presentations/2026/celery/slides/02-progressbar.md

# Les barres de progression dans Celery
<img src="./assets/progressbar.png">
    
- **Intro**
- Communication Worker->Django
- Communication Django->Client
- Accès concurrent



## Intro

### Liste des courses
- Progression en temps réel
- Barre de progression propre à chaque tâche instanciée

### Contraintes
- Tâches possiblement longues
- Éviter le polling (fonctionner sur évènements)
- Pas de résidus en cas de plantage



### Situation initiale

- Fonctionnalité non existante dans Celery
- Solutions sur le net : progression par comptage de micro-tâches
- Une bibliothèque existante sur le net : celery-progress



### Complexité supplémentaire

<div data-mermaid>
    <script type="text/mermaid">
        %%{init: {'theme': 'light', 'themeVariables': { 'darkMode': false }}}%%
        sequenceDiagram
            actor User
            participant Django
            participant Broker@{ "type" : "database" }
            participant Worker
            participant Postgres@{ "type" : "database" }
            User->>Django: Click button
            Django->>Broker: Send message : schedule task
            Broker->>Worker: Acquire task
            activate Worker
            Worker->>Postgres: Save results
            deactivate Worker
            User ->>Django: Query task result
            Django->>Postgres: Query task result
            Postgres-->>Django: task result
            Django-->>User: task result
    </script>
</div>




<div data-mermaid data-auto-animate>
    <script type="text/mermaid">
    sequenceDiagram
        actor User
        participant Django
        participant Broker@{ "type" : "database" }
        participant Worker
        participant Postgres@{ "type" : "database" }
        User->>Django: Click button
        Django->>Broker: Send message : schedule task
        Broker->>Worker: Acquire task
        activate Worker
        rect rgb(255, 224, 224)
            loop
                Worker->>Worker: Work
                Worker->>Broker: Progress event
                Broker->>Django: Progress event
                Django->>User: Progress event
            end
        end
        Worker->>Postgres: Save results
        deactivate Worker
        User ->>Django: Query task result
        Django->>Postgres: Query task result
        Postgres-->>Django: task result
        Django-->>User: task result
    </script>
</div>





<div data-mermaid >
    <script type="text/mermaid">
    sequenceDiagram
        actor User
        participant Django
        participant Worker
        participant Postgres@{ "type" : "database" }
        User->>Django: Click button
        Django->>()Worker: Send message : schedule task
        activate Worker
        rect rgb(255, 224, 224)
            loop
                Worker->>Worker: Work
                Worker->>()Django: Progress event
                Django->>User: Progress event
            end
        end
        Worker->>Postgres: Save results
        deactivate Worker
        User ->>Django: Query task result
        Django->>Postgres: Query task result
        Postgres-->>Django: task result
        Django-->>User: task result
    </script>
</div>

---

## Communication worker -> server

- Coté Worker
- Coté Django
- Accès concurrent

---

### Coté Worker
https://docs.celeryq.dev/en/main/reference/celery.events.html
> The worker has the ability to send a message whenever some event happens. These events are then captured by tools like Flower, and celery events to monitor the cluster.



### Coté Worker
- celery.events.EventDispatcher 📢
- celery.events.EventReceiver 👂




### Coté Worker
<!-- .slide: data-auto-animate -->

```python[]
class PtfTask(AbortableTask):
    _progress_dispatcher = None

    @property
    def progress_dispatcher(self):
        # Singleton
        ...
        
    def update_state(self, task_id, state, meta, **kwargs):
        self.progress_dispatcher.send(
            "ptf-task-progress",
            data=meta,
            uuid=task_id or self.request.id,
        )
        return super().update_state(task_id, state, meta, **kwargs)

```
<!-- .element.code: data-id="code" -->
> On envoie un évènement `ptf-task-progress` quand on met à jour l'état de la tâche



### Coté Worker
<!-- .slide: data-auto-animate -->
```python[13]
class PtfTask(AbortableTask):
    _progress_dispatcher = None

    @property
    def progress_dispatcher(self):
        # Singleton
        ...
        
    def update_state(self, task_id, state, meta, **kwargs):
        self.progress_dispatcher.send(
            "ptf-task-progress",
            data=meta,
            uuid=task_id or self.request.id,
        )
        return super().update_state(task_id, state, meta, **kwargs)

```
<!-- .element: data-id="code" -->
> Point important: on peut faire avancer la progression de n'importe quelle tâche tant qu'on a son ID!



### Coté Worker

```python
@shared_task(name="crawler.tasks.toto", base=PtfTask, bind=True)
def print_toto(self, toto = "toto"):   
    meta = {"current": 0, "total": len(toto)}
    # Initialize progress bar at 0
    self.update_state(
        meta=meta,
        state=states.STARTED,
    )
    
    for index, letter in enumerate(toto):
        # Do work
        print(letter)
        
        # Update progress bar
        meta["current"] = index
        self.update_state(
            meta=meta,
            state=states.STARTED,
        )
```

---

### Coté Django

```python
from celery import Celery, current_app

def send_sse_message(event):
    ...
    
handlers = {
    "ptf-task-progress": send_sse_message,
}

receiver = current_app.events.Receiver(
    channel=current_app.connection_for_read(), app=current_app, handlers=handlers
)
```
> On envoie un message au client quand on reçoit un évènement `ptf-task-progress`



### Coté Django
Django 4.2 Workaround
<img src="./assets/disconnections.png">