amaechler commented on code in PR #20404:
URL: https://github.com/apache/druid/pull/20404#discussion_r4084077787
##########
docs/operations/dynamic-config-provider.md:
##########
@@ -76,4 +76,96 @@ When you define the consumer properties in the supervisor
spec, use the dynamic
},
...
```
-When connecting to Kafka, Druid replaces the environment variables with their
corresponding values.
\ No newline at end of file
+When connecting to Kafka, Druid replaces the environment variables with their
corresponding values.
+
+## Kubernetes node label dynamic config provider
+
+The
[`druid-kubernetes-extensions`](../development/extensions-core/kubernetes.md)
extension provides a dynamic config provider
(`K8sNodeLabelDynamicConfigProvider`) that reads configuration values from the
labels of the Kubernetes node that the Druid process runs on. Use it for values
that depend on Kubernetes runtime information, for example where a process was
scheduled, which cannot be written into a spec ahead of time.
+
+The Kubernetes node label dynamic config provider uses the following syntax:
+
+```json
+druid.dynamic.config.provider={"type": "k8sNodeLabel","labels":{"property1":
"example.com/label-one","property2": "example.com/label-two"}}
+```
+
+|Field|Type|Description|Required|
+|-----|----|-----------|--------|
+|`type`|String|dynamic config provider type|Yes: `k8sNodeLabel`|
+|`labels`|Map|configuration keys to resolve, each mapped to the name of the
node label that holds its value|Yes|
+|`nodeNameVariable`|String|environment variable that holds the name of the
node the process runs on|No (default: `HOST_NODE_NAME`)|
+
+You can use it anywhere Druid accepts a dynamic config provider, for example
Kafka [`consumerProperties`](../ingestion/kafka-ingestion.md), Iceberg
[`catalogProperties`](../development/extensions-contrib/iceberg.md), and Schema
Registry [`config` and `headers`](../ingestion/data-formats.md).
+
+### Prerequisites
+
+Include `druid-kubernetes-extensions` in the [extensions load
list](../configuration/extensions.md#loading-extensions) of every service that
resolves the spec. For a supervisor spec, that is the Overlord and the Peon
services.
+
+Each pod must know the node it runs on. Expose the node name from the downward
API as the environment variable named by `nodeNameVariable`, or set
`nodeNameVariable` to a variable your pod template already provides:
+
+```yaml
+env:
+ - name: HOST_NODE_NAME
+ valueFrom:
+ fieldRef:
+ fieldPath: spec.nodeName
+```
+
+Nodes are cluster-scoped, so the service account the Druid pods run as needs a
ClusterRole granting `get` on `nodes`:
+
+```yaml
+kind: ClusterRole
+apiVersion: rbac.authorization.k8s.io/v1
+metadata:
+ name: druid-node-reader
+rules:
+- apiGroups:
+ - ""
+ resources:
+ - nodes
+ verbs:
+ - get
+---
+kind: ClusterRoleBinding
+apiVersion: rbac.authorization.k8s.io/v1
+metadata:
+ name: druid-node-reader
+subjects:
+- kind: ServiceAccount
+ name: default # the service account your Druid pods use
+ namespace: druid # their namespace
+roleRef:
+ kind: ClusterRole
+ name: druid-node-reader
+ apiGroup: rbac.authorization.k8s.io
+```
+
+### Behavior
+
+- If the node name variable is unset, the API server cannot be reached or
refuses the request, or the node does not carry the label, Druid logs a warning
and omits the key. The consumer then applies its own default. A label with an
empty value counts as missing. As with other dynamic config providers, a
resolved value takes precedence over the same key specified directly.
+- Druid reads the node once per process and keeps its labels until the process
restarts. A failed lookup is not kept, so it is retried the next time the
configuration is resolved.
+
+### Examples
+
+Kafka consumers fetch from the nearest replica when `client.rack` matches a
broker's `broker.rack`
([KIP-392](https://cwiki.apache.org/confluence/display/KAFKA/KIP-392%3A+Allow+consumers+to+fetch+from+closest+replica)).
On EKS, `topology.k8s.aws/zone-id` holds the zone ID that MSK uses for
`broker.rack`; on GKE, `topology.kubernetes.io/zone` holds the zone name that
Google Cloud Managed Service for Apache Kafka uses:
+
+```json
+"consumerProperties": {
+ "bootstrap.servers": "localhost:9092",
+ "druid.dynamic.config.provider": {
+ "type": "k8sNodeLabel",
+ "labels": { "client.rack": "topology.k8s.aws/zone-id" }
+ }
+}
+```
+
+An Iceberg REST catalog can read from S3 in the task's own region by taking
`client.region` from the node's region label:
+
+```json
+"catalogProperties": {
+ "uri": "https://iceberg-rest.example.com";,
Review Comment:
Good find, updated.
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