Reactive Python - Cross-language RPC conversation

@xav-b|October 10, 2016 (8y ago)11 views

I had the opportunity to study the data science market lately and discovered some appealing products. We hunted a service that would let my team focus on actual data exploration and high business value models. You know, getting the DevOps stuff out of the way.

Most of the tools offered a way to code your own data pipeline and expose it behind an hosted REST API. I think it is a simple and reasonable approach:

Plug-and-Play models that scale automatically lowers the risk of deploying cutting edge technologies
You still have full control of the business/technical logic
Most of developers are comfortable interacting with APIs

As desperate victims of the NIH syndrom (Not Invented Here) let's build such service for fun and profit !

The scene

Allow me to make up a realistic business case. We have of mobile application firing a storm of events on a dedicated API written in Node.js for performance, industry trending and asynchronous kind of motivations.

It forwards those events (like clicks, downloads, ...) to an online machine learning model to feed its comprehension of our users' behavior.

This setup brings near real-time representation of the world and aims for a dynamic and accurate prediction engine. Python is a reasonable choice given its battle-tested data science ecosystem and highly productive environment. The challenge yet is to link the API to the Python backend which needs to react to each single new event. In this article I will draft the first critical steps of a solution to this modern mission.

Side note : while we laid the foundation of a specific business case, I hope you will find interesting inspirations for more general reactive Python projects.

The stack

I will spare you the full research and jump to the interesting RPC (Remote Procedure Call) part. Microservices rise found in this protocole a reliable and elegant approach for inter-communication. Indeed we will see how a service can expose its methods to language-agnostic clients.

Google released a comprehensive framework using Protocol Buffers that certainely suits massive and demanding distributed infrastructures. I found, however, a simpler alternative in ZeroRPC while still production-tested (at Dotcloud in the old days before Docker). The project uses ZMQ to bridge services through RPC and support Node.js and Python.

Enough with tools of the trade, let's get started.

The processing layer

We are going to develop and expose a model describing the impact of different media ads on sales. To spice up the challenge, it will be continuously updated when new data is available. Dependencies are heavy but easy to install :

$ python --version
Python 2.7.10
$ # It should work with Python 3 but installation is tricky. See:
$ # https://github.com/0rpc/zerorpc-python/issues/108
 
$ # depending on your Python setup you might need sudo
$ (sudo) pip install
  statsmodels==0.6.1 \
  pandas==0.18.1 \
  scipy==0.17.1 \
  patsy==0.4.1 \
  zerorpc==0.5.2

This part of the application is the server-side of ZeroRPC services. It serves methods to clients becoming able to call them.

#! /usr/bin/env python
# -*- coding: utf-8 -*-
# vim:fenc=utf-8
#
# filename: predict.py
 
import random
import zerorpc
 
 
class OnlineLinearRegression(object):
    """Remote machine learning process."""
 
    def predict(self, data):
        return random.random()
 
 
if __name__ == '__main__':
    engine = zerorpc.Server(OnlineLinearRegression())
    engine.bind('tcp://0.0.0.0:4242')
 
    try:
        engine.run()
    except KeyboardInterrupt:
        print('caught CTRL-C, shutting down')

Running python predict.py will allow client to connect on 0.0.0.0:4222 and call the predict() class function. ZeroRPC takes care of serializing arguments and return values, which is comfortable but forbid anything other than primitive types (integers, strings, ... but no dataframes for example).

Now that we have the communication skeleton, let's implement the continuous learning model. To keep things digest, we will stick to a linear regression to analyze sales data. We can answer a handful of business questions with this approach and it will bound lines of code. You will find more details in this playbook.

import pandas as pd
import statsmodels.formula.api as smf
 
DATA_URL = 'http://www-bcf.usc.edu/~gareth/ISL/Advertising.csv'
 
 
class OnlineLinearRegression(object):
    """Remote machine learning process."""
 
    def __init__(self):
        self._data = pd.read_csv(DATA_URL, index_col=0)
        print('downloaded data:\n{}'.format(self._data.head()))
        print('initializing model...')
        self.lm = smf.ols(formula='Sales ~ TV', data=self._data).fit()
 
    def train(self):
        self.lm = smf.ols(formula='Sales ~ TV', data=self._data).fit()
        print('re-trained model (new params: {})'.format(self.lm.params))
        return self.lm.summary().as_html()
 
    def learn(self, knowledge):
        row = pd.DataFrame([knowledge], columns=self._data.columns)
        self._data = self._data.append(row)
        return self._data.describe().to_json()
 
    def predict(self, data):
        return self.lm.predict(pd.DataFrame(data))[0]

I highly recommend you to play with the dataset in an interpreter :

In [1]: import pandas as pd
 
In [2]: data =
pd.read_csv('http://www-bcf.usc.edu/~gareth/ISL/Advertising.csv',
index_col=0)
 
In [3]: data.head()
Out[3]:
            TV  Radio  Newspaper  Sales
      1  230.1   37.8       69.2   22.1
      2   44.5   39.3       45.1   10.4
      3   17.2   45.9       69.3    9.3
      4  151.5   41.3       58.5   18.5
      5  180.8   10.8       58.4   12.9

Run the script and jump to the next session to consume the API.

