ContextError while in context

[Mystic-Mirage]

I have a random crash while querying. It happens from time to time (once or twice a week) and most of the time things just work. Traceback says about missing context but the code already within NDB client's context. I'm not using threading like in #128 so seems it is a something different.

python-ndb == 0.0.1

Code example

class Settings(ndb.Model):
 name = ndb.StringProperty()
 value = ndb.StringProperty()

 @classmethod
 def get(cls, name):

 with ndb.Client().context():
 entity = cls.query(Settings.name == name).get() # <== exception raises here

 return entity.value

Stack trace

Traceback (most recent call last):
 File "/env/lib/python3.7/site-packages/flask/app.py", line 2292, in wsgi_app
 response = self.full_dispatch_request()
 File "/env/lib/python3.7/site-packages/flask/app.py", line 1815, in full_dispatch_request
 rv = self.handle_user_exception(e)
 File "/env/lib/python3.7/site-packages/flask/app.py", line 1718, in handle_user_exception
 reraise(exc_type, exc_value, tb)
 File "/env/lib/python3.7/site-packages/flask/_compat.py", line 35, in reraise
 raise value
 File "/env/lib/python3.7/site-packages/flask/app.py", line 1813, in full_dispatch_request
 rv = self.dispatch_request()
 File "/env/lib/python3.7/site-packages/flask/app.py", line 1799, in dispatch_request
 return self.view_functions[rule.endpoint](**req.view_args)
 File "/srv/lesivka_www/routes/www.py", line 35, in template_view
 return render_template(template)
 File "/env/lib/python3.7/site-packages/flask/templating.py", line 133, in render_template
 ctx.app.update_template_context(context)
 File "/env/lib/python3.7/site-packages/flask/app.py", line 792, in update_template_context
 context.update(func())
 File "/srv/main.py", line 19, in template_injection
 TRACKING_ID=Settings.get('TRACKING_ID'),
 File "/srv/lesivka_www/storage.py", line 14, in get
 entity = cls.query(Settings.name == name).get()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/query.py", line 1180, in wrapper
 return wrapped(self, *dummy_args, _options=query_options)
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/query.py", line 2019, in get
 return self.get_async(_options=_options).result()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 190, in result
 self.check_success()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 134, in check_success
 self.wait()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 126, in wait
 _eventloop.run1()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/_eventloop.py", line 411, in run1
 loop.run1()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/_eventloop.py", line 344, in run1
 delay = self.run0()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/_eventloop.py", line 333, in run0
 callback(rpc)
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 323, in done_callback
 _eventloop.call_soon(self._advance_tasklet, yielded.result())
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/_eventloop.py", line 380, in call_soon
 loop = get_event_loop()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/_eventloop.py", line 368, in get_event_loop
 context = context_module.get_context()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/context.py", line 78, in get_context
 raise exceptions.ContextError()
google.cloud.ndb.exceptions.ContextError: No current context. NDB calls must be made in context established by google.cloud.ndb.Client.context.

[cguardia]

Thanks for the report. This is a bit mysterious, but we'll see what we can find.

[Mystic-Mirage]

The same app and the same error but slightly different traceback was seen two days ago:

Traceback (most recent call last):
 File "/env/lib/python3.7/site-packages/flask/app.py", line 2292, in wsgi_app
 response = self.full_dispatch_request()
 File "/env/lib/python3.7/site-packages/flask/app.py", line 1815, in full_dispatch_request
 rv = self.handle_user_exception(e)
 File "/env/lib/python3.7/site-packages/flask/app.py", line 1718, in handle_user_exception
 reraise(exc_type, exc_value, tb)
 File "/env/lib/python3.7/site-packages/flask/_compat.py", line 35, in reraise
 raise value
 File "/env/lib/python3.7/site-packages/flask/app.py", line 1813, in full_dispatch_request
 rv = self.dispatch_request()
 File "/env/lib/python3.7/site-packages/flask/app.py", line 1799, in dispatch_request
 return self.view_functions[rule.endpoint](**req.view_args)
 File "/srv/lesivka_www/routes/www.py", line 35, in template_view
 return render_template(template)
 File "/env/lib/python3.7/site-packages/flask/templating.py", line 133, in render_template
 ctx.app.update_template_context(context)
 File "/env/lib/python3.7/site-packages/flask/app.py", line 792, in update_template_context
 context.update(func())
 File "/srv/main.py", line 19, in template_injection
 TRACKING_ID=Settings.get('TRACKING_ID'),
 File "/srv/lesivka_www/storage.py", line 14, in get
 entity = cls.query(Settings.name == name).get()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/query.py", line 1180, in wrapper
 return wrapped(self, *dummy_args, _options=query_options)
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/query.py", line 2019, in get
 return self.get_async(_options=_options).result()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 190, in result
 self.check_success()
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 137, in check_success
 raise self._exception
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/tasklets.py", line 288, in _advance_tasklet
 self.generator.throw(type(error), error)
 File "/env/lib/python3.7/site-packages/google/cloud/ndb/query.py", line 2050, in get_async
 results = yield _datastore_query.fetch(options)
google.cloud.ndb.exceptions.ContextError: No current context. NDB calls must be made in context established by google.cloud.ndb.Client.context.

[chrisrossi]

Hi @Mystic-Mirage ,

I'm going to build a sample app and and try some stress testing to see if I can reproduce this.

Which WSGI container are you using? It's possible that however it does threads/dispatching is incompatible with how NDB uses thread locals to keep track of its context. Either way, I'd like to test with the same container you're using.

Thanks!

[Mystic-Mirage]

I'm using Python 3.7 standard environment without specifying an entrypoint in the app.yaml. The documentation says Gunicorn is used by default.

[chrisrossi]

HI @Mystic-Mirage

I still haven't managed to recreate this error. I did try it with your method of creating the client in the view code and I've noticed a very big performance hit creating the client from scratch on every request. The recommended way of integrating is to create the client once at startup and use a WSGI middleware. Here is the test app I've been using:

import flask
import pprint

from google.cloud import ndb


def ndb_wsgi_middleware(wsgi_app):
 def middleware(environ, start_response):
 with client.context():
 return wsgi_app(environ, start_response)

 return middleware


client = ndb.Client()
app = flask.Flask("NDB Example")
app.wsgi_app = ndb_wsgi_middleware(app.wsgi_app) # Wrap the app in middleware.


class SomeThing(ndb.Model):
 foo = ndb.StringProperty()


@app.route("/")
def show_things():
 things = SomeThing.query()
 return pprint.pformat([thing_json(thing) for thing in things]) + "\n"


@app.route("/make/<foo>")
def make_thing(foo):
 thing = SomeThing(foo=foo)
 key = thing.put()
 return flask.redirect(f"/show/{key.id()}", code=302)


@app.route("/show/<id>")
def show_thing(id):
 key = ndb.Key("SomeThing", int(id))
 thing = key.get()
 if thing is None:
 flask.abort(404)

 return pprint.pformat(thing_json(thing)) + "\n"


def thing_json(thing):
 return {
 "key": thing.key,
 "properties": thing.to_dict(),
 }

I still haven't tracked down the source of your exceptions, but you should see a bit of a performance increase, and be able to write cleaner code, if you switch to instantiating the Client once and using a middleware.

[Mystic-Mirage]

Thank you @chrisrossi. I will change my code and see what happen.

[chrisrossi]

@Mystic-Mirage,

Try as I might, I haven't been able to reproduce this error. Let's make sure that Client.context() is only being called in one place in your code. (The middleware.) Then see if this resurfaces.

Thanks,
Chris

[Mystic-Mirage]

Should I use middleware or I can use something like bellow?

from google.cloud import ndb

NOT_SET = 'NOT SET'
client = ndb.Client()


class Settings(ndb.Model):
 name = ndb.StringProperty()
 value = ndb.StringProperty()

 @classmethod
 def get(cls, name):
 with client.context():
 entity = cls.query(Settings.name == name).get()

 if entity is None:
 entity = cls()
 entity.name = name
 entity.value = NOT_SET
 entity.put()

 if entity.value == NOT_SET:
 raise ValueError(name)

 return entity.value

The ndb.Client instance initialized once during import. This is only the place where I'm getting data from the storage.