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Dependencies

You can declare dependencies between any two cdk8s constructs using the addDependency() method.

ApiObjects

For example, you can force kubernetes to first apply a Namespace before applying the Service in the scope of that namespace:

const namespace = new k8s.Namespace(chart, 'backend');
const service = new k8s.Service(chart, 'Service', { metadata: { namespace: namespace.name }});

// declare the dependency. this is just a syntactic sugar for Node.of(service).addDependency(namespace)
service.addDependency(namespace);

cdk8s will ensure that the Namespace object is placed before the Service object in the resulting manifest:

apiVersion: v1
kind: Namespace
metadata:
  name: chart-backend-a59d2e47
---
apiVersion: v1
kind: Service
metadata:
  name: chart-service-93d02be7
  namespace: chart-backend-a59d2e47

Charts

You can also specify dependencies between charts, in exactly the same manner. For example, if we have a chart that provisions our namespace, we need that chart to be applied first:

const namespaceChart = new NamespaceChart(app, 'namespace');
const applicationChart = new ApplicationChart(app, 'application');

// declare the dependency. this is just a syntactic sugar for Node.of(applicationChart).addDependency(namespaceChart)
applicationChart.addDependency(namespaceChart);

Running cdk8s synth will produce the following dist directory:

> cdk8s synth

dist/0000-namespace.k8s.yaml
dist/0001-application.k8s.yaml

Notice that the namespace chart appears first with the 0000 prefix. This will ensure that a subsequent execution of kubectl apply -f dist/ will apply the namespace first, and the application second.

Custom Constructs

The behavior above applies in the same way to custom constructs that you create or use.

class Database extends Construct {
  constructor(scope: Construct, name: string) {
    super(scope, name);

    new k8s.StatefulSet(this, 'StatefulSet');
    new k8s.ConfigMap(this, 'ConfigMap');
  }
}

const app = new App();

const chart = new Chart(app, 'Chart');

const service = new k8s.Service(chart, 'Service')
const database = new Database(chart, 'Database');

service.addDependency(database);

Declaring such a dependency will cause each ApiObject in the source construct, to depend on every ApiObject in the target construct.

Note that in the example above, the source construct is actually an ApiObject, which is also ok since it is essentially a construct with a single ApiObject.

Note that if the source of your dependency is a custom construct, it won’t have the addDependency syntactic sugar by default, so you’ll have to use Node.of().

The resulting manifest will be:

apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: chart-database-statefulset-4627f8e2
---
apiVersion: v1
kind: ConfigMap
metadata:
  name: chart-database-configmap-676f8640
---
apiVersion: v1
kind: Service
metadata:
  name: chart-service-93d02be7

You can see that all ApiObjects of the Database construct, appear before the Service object.

Things just got cool

If you simply declare a dependency between two ApiObjects (or Constructs), that belong to two different Charts, cdk8s will create the chart dependency automatically for you.

const namespaceChart = new NamespaceChart(app, 'namespace');
const applicationChart = new ApplicationChart(app, 'application');

const namespace = new k8s.Namespace(namespaceChart, 'namespace');
const deployment = new k8s.Deployment(applicationChart, 'Deployment');

// dependency between ApiObjects, not Charts!
deployment.addDependency(namespace);

Running cdk8s synth will produce the same result as if explicit chart dependencies were declared:

> cdk8s synth

dist/0000-namespace.k8s.yaml
dist/0001-application.k8s.yaml

This means you need not be bothered with managing chart dependencies, simply work with the ApiObjects you create, and let cdk8s infer the chart dependencies.