How-To: Enable the transactional outbox pattern

Commit a single transaction across a state store and pub/sub message broker

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.

Learn more about the transactional methods you can use.

Note

Message brokers that work with the competing consumer pattern (for example, Apache Kafka) are encouraged to reduce the chances of duplicate events.

Enable the outbox pattern

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

Additional configurations

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"

Shape the outbox pattern message

You can override the outbox pattern message published to the pub/sub broker by setting another transaction that is not be saved to the database and is explicitly mentioned as a projection. This transaction is added a metadata key named outbox.projection with a value set to true. When added to the state array saved in a transaction, this payload is ignored when the state is written and the data is used as the payload sent to the upstream subscriber.

To use correctly, the key values must match between the operation on the state store and the message projection. If the keys do not match, the whole transaction fails.

If you have two or more outbox.projection enabled state items for the same key, the first one defined is used and the others are ignored.

Learn more about default and custom CloudEvent messages.

• Python
• JavaScript
• .NET
• Java
• Go
• HTTP

In the following Python SDK example of a state transaction, the value of "2" is saved to the database, but the value of "3" is published to the end-user topic.

DAPR_STORE_NAME = "statestore"
async def main():
    client = DaprClient()
    # Define the first state operation to save the value "2"
    op1 = StateItem(
        key="key1",
        value=b"2"
    )
    # Define the second state operation to publish the value "3" with metadata
    op2 = StateItem(
        key="key1",
        value=b"3",
        options=StateOptions(
            metadata={
                "outbox.projection": "true"
            }
        )
    )
    # Create the list of state operations
    ops = [op1, op2]
    # Execute the state transaction
    await client.state.transaction(DAPR_STORE_NAME, operations=ops)
    print("State transaction executed.")

By setting the metadata item "outbox.projection" to "true" and making sure the key values match (key1):

• The first operation is written to the state store and no message is written to the message broker.
• The second operation value is published to the configured pub/sub topic.

In the following JavaScript SDK example of a state transaction, the value of "2" is saved to the database, but the value of "3" is published to the end-user topic.

const { DaprClient, StateOperationType } = require('@dapr/dapr');
const DAPR_STORE_NAME = "statestore";
async function main() {
  const client = new DaprClient();
  // Define the first state operation to save the value "2"
  const op1 = {
    operation: StateOperationType.UPSERT,
    request: {
      key: "key1",
      value: "2"
    }
  };
  // Define the second state operation to publish the value "3" with metadata
  const op2 = {
    operation: StateOperationType.UPSERT,
    request: {
      key: "key1",
      value: "3",
      metadata: {
        "outbox.projection": "true"
      }
    }
  };
  // Create the list of state operations
  const ops = [op1, op2];
  // Execute the state transaction
  await client.state.transaction(DAPR_STORE_NAME, ops);
  console.log("State transaction executed.");
}
main().catch(err => {
  console.error(err);
});

By setting the metadata item "outbox.projection" to "true" and making sure the key values match (key1):

• The first operation is written to the state store and no message is written to the message broker.
• The second operation value is published to the configured pub/sub topic.

In the following .NET SDK example of a state transaction, the value of "2" is saved to the database, but the value of "3" is published to the end-user topic.

public class Program
{
    private const string DAPR_STORE_NAME = "statestore";
    public static async Task Main(string[] args)
    {
        var client = new DaprClientBuilder().Build();
        // Define the first state operation to save the value "2"
        var op1 = new StateTransactionRequest(
            key: "key1",
            value: Encoding.UTF8.GetBytes("2"),
            operationType: StateOperationType.Upsert
        );
        // Define the second state operation to publish the value "3" with metadata
        var metadata = new Dictionary<string, string>
        {
            { "outbox.projection", "true" }
        };
        var op2 = new StateTransactionRequest(
            key: "key1",
            value: Encoding.UTF8.GetBytes("3"),
            operationType: StateOperationType.Upsert,
            metadata: metadata
        );
        // Create the list of state operations
        var ops = new List<StateTransactionRequest> { op1, op2 };
        // Execute the state transaction
        await client.ExecuteStateTransactionAsync(DAPR_STORE_NAME, ops);
        Console.WriteLine("State transaction executed.");
    }
}

By setting the metadata item "outbox.projection" to "true" and making sure the key values match (key1):

• The first operation is written to the state store and no message is written to the message broker.
• The second operation value is published to the configured pub/sub topic.

