Skip to content

Streaming (SSE) demo

One task that demonstrates live progress via Server-Sent Events.

progress

A four-step flow that emits status, progress, and done events as it runs.

# Sync REST (waits for final JSON)
curl -s -X POST localhost:8765/tasks/streaming/progress -d '{}'

# SSE stream (live events)
curl -N -X POST localhost:8765/tasks/streaming/progress \
  -H 'Accept: text/event-stream' -d '{}'

curl -N -X POST http://127.0.0.1:8765/tasks/streaming/progress \
  -H 'Content-Type: application/json' \
  -H 'Accept: text/event-stream' \
  -d '{}'

import requests

with requests.post(
    "http://127.0.0.1:8765/tasks/streaming/progress",
    json={},
    headers={"Accept": "text/event-stream"},
    stream=True,
) as resp:
    for line in resp.iter_lines(decode_unicode=True):
        if line:
            print(line)

Event stream

SSE event stream

Sample output:

event: status
data: {"text":"Step 1 of 4 — navigate","data":null}

event: progress
data: {"text":"navigation complete","data":{"fraction":0.25}}

event: status
data: {"text":"Step 2 of 4 — pause briefly","data":null}

event: progress
data: {"text":"pause finished","data":{"fraction":0.5}}

event: status
data: {"text":"Step 3 of 4 — read title","data":null}

event: progress
data: {"text":"extracted title","data":{"fraction":0.75}}

event: status
data: {"text":"Step 4 of 4 — done","data":null}

event: done
data: {"ok":true,"data":{"page":{"title":"Example Domain"}}}

Recipe (.webtask)

task "streaming/progress"
    pool default
    timeout 30000
    transport rest
    transport sse

    status "Step 1 of 4 — navigate"
    goto "https://example.com"
    emit progress "navigation complete" data { fraction: 0.25 }

    status "Step 2 of 4 — pause briefly"
    wait 1000
    emit progress "pause finished" data { fraction: 0.5 }

    # … steps 3–4 …
end

Concepts:

  • Two transport lines (rest + sse) opt the task into streaming
  • Every status "…" line becomes an SSE status event automatically
  • emit progress "…" data {…} sends custom events with arbitrary JSON
  • The final result arrives as event: done

When to use SSE

Use SSE when… Use sync REST when…
Long-running tasks (minutes) Tasks finish in seconds
UI shows a progress bar Batch/cron jobs
User waits interactively Fire-and-forget with polling

Protocol details: HTTP API → SSE

What's next?