$ python predict.py
downloaded data:
      TV  Radio  Newspaper  Sales
      1  230.1   37.8       69.2   22.1
      2   44.5   39.3       45.1   10.4
      3   17.2   45.9       69.3    9.3
      4  151.5   41.3       58.5   18.5
      5  180.8   10.8       58.4   12.9
Initializing model ...
listening on :4222 ...

The reactive layer

In this section we will write a minimalist express API to bridge HTTP requests and our Prediction service. Again, a few commands should fetch everything the requirements. The code uses modern es6 syntax so you will need latest Node.js or a transpiler like Babel.

$ node --version
v6.0.0
$ npm --version
3.8.6
 
$ # libzmq differs from platform to platform. For Mac OSX users:
$ brew install libzmq
 
$ npm install express zerorpc

The strategy is to map each Python method to an HTTP endpoint, so we can fully access and exchange data witht the model. Since the code fits in a single file, I pasted the content below with (hopefully) descriptive comments, and no error handling (bad, don't reproduce at home).

// import and initialize libraries
const express = require('express')
const bodyParser = require('body-parser')
const zerorpc = require('zerorpc')
 
const app = express()
const client = new zerorpc.Client()
 
// Express server configuration
// parse application/json
app.use(bodyParser.json())
// parse application/x-www-form-urlencoded
app.use(bodyParser.urlencoded({ extended: true  }))
 
// define API routes and controllers
 
app.post('/v1/ia/learn', (req, res) => {
  // read new data to feed the model
  const new_insights = req.body.data
 
  // example: [3.45, 34.6, 12.2, 8.4]
  client.invoke('learn', new_insights, (err, feedback, more) => {
    // send back result for debugging / information puprose
    res.status(202).json({ feedback })
  })
})
 
// tell the model to update (call it after sending some data)
app.get('/v1/ia/train', (req, res) => {
  client.invoke('train', (err, data, more) => {
    // return html rendering, visit localhost:3000/v1/ia/train to
    // execute training and inspect resulting linear regression parameters
    res.status(200).send(data)
  })
})
 
// finally use the resulting model and ask for prediction based on
// current ingested values
app.get('/v1/ia/predict', (req, res) => {
  // let's see how a value of 50 units on TV ads will presumably impact sales
  const example_query = { TV: [50] }
  client.invoke('predict', example_query, (err, data, more) => {
    res.status(200).json({ data })
  })
})
 
client.connect('tcp://127.0.0.1:4242')
 
app.listen(3000, () => console.log('API is listening on port 3000'))

Ready ?

$ node api.js
API is listening on port 3000

Yeah, both services are.

Cross-language real-time predictions

Here we are to the actual reward. Assuming our API and engine backend are still running, let's play with the model from the command line.

First we simulate some new data hitting the API, like new sales statistics as captured by an application.

$ curl -X POST \
  -H "Accept:application/json" \
  -d 'data[]=3.14&data[]=34.6&data[]=12.2&data[]=8.4' \
  localhost:3000/v1/ia/learn
{"feedback":"{\"TV\":{\"count\":202.0,\"unique\":191.0,\"top\":109.8,\"freq\":2.0},\"Radio\":{\"count\":202.0,\"unique\":168.0,\"top\":5.7,\"freq\":3.0},\"Newspaper\":{\"count\":202.0,\"unique\":173.0,\"top\":25.6,\"freq\":3.0},\"Sales\":{\"count\":202.0,\"unique\":122.0,\"top\":9.7,\"freq\":5.0}}"}%

The response is the dataset summary after update. You can run this command several times and see the Json evolves to reflect new insights we sent in the Python internal data representation.

Yet, we need to train it to take into account the data aggregated. Just visit http://localhost:3000/v1/ia/train from your favorite $BROWSER and watch the current OLS regression state. Here is mine :

# OLS Regression Results
Dep. Variable:  Sales
R-squared:  0.612
Model:  OLS Adj.
R-squared: 0.610
Method: Least Squares
F-statistic:  312.1
Date: Mon, 16 May 2016
Prob (F-statistic): 1.47e-42
Time: 13:04:51
Log-Likelihood: -519.05
No. Observations: 200
AIC:  1042.
Df Residuals: 198
BIC:  1049.
Df Model: 1
Covariance Type: |
  nonrobust coef  std err t P>|t| [95.0% Conf. Int.]
  Intercept 7.0326  0.458 15.360  0.000 6.130 7.935
  TV  0.0475  0.003 17.668  0.000 0.042 0.053
Omnibus:  0.531
Durbin-Watson:  1.935
Prob(Omnibus):  0.767
Jarque-Bera (JB): 0.669
Skew: -0.089
Prob(JB): 0.716
Kurtosis: 2.779 Cond. No. 338.

And finally we can request a prediction from this very model.

$ curl localhost:3000/v1/ia/predict
{"prediction":9.409425570778684}

The response tells us that an hypothetic value of 50 on TV ads will get us to 9.4 in sales.

Conclusion

Let's pat our back : we built a cross platform application exposing real-time model fitting and business prediction as a RESTFul service. Before funding a startup, notice we pass silent lot of serious things like API security, error handling, model building dark magic like feature engineering.

Nevertheless the business logic is encapsulated into a Python class that don't need to bother about data interfaces and external constraints, yet still being able to continuously adapt itself to the reality. Data engineers can improve the prediction process behind an API, saving the mobile team from deploying new versions to catch up.