Scope Pub/Sub topic access

Use scopes to limit Pub/Sub topics to specific applications

Introduction

Namespaces or component scopes can be used to limit component access to particular applications. These application scopes added to a component limit only the applications with specific IDs to be able to use the component.

In addition to this general component scope, the following can be limited for pub/sub components:

  • Which topics which can be used (published or subscribed)
  • Which applications are allowed to publish to specific topics
  • Which applications are allowed to subscribe to specific topics

This is called pub/sub topic scoping.

Pub/sub scopes are defined for each pub/sub component. You may have a pub/sub component named pubsub that has one set of scopes, and another pubsub2 with a different set.

To use this topic scoping three metadata properties can be set for a pub/sub component:

  • spec.metadata.publishingScopes
    • A semicolon-separated list of applications & comma-separated topic lists, allowing that app to publish to that list of topics
    • If nothing is specified in publishingScopes (default behavior), all apps can publish to all topics
    • To deny an app the ability to publish to any topic, leave the topics list blank (app1=;app2=topic2)
    • For example, app1=topic1;app2=topic2,topic3;app3= will allow app1 to publish to topic1 and nothing else, app2 to publish to topic2 and topic3 only, and app3 to publish to nothing.
  • spec.metadata.subscriptionScopes
    • A semicolon-separated list of applications & comma-separated topic lists, allowing that app to subscribe to that list of topics
    • If nothing is specified in subscriptionScopes (default behavior), all apps can subscribe to all topics
    • For example, app1=topic1;app2=topic2,topic3 will allow app1 to subscribe to topic1 only and app2 to subscribe to topic2 and topic3
  • spec.metadata.allowedTopics
    • A comma-separated list of allowed topics for all applications.
    • If allowedTopics is not set (default behavior), all topics are valid. subscriptionScopes and publishingScopes still take place if present.
    • publishingScopes or subscriptionScopes can be used in conjuction with allowedTopics to add granular limitations

These metadata properties can be used for all pub/sub components. The following examples use Redis as pub/sub component.

Example 1: Scope topic access

Limiting which applications can publish/subscribe to topics can be useful if you have topics which contain sensitive information and only a subset of your applications are allowed to publish or subscribe to these.

It can also be used for all topics to have always a “ground truth” for which applications are using which topics as publishers/subscribers.

Here is an example of three applications and three topics:

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: pubsub
  namespace: default
spec:
  type: pubsub.redis
  metadata:
  - name: redisHost
    value: "localhost:6379"
  - name: redisPassword
    value: ""
  - name: publishingScopes
    value: "app1=topic1;app2=topic2,topic3;app3="
  - name: subscriptionScopes
    value: "app2=;app3=topic1"

The table below shows which applications are allowed to publish into the topics:

topic1 topic2 topic3
app1 X
app2 X X
app3

The table below shows which applications are allowed to subscribe to the topics:

topic1 topic2 topic3
app1 X X X
app2
app3 X

Note: If an application is not listed (e.g. app1 in subscriptionScopes) it is allowed to subscribe to all topics. Because allowedTopics is not used and app1 does not have any subscription scopes, it can also use additional topics not listed above.

Example 2: Limit allowed topics

A topic is created if a Dapr application sends a message to it. In some scenarios this topic creation should be governed. For example:

  • A bug in a Dapr application on generating the topic name can lead to an unlimited amount of topics created
  • Streamline the topics names and total count and prevent an unlimited growth of topics

In these situations allowedTopics can be used.

Here is an example of three allowed topics:

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: pubsub
  namespace: default
spec:
  type: pubsub.redis
  metadata:
  - name: redisHost
    value: "localhost:6379"
  - name: redisPassword
    value: ""
  - name: allowedTopics
    value: "topic1,topic2,topic3"

All applications can use these topics, but only those topics, no others are allowed.

Example 3: Combine allowedTopics and scopes

Sometimes you want to combine both scopes, thus only having a fixed set of allowed topics and specify scoping to certain applications.

Here is an example of three applications and two topics:

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: pubsub
  namespace: default
spec:
  type: pubsub.redis
  metadata:
  - name: redisHost
    value: "localhost:6379"
  - name: redisPassword
    value: ""
  - name: allowedTopics
    value: "A,B"
  - name: publishingScopes
    value: "app1=A"
  - name: subscriptionScopes
    value: "app1=;app2=A"

Note: The third application is not listed, because if an app is not specified inside the scopes, it is allowed to use all topics.

The table below shows which application is allowed to publish into the topics:

A B C
app1 X
app2 X X
app3 X X

The table below shows which application is allowed to subscribe to the topics:

A B C
app1
app2 X
app3 X X

Demo