In the following Java SDK example of a state transaction, the value of "2" is saved to the database, but the value of "3" is published to the end-user topic.

public class Main {
    private static final String DAPR_STORE_NAME = "statestore";
    public static void main(String[] args) {
        try (DaprClient client = new DaprClientBuilder().build()) {
            // Define the first state operation to save the value "2"
            StateOperation<String> op1 = new StateOperation<>(
                    StateOperationType.UPSERT,
                    "key1",
                    "2"
            );
            // Define the second state operation to publish the value "3" with metadata
            Map<String, String> metadata = new HashMap<>();
            metadata.put("outbox.projection", "true");
            StateOperation<String> op2 = new StateOperation<>(
                    StateOperationType.UPSERT,
                    "key1",
                    "3",
                    metadata
            );
            // Create the list of state operations
            List<StateOperation<?>> ops = new ArrayList<>();
            ops.add(op1);
            ops.add(op2);
            // Execute the state transaction
            client.executeStateTransaction(DAPR_STORE_NAME, ops).block();
            System.out.println("State transaction executed.");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

By setting the metadata item "outbox.projection" to "true" and making sure the key values match (key1):

• The first operation is written to the state store and no message is written to the message broker.
• The second operation value is published to the configured pub/sub topic.

In the following Go SDK example of a state transaction, the value of "2" is saved to the database, but the value of "3" is published to the end-user topic.

ops := make([]*dapr.StateOperation, 0)
op1 := &dapr.StateOperation{
    Type: dapr.StateOperationTypeUpsert,
    Item: &dapr.SetStateItem{
        Key:   "key1",
        Value: []byte("2"),
    },
}
op2 := &dapr.StateOperation{
    Type: dapr.StateOperationTypeUpsert,
    Item: &dapr.SetStateItem{
        Key:   "key1",
                Value: []byte("3"),
         // Override the data payload saved to the database 
                Metadata: map[string]string{
                    "outbox.projection": "true",
        },
    },
}
ops = append(ops, op1, op2)
meta := map[string]string{}
err := testClient.ExecuteStateTransaction(ctx, store, meta, ops)

By setting the metadata item "outbox.projection" to "true" and making sure the key values match (key1):

• The first operation is written to the state store and no message is written to the message broker.
• The second operation value is published to the configured pub/sub topic.

You can pass the message override using the following HTTP request:

curl -X POST http://localhost:3500/v1.0/state/starwars/transaction \
  -H "Content-Type: application/json" \
  -d '{
  "operations": [
    {
      "operation": "upsert",
      "request": {
        "key": "order1",
        "value": {
            "orderId": "7hf8374s",
            "type": "book",
            "name": "The name of the wind"
        }
      }
    },
    {
      "operation": "upsert",
      "request": {
        "key": "order1",
        "value": {
            "orderId": "7hf8374s"
        },
        "metadata": {
           "outbox.projection": "true"
        },
        "contentType": "application/json"
      }
    }
  ]
}'

By setting the metadata item "outbox.projection" to "true" and making sure the key values match (key1):

• The first operation is written to the state store and no message is written to the message broker.
• The second operation value is published to the configured pub/sub topic.

Override Dapr-generated CloudEvent fields

You can override the Dapr-generated CloudEvent fields on the published outbox event with custom CloudEvent metadata.

• Python
• JavaScript
• .NET
• Java
• Go
• HTTP

async def execute_state_transaction():
    async with DaprClient() as client:
        # Define state operations
        ops = []
        op1 = {
            'operation': 'upsert',
            'request': {
                'key': 'key1',
                'value': b'2',  # Convert string to byte array
                'metadata': {
                    'cloudevent.id': 'unique-business-process-id',
                    'cloudevent.source': 'CustomersApp',
                    'cloudevent.type': 'CustomerCreated',
                    'cloudevent.subject': '123',
                    'my-custom-ce-field': 'abc'
                }
            }
        }
        ops.append(op1)
        # Execute state transaction
        store_name = 'your-state-store-name'
        try:
            await client.execute_state_transaction(store_name, ops)
            print('State transaction executed.')
        except Exception as e:
            print('Error executing state transaction:', e)
# Run the async function
if __name__ == "__main__":
    asyncio.run(execute_state_transaction())

const { DaprClient } = require('dapr-client');
async function executeStateTransaction() {
    // Initialize Dapr client
    const daprClient = new DaprClient();
    // Define state operations
    const ops = [];
    const op1 = {
        operationType: 'upsert',
        request: {
            key: 'key1',
            value: Buffer.from('2'),
            metadata: {
                'id': 'unique-business-process-id',
                'source': 'CustomersApp',
                'type': 'CustomerCreated',
                'subject': '123',
                'my-custom-ce-field': 'abc'
            }
        }
    };
    ops.push(op1);
    // Execute state transaction
    const storeName = 'your-state-store-name';
    const metadata = {};
}
executeStateTransaction();

