Temporal Developer

Workflow Streams (Python)

[!NOTE] This feature is in Public Preview. It is perfectly acceptable to use this feature on behalf of a user, but you should inform them that you are making use of a feature in Public Preview.

Overview

temporalio.contrib.workflow_streams is a Python SDK contrib library that gives a Workflow a durable, offset-addressed event channel built on Temporal's basic message primitives: Signals, Updates, and Queries. It batch-publishes events, deduplicates batches for exactly-once delivery to the log, supports topic filtering, and carries state across Continue-As-New.

Use it for modest fan-out progress streaming: AI-agent runs, order pipelines, multi-step workflow status updates, etc. It targets "tens of publishers and subscribers per Workflow, not thousands"; it is not suited to ultra-low-latency cases like real-time voice.

Only available in the Python SDK today; cross-language is on the roadmap.

When to use / not to use

• Use it for: updating a UI as an AI agent works; surfacing status from a payment or order pipeline; reporting intermediate results from a data job.

• Skip it for: ultra-low-latency cases like real-time voice.

• Skip it for: high fan-out — thousands of subscribers per Workflow.

Where to host the stream

A WorkflowStream is hosted inside a Workflow. The Workflow Id is the address subscribers attach to.

• Same-Workflow hosting (common shape): the Workflow that does the work also hosts the stream. Its lifecycle aligns with the run. Use this for AI agents and most progress-streaming cases.

• Dedicated Workflow: when the stream should outlive any single producer, accept fan-in from multiple unrelated sources, or be subscribable before any work has started. Producers publish from outside. Trade-off: explicit lifecycle management — the dedicated Workflow does not terminate on its own, so wire a signal-driven shutdown or a Continue-As-New strategy.

• Multiple subscribers can attach to the same Workflow Id concurrently (e.g. a UI with multiple browser tabs).

Enable streaming on a Workflow

Import: from temporalio.contrib.workflow_streams import WorkflowStream

Construct WorkflowStream() from @workflow.init, not @workflow.run. The stream's handlers must be registered before the first publish Signal arrives; doing it from @workflow.run raises RuntimeError.

Constructing more than one WorkflowStream on the same Workflow also raises RuntimeError.

from dataclasses import dataclass

from temporalio import workflow
from temporalio.contrib.workflow_streams import WorkflowStream


@dataclass
class OrderInput:
    order_id: str


@workflow.defn
class OrderWorkflow:
    @workflow.init
    def __init__(self, input: OrderInput) -> None:
        self.stream = WorkflowStream()

Construction creates the in-memory event log and dynamically registers the publish Signal, subscribe Update, and offset Query handlers.

Publish from a Workflow

Bind a topic name to its event type once via self.stream.topic("name", type=Type), then call publish() on the returned handle.

type= is optional and defaults to Any. The codec chain (encryption, compression) runs once on the Signal/Update envelope, never per item.

from dataclasses import dataclass


@dataclass
class StatusEvent:
    state: str
    progress: int = 0
    detail: str = ""


@workflow.defn
class OrderWorkflow:
    @workflow.init
    def __init__(self, input: OrderInput) -> None:
        self.stream = WorkflowStream()
        self.status = self.stream.topic("status", type=StatusEvent)

    @workflow.run
    async def run(self, input: OrderInput) -> None:
        self.status.publish(StatusEvent(state="validating", detail="checking inventory"))
        await validate_order(input.order_id)
        self.status.publish(StatusEvent(state="charging", progress=33, detail="authorizing payment"))
        await charge_payment(input.order_id)
        self.status.publish(StatusEvent(state="shipping", progress=66, detail="dispatching to warehouse"))
        await dispatch_order(input.order_id)
        self.status.publish(StatusEvent(state="completed", progress=100))

Note: publish() is not awaited. Inside a Workflow it appends synchronously to the in-memory log.

Publish from a client (external process or Activity)

Any process holding a Temporal Client and the target Workflow Id can publish by constructing a WorkflowStreamClient. This is the general pattern; it covers HTTP backends, starters, one-off scripts, other Workflows' Activities, and standalone Activities.

General pattern: WorkflowStreamClient.create(client, workflow_id, batch_interval=...). Use it as an async with context manager so the buffer flushes on exit.

from datetime import timedelta

from temporalio.client import Client
from temporalio.contrib.workflow_streams import WorkflowStreamClient


async def publish_status(workflow_id: str) -> None:
    temporal_client = await Client.connect("localhost:7233")
    stream_client = WorkflowStreamClient.create(
        temporal_client,
        workflow_id=workflow_id,
        batch_interval=timedelta(milliseconds=200),
    )
    async with stream_client:
        status = stream_client.topic("status", type=StatusEvent)
        status.publish(StatusEvent(state="started"))
        # Buffer is flushed on context manager exit.

