How to run custom code on notifications?

Kinto is able to execute some custom code when a particular event occurs. For example, when a record is created or updated in a particular collection.

Kinto uses the same thread to trigger notifications on events, so any custom code that is executed through a notification will block the incoming request until it’s done.

This design is useful when we want to ensure that something is done on the server before we send back the result to the client. But sometimes it’s preferrable to run the notifications asynchronously.

For the latter, the simplest way to run our custom code asynchronously is to use separate process workers that are notified via a job queue

This tutorial presents the basic steps to run code both ways:

  • synchronously in Python;
  • asynchronously using a Redis queue, consumed via any third-party application.

Run synchronous code

In this example, we will send an email to an administrator every time a new bucket is created.

To run this in production, we would rely on a local email server acting as a relay in order to avoid bottlenecks. Or use the asynchronous approach otherwise.

Implement a listener

Create a file kinto_email.py with the following scaffold:

from kinto.core.listeners import ListenerBase

class Listener(ListenerBase):
    def __call__(self, event):
        print(event.payload)

def load_from_config(config, prefix=''):
    return Listener()

Then, we will read the email server configuration and recipients from the settings.

from kinto.core.listeners import ListenerBase
from pyramid.settings import aslist, asbool

class Listener(ListenerBase):
    def __init__(self, server, tls, username, password, sender, recipients):
        self.server = server
        self.tls = tls
        self.username = username
        self.password = password
        self.sender = sender
        self.recipients = recipients

    def __call__(self, event):
        print(event.payload)

def load_from_config(config, prefix=''):
    settings = config.get_settings()

    server = settings[prefix + 'server']
    tls = asbool(settings[prefix + 'tls'])
    username = settings[prefix + 'username']
    password = settings[prefix + 'password']
    sender = settings[prefix + 'from']
    recipients = aslist(settings[prefix + 'recipients'])

    return Listener(server, tls, username, password, sender, recipients)

Now, every time a new event occurs, we send an email:

import smtplib
from email.mime.text import MIMEText

from kinto.core.listeners import ListenerBase
from pyramid.settings import aslist, asbool

class Listener(ListenerBase):
    def __init__(self, server, tls, username, password, sender, recipients):
        self.server = server
        self.tls = tls
        self.username = username
        self.password = password
        self.sender = sender
        self.recipients = recipients

    def __call__(self, event):
        subject = "%s %sd" % (event.payload['resource_name'],
                              event.payload['action'])
        text = "User id: %s" % event.request.prefixed_userid

        message = MIMEText(text)
        message['Subject'] = subject
        message['From'] = self.sender
        message['To'] = ", ".join(self.recipients)

        server = smtplib.SMTP(self.server)
        if self.tls:
            server.starttls()
        if self.username and self.password:
            server.login(self.username, self.password)
        server.sendmail(self.sender, self.recipients, message.as_string())
        server.quit()

def load_from_config(config, prefix=''):
    settings = config.get_settings()

    server = settings[prefix + 'server']
    tls = asbool(settings[prefix + 'tls'])
    username = settings[prefix + 'username']
    password = settings[prefix + 'password']
    sender = settings[prefix + 'from']
    recipients = aslist(settings[prefix + 'recipients'])

    return Listener(server, tls, username, password, sender, recipients)

Add it to Python path

For the simplicity in this tutorial, we will just alter the PYTHONPATH system environment variable. Specify the path to the folder containing the kinto_email.py:

$ export PYTHONPATH="/path/to/folder:${PYTHONPATH}"

In order to test that it works, simply try to import it from a python script:

$ python
Python 2.7.9 (default, Apr  2 2015, 15:33:21)
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import kinto_email
>>>

Enable in configuration

As explained in the settings section, just enable a new listener pointing to your python module:

kinto.event_listeners = send_email

kinto.event_listeners.send_email.use = kinto_email
kinto.event_listeners.send_email.server = localhost:1025
kinto.event_listeners.send_email.tls = false
kinto.event_listeners.send_email.username =
kinto.event_listeners.send_email.password =
kinto.event_listeners.send_email.from = [email protected]
kinto.event_listeners.send_email.recipients = [email protected]

Kinto should load the listeners without errors:

$ kinto start
Starting subprocess with file monitor
2016-01-21 16:21:59,941 INFO  [kinto.core.initialization][MainThread] Setting up 'send_email' listener

Test it

In a separate terminal, run a fake SMTP server on localhost:1025:

$ python -m smtpd -n -c DebuggingServer localhost:1025

Create a record (using HTTPie):

$ echo '{"data": {"note": "kinto"}}' | \
    http --auth token:alice-token --verbose POST http://localhost:8888/v1/buckets/default/collections/notes/records

And observe the fake server output:

---------- MESSAGE FOLLOWS ----------
Content-Type: text/plain; charset="us-ascii"
MIME-Version: 1.0
Content-Transfer-Encoding: 7bit
Subject: record created
From: [email protected]
To: [email protected]
X-Peer: 127.0.0.1

User id: basicauth:fea1e21d339299506d89e60f048cefd5b424ea641ba48267c35a4ce921439fa4
------------ END MESSAGE ------------

It worked!

Run asynchronous code

Note

You will need to install kinto-redis to use this listener.

In this part, we will take advantage of the built-in listener that delivers the events into a Redis queue. Separate scripts, also as known as “workers”, then consume the queue to execute custom asynchronous code.

Run Redis

Redis is available in most Linux distributions or Mac OS brew. Using Docker it is also very easy to run a server on localhost:6379:

$ sudo docker run -p 6379:6379 redis

Setup Kinto queue

In configuration, we setup the listener to post the message to a queue named eventqueue:

kinto.event_listeners = redis

kinto.event_listeners.redis.use = kinto_redis.listeners
kinto.event_listeners.redis.url = redis://localhost:6379/0
kinto.event_listeners.redis.pool_size = 5
kinto.event_listeners.redis.listname = eventqueue

Kinto should load the listeners without errors:

$ kinto start
Starting subprocess with file monitor
2016-01-21 16:21:59,941 INFO  [kinto.core.initialization][MainThread] Setting up 'redis' listener

Run worker(s)

The simplest worker would look like that:

import time
import json

import redis

def main():
    db = redis.Redis()
    # Run indefinitely.
    while True:
        # Wait for new messages (blocking).
        key, payload = db.blpop("eventqueue")
        # Decode JSON payload.
        message = json.loads(payload)
        # Simulate long task.
        time.sleep(2)
        print(message)

if __name__ == "__main__":
    main()

Run it in a separate terminal:

$ python worker.py

Test it!

Create a record (using HTTPie):

$ echo '{"data": {"note": "kinto"}}' | \
    http --auth token:alice-token --verbose POST http://localhost:8888/v1/buckets/default/collections/notes/records

The server response is returned immediately.

But 2 seconds later, look at the worker output:

{u'resource_name': u'record', u'user_id': u'basicauth:fea1e21d339299506d89e60f048cefd5b424ea641ba48267c35a4ce921439fa4', u'timestamp': 1453459942672, u'uri': u'/buckets/c8c94a74-5bf6-9fb0-5b72-b0777da6718e/collections/assets/records', u'bucket_id': u'c8c94a74-5bf6-9fb0-5b72-b0777da6718e', u'action': u'create', u'collection_id': u'assets'}

It worked!