Dagger

Dagger is a distributed, scalable, durable, and highly available orchestration engine to execute asynchronous and synchronous long-running business logic in a scalable and resilient way. Dagger requires Python 3.7 or later for the new async/await_ syntax, and variable type annotations.

Pizza Ordering and Delivery Workflow Example

Here's an example of how to use the library to build and run a Pizza Ordering Workflow:

The PizzaWorkflow consists of 2 Processes:

• Order : Responsible for communicating with the order service to place a pizza order(CommandTask) and wait for the order to be ready(ListenerTask)
• Delivery: Once the order is ready, this process communicates with the delivery service to start delivery of the pizza order(CommandTask)

Step 1 Instantiate Dagger

workflow_engine = Dagger(
                    broker=KAFKA_ADMIN_CLIENT_URL,
                    store="aerospike://",
                    consumer_auto_offset_reset="latest",
                    task_update_topic="task_update_topic",
                    trigger_interval=600,
                    aerospike_config=aerospike_config,
                    enable_changelog=False,
                    web_port=6066,
                    serializer="raw",
                )

Step 2 Define Leaf Command and Listener Tasks For Order Process

Let's assume that the Order Microservice processes incoming orders over a Kafka Topic - pizza_order_topic with the JSON schema

{
  "order_id": "id",
  "pizza_type": "1",
  "customer_id": "customer_id"
}

Using this information let's build out the OrderCommandTask by overriding the execute method that implements the business logic on how to send the payload to the Order Service over a Kafka topic

class OrderCommandTask(KafkaCommandTask[str, str]):
    async def execute(
        self,
        runtime_parameters: Dict[str, str],
        workflow_instance: ITemplateDAGInstance,
    ) -> None:
        payload = {
            "order_id": runtime_parameters["order_id"],
            "customer_id": runtime_parameters["customer_id"],
            "pizza_type": runtime_parameters["pizza_type"],
        }
        await workflow_engine.topics[self.topic].send(
            value=json.dumps(payload)
        )

After executing the OrderCommandTask, the workflow should enter a WAIT_STATE until it receives a message from the OrderService about the status of the order. Let's assume that Order Service sends a message on a Kafka Topic: order_status_topic when the order is ready in the following JSON format

{
  "order_id": "id",
  "status": "READY"
}

Let's model the OrderListenerTask to process this message on the order_status_topic by implementing the get_correlatable_keys_from_payload and on_message methods on the Listener. It also needs to specify the correletable_key as order_id to look up the payload

class PizzaWaitForReadyListener(KafkaListenerTask[str, str]):
    correlatable_key = "order_id"

    async def get_correlatable_keys_from_payload(
        self, payload: Any
    ) -> List[TaskLookupKey]:
        tpayload = json.loads(payload)
        key = tpayload[self.correlatable_key]
        return [(self.correlatable_key, key)]

    async def on_message(
            self, runtime_parameters: Dict[str, VT], *args: Any, **kwargs: Any
    ) -> bool :
        logger.info(f"Pizza Order is Ready")
        return True

When the order service sends a status message on the order_status_topic, Dagger invokes the get_correlatable_keys_from_payload to determine which workflow instance that message belongs to. Once it determines the workflow instance, it invokes on_message on the corresponding ListenerTask

Now that we have the LEAF tasks modeled, lets attach them to the parent Order Process

def pizza_ordering_process(
    process_name: str = "Order"
) -> IProcessTemplateDAGBuilder:
    dag_builder = DAGBuilderHelper(dagger_app=workflow_engine)
    root_task = dag_builder.build_and_link_tasks(
        [
           dag_builder.generic_command_task_builder(
                topic="pizza_order_topic",
                task_type=OrderCommandTask,
                process_name=process_name,
            ),
            dag_builder.generic_listener_task_builder(
                topic="PizzaWaitForReadyListener",
                task_type=PizzaWaitForReadyListener,
                process_name=process_name,
            ),
        ]
    )
    return dag_builder.generic_process_builder(process_name=process_name, root_task=root_task)

The Order Process is in COMPLETED when both the CommandTask and the PizzaWaitForReadyListener are COMPLETED and then the workflow transitions to execute the next Process Delivery