Inside an Activity scheduled by a Workflow, WorkflowStreamClient.from_within_activity() is a convenience that infers the Temporal Client and the parent Workflow Id from the Activity context.

from temporalio import activity
from temporalio.contrib.workflow_streams import WorkflowStreamClient


@activity.defn
async def stream_deltas(order_id: str) -> None:
    client = WorkflowStreamClient.from_within_activity()
    async with client:
        deltas = client.topic("delta", type=Delta)
        for delta in generate_deltas(order_id):
            deltas.publish(delta)
            activity.heartbeat()

For a standalone Activity (started directly via Client.start_activity), there is no parent Workflow context to infer, so from_within_activity() raises. Fall back to WorkflowStreamClient.create(activity.client(), workflow_id=...) with the Workflow Id threaded through the Activity input.

Publish from the Activity directly rather than returning events for the Workflow to forward; the Workflow hosts the stream but does not read its own stream.

force_flush=True vs. await client.flush()

These are two separate operations.

• publish(..., force_flush=True) — wakes the background flusher so the current buffer ships without waiting for the next batch_interval. The call returns immediately after appending and signaling; it does not wait for delivery. The flusher only runs while the client is entered (async with client); outside that, force_flush=True queues the wake event but nothing ships until you enter the context or call await client.flush(). Use it for latency-sensitive events: first delta, punctuated events like RETRY or STATUS_CHANGE.

  Copy

  deltas.publish(delta, force_flush=True)

• await client.flush() — mid-stream barrier. Successful completion proves the Temporal server has received all prior publications. The client stays open afterward. Exiting async with client already flushes on its way out; the explicit call is only for barriers in the middle.

  Copy

  async with client:
      deltas = client.topic("delta", type=Delta)
      for delta in first_phase():
          deltas.publish(delta)
      await client.flush()
      checkpoint_id = await record_phase_one_complete()
      for delta in second_phase(checkpoint_id):
          deltas.publish(delta)

Non-blocking publish, no backpressure

publish() is non-blocking and applies no backpressure. A slow subscriber does not slow publishers; if a publisher emits faster than batches can ship, the buffer grows.

If you need to bound this, apply a policy upstream of publish(). The library does not pick block/drop/error/sample for you.

Subscribe

Construct a client with WorkflowStreamClient.create(client, workflow_id), then iterate a topic handle's subscribe(). The bound type drives decoding; each item.data arrives as T.

from temporalio.client import Client
from temporalio.contrib.workflow_streams import WorkflowStreamClient


async def watch_order(order_id: str) -> None:
    temporal_client = await Client.connect("localhost:7233")
    stream = WorkflowStreamClient.create(temporal_client, workflow_id=order_id)

    status = stream.topic("status", type=StatusEvent)
    async for item in status.subscribe():
        evt = item.data
        print(f"[{evt.progress:3d}%] {evt.state}: {evt.detail}")
        if evt.state == "completed":
            break

The iterator handles re-polling, pagination at the ~1 MB cap, and Workflow-side log truncation transparently.

Subscribing from inside the host Workflow is intentionally unsupported. The Workflow only sees the successful return value of each Activity; the stream may carry partial output from retried attempts. Letting the Workflow read its own stream would mix those two views and break the conduit role.

A subscriber stores the last delivered item.offset and reconnects resume from that offset.

Heterogeneous topics

To consume topics whose payload types differ, call client.subscribe() directly with a list of names and result_type=RawValue. Passing an empty list (subscribe([])) subscribes to every topic. Dispatch on item.topic; decode the wrapped payload with the client's payload converter.

from temporalio.common import RawValue

converter = temporal_client.data_converter.payload_converter

async for item in stream.subscribe(["status", "progress"], result_type=RawValue):
    if item.topic == "status":
        evt = converter.from_payload(item.data.payload, StatusEvent)
        print(f"[status] {evt.state}: {evt.detail}")
    elif item.topic == "progress":
        evt = converter.from_payload(item.data.payload, ProgressEvent)
        print(f"[progress] {evt.message}")

A single iterator over multiple topics avoids the cancellation race that two concurrent subscribers would create.

