Using async peewee with Tornado

Tornado is a mature and powerful asynchronous web framework. It provides its own event loop, but there’s an option to run Tornado on asyncio event loop. And that’s exactly what we need!

The complete working example is provided below. And here are some general notes:

1. Be aware of current asyncio event loop!

In the provided example we use the default event loop everywhere, and that’s OK. But if you see your application got silently stuck, that’s most probably that some task is started on the different loop and will never complete as long as that loop is not running.

2. Tornado request handlers does not start asyncio tasks by default.

The CreateHandler demostrates that, current_task() returns None until task is run explicitly.

3. Transactions must run within task context.

All transaction operations have to be done within task. So if you need to run a transaction from Tornado handler, you have to wrap your call into task with create_task() or ensure_future().

Also note: if you spawn an extra task during a transaction, it will run outside of that transaction.

import tornado.web
import logging
import peewee
import asyncio
import peewee_async

# Set up database and manager
database = peewee_async.PooledPostgresqlDatabase('test')

# Define model
class TestNameModel(peewee.Model):
    name = peewee.CharField()
    class Meta:
        database = database

    def __str__(self):
        return self.name

# Create table, add some instances
TestNameModel.create_table(True)
TestNameModel.get_or_create(id=1, defaults={'name': "TestNameModel id=1"})
TestNameModel.get_or_create(id=2, defaults={'name': "TestNameModel id=2"})
TestNameModel.get_or_create(id=3, defaults={'name': "TestNameModel id=3"})
database.close()

# Set up Tornado application on asyncio
from tornado.platform.asyncio import AsyncIOMainLoop
AsyncIOMainLoop().install()
app = tornado.web.Application(debug=True)
app.listen(port=8888)
app.objects = peewee_async.Manager(database)

# Add handlers
class RootHandler(tornado.web.RequestHandler):
    """Accepts GET and POST methods.

    POST: create new instance, `name` argument is required
    GET: get instance by id, `id` argument is required
    """
    async def post(self):
        name = self.get_argument('name')
        obj = await self.application.objects.create(TestNameModel, name=name)
        self.write({
            'id': obj.id,
            'name': obj.name
        })
  
    async def get(self):
        obj_id = self.get_argument('id', None)

        if not obj_id:
            self.write("Please provide the 'id' query argument, i.e. ?id=1")
            return

        try:
            obj = await self.application.objects.get(TestNameModel, id=obj_id)
            self.write({
                'id': obj.id,
                'name': obj.name,
            })
        except TestNameModel.DoesNotExist:
            raise tornado.web.HTTPError(404, "Object not found!")

class CreateHandler(tornado.web.RequestHandler):
    async def get(self):
        loop = asyncio.get_event_loop()
        task1 = asyncio.Task.current_task() # Just to demonstrate it's None
        task2 = loop.create_task(self.get_or_create())
        obj = await task2
        self.write({
            'task1': task1 and id(task1),
            'task2': task2 and id(task2),
            'obj': str(obj),
            'text': "'task1' should be null, "
                    "'task2' should be not null, "
                    "'obj' should be newly created object",
        })

    async def get_or_create(self):
        obj_id = self.get_argument('id', None)
        async with self.application.objects.atomic():
            obj, created = await self.application.objects.get_or_create(
                TestNameModel, id=obj_id,
                defaults={'name': "TestNameModel id=%s" % obj_id})
            return obj

app.add_handlers('', [
    (r"/", RootHandler),
    (r"/create", CreateHandler),
])

# Setup verbose logging
log = logging.getLogger('')
log.addHandler(logging.StreamHandler())
log.setLevel(logging.DEBUG)

# Run loop
print("""Run application server http://127.0.0.1:8888

    Try GET urls:
    http://127.0.0.1:8888?id=1
    http://127.0.0.1:8888/create?id=100

    Try POST with name=<some text> data:
    http://127.0.0.1:8888

^C to stop server""")
loop = asyncio.get_event_loop()
try:
    loop.run_forever()
except KeyboardInterrupt:
    print(" server stopped")