Concurrent execution of workflows

In addition to looping and branching, workflows can run steps concurrently. This is useful when you have multiple steps that can be run independently of each other and they have time-consuming operations that they await, allowing other steps to run in parallel.

Emitting multiple events

In our examples so far, we've only emitted one event from each step. But there are many cases where you would want to run steps in parallel. To do this, you need to emit multiple events. You can do this using send_event:

class ParallelFlow(Workflow):
    @step
    async def start(self, ctx: Context, ev: StartEvent) -> StepTwoEvent:
        ctx.send_event(StepTwoEvent(query="Query 1"))
        ctx.send_event(StepTwoEvent(query="Query 2"))
        ctx.send_event(StepTwoEvent(query="Query 3"))

    @step(num_workers=4)
    async def step_two(self, ctx: Context, ev: StepTwoEvent) -> StopEvent:
        print("Running slow query ", ev.query)
        await asyncio.sleep(random.randint(1, 5))

        return StopEvent(result=ev.query)

In this example, our start step emits 3 StepTwoEvents. The step_two step is decorated with num_workers=4, which tells the workflow to run up to 4 instances of this step concurrently (this is the default).

Collecting events

If you execute the previous example, you'll note that the workflow stops after whichever query is first to complete. Sometimes that's useful, but other times you'll want to wait for all your slow operations to complete before moving on to another step. You can do this using collect_events:

class ConcurrentFlow(Workflow):
    @step
    async def start(self, ctx: Context, ev: StartEvent) -> StepTwoEvent:
        ctx.send_event(StepTwoEvent(query="Query 1"))
        ctx.send_event(StepTwoEvent(query="Query 2"))
        ctx.send_event(StepTwoEvent(query="Query 3"))

    @step(num_workers=4)
    async def step_two(self, ctx: Context, ev: StepTwoEvent) -> StepThreeEvent:
        print("Running query ", ev.query)
        await asyncio.sleep(random.randint(1, 5))
        return StepThreeEvent(result=ev.query)

    @step
    async def step_three(self, ctx: Context, ev: StepThreeEvent) -> StopEvent:
        # wait until we receive 3 events
        result = ctx.collect_events(ev, [StepThreeEvent] * 3)
        if result is None:
            return None

        # do something with all 3 results together
        print(result)
        return StopEvent(result="Done")

The collect_events method lives on the Context and takes the event that triggered the step and an array of event types to wait for. In this case, we are awaiting 3 events of the same StepThreeEvent type.

The step_three step is fired every time a StepThreeEvent is received, but collect_events will return None until all 3 events have been received. At that point, the step will continue and you can do something with all 3 results together.

The result returned from collect_events is an array of the events that were collected, in the order that they were received.

Multiple event types

Of course, you do not need to wait for the same type of event. You can wait for any combination of events you like, such as in this example:

class ConcurrentFlow(Workflow):
    @step
    async def start(
        self, ctx: Context, ev: StartEvent
    ) -> StepAEvent | StepBEvent | StepCEvent:
        ctx.send_event(StepAEvent(query="Query 1"))
        ctx.send_event(StepBEvent(query="Query 2"))
        ctx.send_event(StepCEvent(query="Query 3"))

    @step
    async def step_a(self, ctx: Context, ev: StepAEvent) -> StepACompleteEvent:
        print("Doing something A-ish")
        return StepACompleteEvent(result=ev.query)

    @step
    async def step_b(self, ctx: Context, ev: StepBEvent) -> StepBCompleteEvent:
        print("Doing something B-ish")
        return StepBCompleteEvent(result=ev.query)

    @step
    async def step_c(self, ctx: Context, ev: StepCEvent) -> StepCCompleteEvent:
        print("Doing something C-ish")
        return StepCCompleteEvent(result=ev.query)

    @step
    async def step_three(
        self,
        ctx: Context,
        ev: StepACompleteEvent | StepBCompleteEvent | StepCCompleteEvent,
    ) -> StopEvent:
        print("Received event ", ev.result)

        # wait until we receive 3 events
        if (
            ctx.collect_events(
                ev,
                [StepCCompleteEvent, StepACompleteEvent, StepBCompleteEvent],
            )
            is None
        ):
            return None

        # do something with all 3 results together
        return StopEvent(result="Done")

There are several changes we've made to handle multiple event types:

• start is now declared as emitting 3 different event types
• step_three is now declared as accepting 3 different event types
• collect_events now takes an array of the event types to wait for

Note that the order of the event types in the array passed to collect_events is important. The events will be returned in the order they are passed to collect_events, regardless of when they were received.

The visualization of this workflow is quite pleasing:

Now let's look at how we can extend workflows with subclassing and other techniques.