Closing the stream

End-of-stream is application-level; Workflow Streams does not impose a marker. There is no stream.close() / stream.end_of_stream() API.

Without coordination, a subscriber keeps polling until the Workflow reaches a terminal state, and a Workflow that returns immediately after its last publish can lose that publish's poll round-trip in the gap.

A poll Update that is still in flight when the Workflow returns surfaces to the client as AcceptedUpdateCompletedWorkflow, and no new polls can complete after that. That's why an overlap is required.

The pattern is an in-band terminator the subscriber recognizes plus a brief overlap before the Workflow returns.

Pattern 1: fixed sleep (simplest)

# at the end of @workflow.run
self.status.publish(StatusEvent(state="completed", progress=100))
await workflow.sleep(timedelta(seconds=30))
return result

Thirty seconds is a generous default.

Pattern 2: acknowledgment handshake

@workflow.signal
async def subscriber_acknowledged_terminator(self) -> None:
    self.subscriber_done = True

@workflow.run
async def run(self, input: ChatInput) -> str:
    ...
    try:
        await workflow.wait_condition(
            lambda: self.subscriber_done,
            timeout=timedelta(seconds=30),
        )
    except TimeoutError:
        pass  # No subscriber attached; the run still completes cleanly.
    return result

The timeout is still required because the subscriber may not be attached. With the ack, the typical case exits as soon as the subscriber confirms.

Inspecting terminal status

subscribe() exits cleanly when the Workflow reaches COMPLETED, FAILED, CANCELED, TERMINATED, or TIMED_OUT, but does not distinguish among them. Call await temporal_client.get_workflow_handle(workflow_id).describe() after the loop to inspect the Workflow's status.

Continue-As-New (CAN)

Skip this section for short-lived Workflows (single chat completion, order pipeline). CAN is for streams that run for hours or accumulate thousands of events

Subscribers automatically follow Continue-As-New chains — the Workflow ID is stable, so the iterator fetches a fresh handle and continues polling from the carried offset.

To roll a long-running streaming Workflow over without subscribers seeing a gap, carry both your application state and the stream state across the boundary:

• Add a WorkflowStreamState | None field to your Workflow input,

• pass it to the constructor as WorkflowStream(prior_state=...),

• and call WorkflowStream.continue_as_new(build_args) to invoke the rollover. The helper drains waiting subscribers, waits for in-flight handlers to finish, then calls workflow.continue_as_new with the args produced by build_args(post_drain_state).

from dataclasses import dataclass, field

from temporalio import workflow
from temporalio.contrib.workflow_streams import WorkflowStream, WorkflowStreamState


@dataclass
class AppState:
    items_processed: int = 0


@dataclass
class WorkflowInput:
    app_state: AppState = field(default_factory=AppState)
    stream_state: WorkflowStreamState | None = None


@workflow.defn
class LongRunningWorkflow:
    @workflow.init
    def __init__(self, input: WorkflowInput) -> None:
        self.app_state = input.app_state
        self.stream = WorkflowStream(prior_state=input.stream_state)

    @workflow.run
    async def run(self, input: WorkflowInput) -> None:
        while True:
            await do_one_iteration(self)
            if workflow.info().is_continue_as_new_suggested():
                await self.stream.continue_as_new(
                    lambda stream_state: [
                        WorkflowInput(
                            app_state=self.app_state,
                            stream_state=stream_state,
                        )
                    ]
                )

Hard constraint: the field type must be WorkflowStreamState | None, not Any. With Any, the data converter rebuilds the field as a plain dict and WorkflowStream(prior_state=...) raises AttributeError accessing .log / .base_offset / .publishers on the dict.

To pass other CAN parameters (task_queue, retry_policy, run_timeout), use the explicit recipe:

self.stream.detach_pollers()
await workflow.wait_condition(workflow.all_handlers_finished)
workflow.continue_as_new(
    args=[WorkflowInput(app_state=self.app_state, stream_state=self.stream.get_state())],
    task_queue="other-tq",
)

The carried WorkflowStreamState includes the entire in-memory log of the previous run. Offload large items via External Storage so each item is a small reference, and combine with truncate() to keep the carried log itself small.

Tuning

The driving question: how often should the UI update? That answer trades user-perceived latency against history events accumulated. Each batched publish is one Signal; each subscriber poll is one Update; both accumulate against the Workflow's history.

