# How to Call a Celery Task From Outside its Code Base

The standard Celery docs and tutorials usually assume  
that your Celery and your API source code live in the same code base.  
In that scenario you simply import the the task into your API module  
and invoke the Celery task asynchronously.

```python
from flask import Flask, jsonify
from tasks import fetch_data

app = Flask(__name__)

@app.route('/', methods=['POST'])
def index():
    fetch_data.s(url=request.json['url']).delay()
    return jsonify({'url': request.json['url']}), 201
```

This only works as long as the task is registered in your current process.  
You need an alternative strategy when your Celery and your API source code  
are not part of the same code base. Or when you have multiple  
Celery microservices and need to call a Celery task  
from within another Celery microservice.

## **Option 1: app.send\_task**

The Celery app instance has a [send\_task](http://docs.celeryproject.org/en/latest/reference/celery.html?ref=celery-school.ghost.io#celery.Celery.send_task)  
method that can be use to call a task by its name.

```python
from flask import Flask, jsonify
from worker import app as celery_app

app = Flask(__name__)

@app.route('/', methods=['POST'])
def index():
    celery_app.send_task('fetch_data', kwargs={'url': request.json['url']})
    return jsonify({'url': request.json['url']}), 201
```

## **Option 2: app.signature**

In our initial example, the `.s(...)` method on the Celery task creates a called Celery [Signature](http://docs.celeryproject.org/en/latest/reference/celery.html?ref=celery-school.ghost.io#celery.Signature) object. A Celery `Signature` essentially wraps the arguments, keyword arguments, and execution options  
of a single Celery task invocation so that it can be passed to functions or serialized and  
sent across the wire.

An alternative strategy therefore is to explicitly create the `Signature` object via the Celery app's  
[signature](http://docs.celeryproject.org/en/latest/reference/celery.html?ref=celery-school.ghost.io#celery.signature) method,  
passing the task name and its kwargs. The resulting Signature object can then be asynchronously executed by the `delay()` method in the same way we did it in the standard example.

```python
from flask import Flask, jsonify
from worker import app as celery_app

app = Flask(__name__)

@app.route('/', methods=['POST'])
def index():
    celery_app.signature('fetch_data', kwargs={'url': request.json['url']).delay()
    return jsonify({'url': request.json['url']}), 201
```

## **Summary**

Celery does not require access to a task's code base in order to invoke it.  
The trick is to invoke a task by its name, either directly via `celery_app.send_task(...)`  
or by creating a Signature object `celery_app.signature(...)` which  
is the equivalent of calling `task.s(...)` when you do have access  
to the task's code base.

Which approach you prefer is ultimately a question of taste, there are minor  
implictions on how you test your task invocations, which we will cover in a another blog post.