Step 3 Define Leaf Command Tasks For Delivery Process

Let's assume that the delivery service just requires an HTTP POST request with the following schema

{
  "order_id": "id",
  "customer_id": "customer_id"
}

We can model the DeliveryCommandTask to POST this payload by implementing the execute method as follows

class DeliveryCommandTask(ExecutorTask[str, str]):
    async def execute(
        self, runtime_parameters: Dict[str, VT], workflow_instance: ITask = None
    ) -> None:
        payload = {
            "order_id": runtime_parameters["order_id"],
            "customer_id": runtime_parameters["customer_id"],
        }
        async with aiohttp.ClientSession() as session:
            async with session.post(url="http://www.deliverysvc.com", json=payload):
                pass

Let's attach this to the parent Delivery Process

def pizza_delivery_process(
    process_name: str = "Delivery",
) -> IProcessTemplateDAGBuilder:
    dag_builder = DAGBuilderHelper(dagger_app=workflow_engine)
    root_task = dag_builder.build_and_link_tasks(
        [
            dag_builder.generic_executor_task_builder(
                task_type=DeliveryCommandTask,
                name=process_name,
            )
        ]
    )
    return dag_builder.generic_process_builder(
        process_name=process_name, root_task=root_task
    )

Step 4 Define the Sequence of Process Execution and register the workflow definition using register_template

Based on the workflow, we want the Order Process to execute first before the Delivery Process. The workflow ensures that the Delivery tasks are executed only after both the tasks in the Order process are in a terminal state

@Dagger.register_template("PizzaWorkflow")
def register_pizza_workflow(template_name: str) -> ITemplateDAG:
    dag_builder_helper = DAGBuilderHelper(workflow_engine)
    order_process = dag_builder_helper.build_and_link_processes(
        [
            pizza_ordering_process(process_name="Order"),
            pizza_delivery_process(process_name="Delivery"),
        ]
    )
    return dag_builder_helper.generic_template(
        template_name=template_name, root_process=order_process
    )

Step 5 Define an API to instantiate and execute pizza order workflows

async def create_and_submit_pizza_delivery_workflow(
    order_id: str, customer_id: str, pizza_type: int
):
    pizza_workflow_template = workflow_engine.template_dags["PizzaWorkflow"]
    pizza_workflow_instance = await pizza_workflow_template.create_instance(
        uuid.uuid1(),
        repartition=False,  # Create this instance on the current worker
        order_id=order_id,
        customer_id=customer_id,
        pizza_type=pizza_type,
    )
    await workflow_engine.submit(pizza_workflow_instance, repartition=False)

Step 6 Start the worker

workflow_engine.main()

Dagger is

Simple

Dagger is extremely easy to use. To get started applications need to install this library, define a DAG using the default templates or extending them based on the use case, creating instances of these DAG's and scheduling them for execution. The library hides all the complexity of producing and consuming from Kafka, maintaining Kafka Streams topology processing and also persistence and recovery of created tasks

Highly Available

Dagger is highly available and can survive network problems and server crashes. In the case of node failure, it can automatically recover the state store(representing task data) or failover to a standby node

Distributed

Start more instances of your application as needed to distribute the load on the system

Fast

A single-core worker instance can already process tens of thousands of tasks every second. Dagger uses a fast key-value lookup store based on rocksDB replicated to kafka topics for fault tolerance

Installation

You can install dagger via the Wayfair artifactory or from source.

To install using pip:

pip install wf-dagger

dagger has a dependency on faust-streaming for kafka stream processing

FAQ

Which version of python is supported?

dagger supports python version >= 3.7

What kafka versions are supported?

dagger supports kafka with version >= 0.10.

Roadmap

See the open issues for a list of proposed features (and known issues).

Contributing

Contributions are what make the open source community such an amazing place to learn, inspire, and create. Any contributions you make are greatly appreciated. For detailed contributing guidelines, please see CONTRIBUTING.md

License

Distributed under the MIT LICENSE License. See LICENSE for more information.

Contact

Vikram Patki - vpatki@wayfair.com

Project Link: https://github.com/wayfair-incubator/dagger

Acknowledgements

This template was adapted from https://github.com/othneildrew/Best-README-Template.

Documentation

Check out the project documentation.

For an overview on the repository structure and how to work with the code base, read the Development Guide.