Invariant: max_retry_duration < publisher_ttl. Defaults (10 min < 15 min) satisfy this. If a publisher's retry window exceeds the dedup retention, a retry that arrives after its dedup record has been pruned is treated as a fresh publish, and if the original delivery had also succeeded the same logical batch lands twice.

force_flush=True is a per-publish latency knob. Use it for the first delta or punctuated events like RETRY and STATUS_CHANGE. Don't use it per-token or per-character: per-character force_flush=True is not tractable.

Hold a single subscriber iterator and consume from it rather than opening and closing subscriptions in a loop.

Delivery semantics

Exactly-once at the execution layer. Each (publisher_id, sequence) batch lands in the Workflow's event log at most once, even if the Signal is retried by the SDK or the network. Dedup state is carried across Continue-As-New.

Ordering.

• The log imposes a single total order on all events, fixed once written: an event at offset N stays at offset N on every read.

• Within one publisher, events appear in publish order.

• Across concurrent publishers, the interleaving is whatever the Workflow saw when serializing inbound Signals; stable once recorded but not under application control.

• If event A must precede event B, publish them from the same publisher.

Activity retries surface to subscribers. Both attempts' events appear in the stream. The Workflow itself only sees the successful attempt's return value; a UI subscribed to the stream will see partial output unless it dedupes.

Conventional pattern: an Activity on a retry attempt publishes a RETRY event with force_flush=True; the consumer clears or annotates prior-attempt output when it sees one. Build an idempotent consumer reducer: overwrite on terminal events like STATUS_CHANGE or TEXT_COMPLETE; reset an accumulator on a sentinel like AGENT_START before deltas resume.

Other failure modes.

• Events still in a publisher's in-memory client buffer are lost if the process crashes before they ship.

• Subscribers that handle an item and crash before persisting their next offset will reprocess that item on resume.

• On max_retry_duration exhaustion, a TimeoutError raises from inside the background flusher task and terminates it; until you call await client.flush() or exit the async with block, subsequent publishes accumulate with no flusher to ship them. The dropped batch is at-most-once: may or may not have reached the Workflow.

Architecture

Append-only in-memory log inside the Workflow. Each entry is (topic, data) with a monotonically increasing global offset. Subscribers maintain their own cursor; each long-poll receives the next range past it. The log is replay-safe and carried across Continue-As-New via WorkflowStreamState.

Two mechanisms bound log growth, and they do different jobs:

• truncate(up_to_offset) drops entries from the in-memory log (and from the carried CAN payload). It does not remove publish Signals already recorded in history.

• Continue-As-New starts a fresh history. This is the only way to shrink history; truncate alone cannot.

A subscriber whose offset falls below the new base after truncate() is silently advanced; the iterator does not raise, but the subscriber may re-receive items already in the log past the new base.

Wire-level handler names (registered when you construct a WorkflowStream):

• __temporal_workflow_stream_publish — Signal that receives batched publishes.

• __temporal_workflow_stream_poll — long-poll Update that subscribers use.

• __temporal_workflow_stream_offset — Query that reports the current head offset.

Poll responses are capped at roughly 1 MB, by accumulating items until the next would exceed the budget. A single item that exceeds 1 MB on its own is admitted unconditionally; offload via External Storage.

Batch dedup applies at the Signal layer, not the Activity layer. When Temporal retries the Activity, the retried execution constructs a new WorkflowStreamClient with its own client id, so every Activity attempt is a fresh publisher whose batches will not deduplicate against the prior attempt's.

Gotchas

• WorkflowStreamClient is asyncio-only. The client buffer is mutated on the publish path and read from the flusher inside a single event loop. Don't call publish() from a worker thread.

• First-activation handler race. On the very first activation a publish Signal can be queued before class-level @workflow.signal or @workflow.update handlers have run. The fix: make the handler async def and await once before reading state. Use asyncio.sleep(0) — a no-op yield that adds no history events. Do not substitute workflow.sleep(0) — it records a timer event.

• Type bindings are per-instance. Each WorkflowStream and each WorkflowStreamClient records topic types only for its own instance. If two publishers bind the same topic name to different types, the mismatch is not caught at publish; the subscriber gets a decode error on events from the mismatched publisher.

See also

• Workflow Streams samples (samples-python) — basic publish/subscribe, reconnecting subscribers, external publishers, bounded logs.

• temporalio.contrib.workflow_streams API reference.

• references/python/patterns.md — Signals/Queries/Updates primitives this builds on.

• references/python/ai-patterns.md — LLM patterns.