How-To: Enable the transactional outbox pattern

The transactional outbox pattern is a well known design pattern for sending notifications regarding changes in an application’s state. The transactional outbox pattern uses a single transaction that spans across the database and the message broker delivering the notification.

Developers are faced with many difficult technical challenges when trying to implement this pattern on their own, which often involves writing error-prone central coordination managers that, at most, support a combination of one or two databases and message brokers.

For example, you can use the outbox pattern to:

1. Write a new user record to an account database.
2. Send a notification message that the account was successfully created.

With Dapr’s outbox support, you can notify subscribers when an application’s state is created or updated when calling Dapr’s transactions API.

The diagram below is an overview of how the outbox feature works:

1. Service A saves/updates state to the state store using a transaction.
2. A message is written to the broker under the same transaction. When the message is successfully delivered to the message broker, the transaction completes, ensuring the state and message are transacted together.
3. The message broker delivers the message topic to any subscribers - in this case, Service B.

Requirements

The outbox feature can be used with using any transactional state store supported by Dapr. All pub/sub brokers are supported with the outbox feature.

Usage

To enable the outbox feature, add the following required and optional fields on a state store component:

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: mysql-outbox
spec:
  type: state.mysql
  version: v1
  metadata:
  - name: connectionString
    value: "<CONNECTION STRING>"
  - name: outboxPublishPubsub # Required
    value: "mypubsub"
  - name: outboxPublishTopic # Required
    value: "newOrder"
  - name: outboxPubsub # Optional
    value: "myOutboxPubsub"
  - name: outboxDiscardWhenMissingState #Optional. Defaults to false
    value: false

Metadata fields

Combining outbox and non-outbox messages on the same state store

If you want to use the same state store for sending both outbox and non-outbox messages, simply define two state store components that connect to the same state store, where one has the outbox feature and the other does not.

MySQL state store without outbox

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: mysql
spec:
  type: state.mysql
  version: v1
  metadata:
  - name: connectionString
    value: "<CONNECTION STRING>"

MySQL state store with outbox

apiVersion: dapr.io/v1alpha1
kind: Component
metadata:
  name: mysql-outbox
spec:
  type: state.mysql
  version: v1
  metadata:
  - name: connectionString
    value: "<CONNECTION STRING>"
  - name: outboxPublishPubsub # Required
    value: "mypubsub"
  - name: outboxPublishTopic # Required
    value: "newOrder"

Demo

Watch this video for an overview of the outbox pattern: