored. All nodes are included.\n\nIf this value is nil, the behavior is equivalent to the Ignore policy. This is a beta-level feature default enabled by the NodeInclusionPolicyInPodTopologySpread feature flag.",
+                                "type": "string"
+                            },
+                            "topologyKey": {
+                                "description": "TopologyKey is the key of node labels. Nodes that have a label with this key and identical values are considered to be in the same topology. We consider each <key, value> as a \"bucket\", and try to put balanced number of pods into each bucket. We define a domain as a particular instance of a topology. Also, we define an eligible domain as a domain whose nodes meet the requirements of nodeAffinityPolicy and nodeTaintsPolicy. e.g. If TopologyKey is \"kubernetes.io/hostname\", each Node is a domain of that topology. And, if TopologyKey is \"topology.kubernetes.io/zone\", each zone is a domain of that topology. It's a required field.",
+                                "type": "string"
+                            },
+                            "whenUnsatisfiable": {
+                                "description": "WhenUnsatisfiable indicates how to deal with a pod if it doesn't satisfy the spread constraint. - DoNotSchedule (default) tells the scheduler not to schedule it. - ScheduleAnyway tells the scheduler to schedule the pod in any location,\n  but giving higher precedence to topologies that would help reduce the\n  skew.\nA constraint is considered \"Unsatisfiable\" for an incoming pod if and only if every possible node assignment for that pod would violate \"MaxSkew\" on some topology. For example, in a 3-zone cluster, MaxSkew is set to 1, and pods with the same labelSelector spread as 3/1/1: | zone1 | zone2 | zone3 | | P P P |   P   |   P   | If WhenUnsatisfiable is set to DoNotSchedule, incoming pod can only be scheduled to zone2(zone3) to become 3/2/1(3/1/2) as ActualSkew(2-1) on zone2(zone3) satisfies MaxSkew(1). In other words, the cluster can still be imbalanced, but scheduler won't make it *more* imbalanced. It's a required field.",
+                                "type": "string"
+                            }
+                        },
+                        "required": [
+                            "maxSkew",
+                            "topologyKey",
+                            "whenUnsatisfiable"
+                        ],
+                        "type": "object"
+                    },
+                    "title": "Topology Spread Constraint for pod assignment",
+                    "type": "array"
                 }
             },
             "title": "Backstage parameters",

charts/backstage/values.schema.tmpl.json

@@ -318,6 +318,15 @@
                     "title": "Affinity for pod assignment",
                     "type": "object"
                 },
+                "topologySpreadConstraints": {
+                    "default": [],
+                    "description": "Ref: https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#pod-topology-spread-constraints",
+                    "items": {
+                        "$ref": "https://raw.githubusercontent.com/yannh/kubernetes-json-schema/master/master/_definitions.json#/definitions/io.k8s.api.core.v1.TopologySpreadConstraint"
+                    },
+                    "title": "Topology Spread Constraint for pod assignment",
+                    "type": "array"
+                },
                 "args": {
                     "title": "Backstage container command arguments",
                     "type": "array",

charts/backstage/values.yaml

@@ -253,6 +253,10 @@ backstage:
   # <br /> Ref: https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#affinity-and-anti-affinity
   affinity: {}
 
+  # -- Topology Spread Constraints for pod assignment
+  # <br /> Ref: https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#pod-topology-spread-constraints
+  topologySpreadConstraints: []
+
   # -- Node labels for pod assignment
   # <br /> Ref: https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#nodeselector
   nodeSelector: {}