public class StateOperationExample
{
    public async Task ExecuteStateTransactionAsync()
    {
        var daprClient = new DaprClientBuilder().Build();
        // Define the value "2" as a string and serialize it to a byte array
        var value = "2";
        var valueBytes = JsonSerializer.SerializeToUtf8Bytes(value);
        // Define the first state operation to save the value "2" with metadata
       // Override Cloudevent metadata
        var metadata = new Dictionary<string, string>
        {
            { "cloudevent.id", "unique-business-process-id" },
            { "cloudevent.source", "CustomersApp" },
            { "cloudevent.type", "CustomerCreated" },
            { "cloudevent.subject", "123" },
            { "my-custom-ce-field", "abc" }
        };
        var op1 = new StateTransactionRequest(
            key: "key1",
            value: valueBytes,
            operationType: StateOperationType.Upsert,
            metadata: metadata
        );
        // Create the list of state operations
        var ops = new List<StateTransactionRequest> { op1 };
        // Execute the state transaction
        var storeName = "your-state-store-name";
        await daprClient.ExecuteStateTransactionAsync(storeName, ops);
        Console.WriteLine("State transaction executed.");
    }
    public static async Task Main(string[] args)
    {
        var example = new StateOperationExample();
        await example.ExecuteStateTransactionAsync();
    }
}

public class StateOperationExample {
    public static void main(String[] args) {
        executeStateTransaction();
    }
    public static void executeStateTransaction() {
        // Build Dapr client
        try (DaprClient daprClient = new DaprClientBuilder().build()) {
            // Define the value "2"
            String value = "2";
            // Override CloudEvent metadata
            Map<String, String> metadata = new HashMap<>();
            metadata.put("cloudevent.id", "unique-business-process-id");
            metadata.put("cloudevent.source", "CustomersApp");
            metadata.put("cloudevent.type", "CustomerCreated");
            metadata.put("cloudevent.subject", "123");
            metadata.put("my-custom-ce-field", "abc");
            // Define state operations
            List<StateOperation<?>> ops = new ArrayList<>();
            StateOperation<String> op1 = new StateOperation<>(
                    StateOperationType.UPSERT,
                    "key1",
                    value,
                    metadata
            );
            ops.add(op1);
            // Execute state transaction
            String storeName = "your-state-store-name";
            daprClient.executeStateTransaction(storeName, ops).block();
            System.out.println("State transaction executed.");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

func main() {
    // Create a Dapr client
    client, err := dapr.NewClient()
    if err != nil {
        log.Fatalf("failed to create Dapr client: %v", err)
    }
    defer client.Close()
    ctx := context.Background()
    store := "your-state-store-name"
    // Define state operations
    ops := make([]*dapr.StateOperation, 0)
    op1 := &dapr.StateOperation{
        Type: dapr.StateOperationTypeUpsert,
        Item: &dapr.SetStateItem{
            Key:   "key1",
            Value: []byte("2"),
            // Override Cloudevent metadata
            Metadata: map[string]string{
                "cloudevent.id":                "unique-business-process-id",
                "cloudevent.source":            "CustomersApp",
                "cloudevent.type":              "CustomerCreated",
                "cloudevent.subject":           "123",
                "my-custom-ce-field":           "abc",
            },
        },
    }
    ops = append(ops, op1)
    // Metadata for the transaction (if any)
    meta := map[string]string{}
    // Execute state transaction
    err = client.ExecuteStateTransaction(ctx, store, meta, ops)
    if err != nil {
        log.Fatalf("failed to execute state transaction: %v", err)
    }
    log.Println("State transaction executed.")
}

curl -X POST http://localhost:3500/v1.0/state/starwars/transaction \
  -H "Content-Type: application/json" \
  -d '{
        "operations": [
          {
            "operation": "upsert",
            "request": {
              "key": "key1",
              "value": "2"
            }
          },
        ],
        "metadata": {
          "id": "unique-business-process-id",
          "source": "CustomersApp",
          "type": "CustomerCreated",
          "subject": "123",
          "my-custom-ce-field": "abc",
        }
      }'

Note

The data CloudEvent field is reserved for Dapr’s use only, and is non-customizable.

Demo

Watch this video for an overview of the outbox pattern:

Last modified October 11, 2024: Fixed typo (#4389) (fe17926)