Sometimes the analytical power of built-in Hive functions is just not enough. In this case it is possible to write hand-tailored User-Defined Functions (UDFs) for transformations and even aggregations which are therefore called User-Defined Aggregation Functions (UDAFs). In this post we focus on how to write sophisticated UDFs and UDAFs in Python. By sophisticated we mean that our UD(A)Fs should also be able to leverage external libraries like Numpy, Scipy, Pandas etc. This makes things a lot more complicated since we have to provide not only some Python script but also a full-blown virtual environment including the external libraries since they may not be available on the cluster nodes. Therefore, in this tutorial we require only that a basic installation of Python is available on the data nodes of the Hive cluster.

## General information

To keep the idea behind UD(A)Fs short, only some general notes are mentioned here. With the help of the Transform/Map-Reduce syntax, i.e. TRANSFORM, it is possible to plug in your own custom mappers and reducers. This is where we gonna hook in our Python script. A UDF is basically only a transformation done by a mapper meaning that each row should be mapped to exactly one row. A UDAF on the other hand allows us to transform a group of rows into one or more rows, meaning that we can reduce the number of input rows to a single output row by some custom aggregation. We can control if the script is run in a mapper or reducer step by the way we formulate our HiveQL query. The statements DISTRIBUTE BY and CLUSTER BY allow us to indicate that we want to actually perform an aggregation. HiveQL feeds the data to the Python script or any other custom script by using the standard input and reads the result from its standard out. All messages from standard error are ignored and can therefore be used for debugging. Since a UDAF is more complex than a UDF and actually can be seen as a generalization of it, the development of a UDAF is demonstrated here.

## Overview and a little task

In order to not get lost in the details, here is what we want to achieve from a high-level perspective.

1. Set up small example Hive table within some database.
2. Create a virtual environment and upload it to Hive’s distributed cache.
3. Write the actual UDAF as Python script and a little helper shell script.
4. Write a HiveQL query that feeds our example table into the Python script.

Our dummy data consists of different types of vehicles (car or bike) and a price. For each category we want to calculate the mean and the standard deviation with the help of Pandas to keep things simple. It should not be necessary to mention that this task can be handled in HiveQL directly, so this is really only for demonstration.

## 1. Setting up our dummy table

With the following query we generate our sample data:

CREATE DATABASE tmp;
USE tmp;
CREATE TABLE foo (id INT, vtype STRING, price FLOAT);
INSERT INTO TABLE foo VALUES (1, "car", 1000.);
INSERT INTO TABLE foo VALUES (2, "car", 42.);
INSERT INTO TABLE foo VALUES (3, "car", 10000.);
INSERT INTO TABLE foo VALUES (4, "car", 69.);
INSERT INTO TABLE foo VALUES (5, "bike", 1426.);
INSERT INTO TABLE foo VALUES (6, "bike", 32.);
INSERT INTO TABLE foo VALUES (7, "bike", 1234.);
INSERT INTO TABLE foo VALUES (8, "bike", null);


Note that the last row even contains a null value that we need to handle later.

In order to prepare a proper virtual environment we need to execute the following steps on an OS that is binary compatible to the OS on the Hive cluster. Typically any recent 64bit Linux distribution will do.

We start by creating an empty virtual environment with:

virtualenv —no-site-packages -p /usr/bin/python3 venv

assuming that virtualenv was already installed with the help of pip. Note that we explicitly ask for Python 3. Who uses Python 2 these days anyhow?

The problem with the activate script of a virtual environment is that its path is hard-coded. We change that by replacing the line

VIRTUAL_ENV="your/path/to/venv"


with

HERE="$( cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd )" VIRTUAL_ENV="$( readlink -f "\${HERE}/../" )"


in ./venv/bin/activate. Additionally, we replace in pip the shebang line, i.e. replacing

#!/your/path/to/venv/venv/bin/python3


with

#!/usr/bin/env python


This will help us later when we call pip list for debugging reasons.

We activate the virtual environment and install Pandas in it.

source venv/bin/activate

pip install numpy pandas

This should install Pandas and all its dependencies into our virtual environment. No we package the virtual environment for later deployment in the distributed cache:

cd venv

tar cvfhz ../venv.tgz ./

cd ..

Be aware that the archive was created with the actual content at its root so when unpacking there will be no directory holding the actual content. We also used the parameter h to package linked files.

Now we push the archive to HDFS so that later Hive’s data nodes will be able to find it:

hdfs dfs -put venv.tgz /tmp

The directory /tmp should be changed accordingly. One should also note that in principle the same procedure should also be possible with conda environments. In practice though, it might be a bit more involved since the activation of a conda environment (what we need to do later) assumes an installation of at least miniconda which might not be available on the data nodes.

We start by writing a simple Python script udaf.py:

import sys
import logging
from itertools import groupby
from operator import itemgetter
import numpy as np
import pandas as pd

SEP = '\t'
NULL = '\\N'

_logger = logging.getLogger(__name__)

for line in input_data:
yield line.strip().split(SEP)

def main():
logging.basicConfig(level=logging.INFO, stream=sys.stderr)
for vtype, group in groupby(data, itemgetter(1)):
group = [(int(rowid), vtype, np.nan if price == NULL else float(price))
for rowid, vtype, price in group]
df = pd.DataFrame(group, columns=('id', 'vtype', 'price'))
output = [vtype, df['price'].mean(), df['price'].std()]
print(SEP.join(str(o) for o in output))

if __name__ == '__main__':
main()


The script should be pretty much self-explanatory. We read from the standard input with the help of a generator that strips and splits the lines by the separator \t. At any point we want to avoid to have more data in memory as needed to perform the actual computation. We use the groupby function that is shipped with Python to iterate over our two types of vehicles. For each group we convert the read values to their respective data types and at that point also take care of null values which are encoded as \N. After this preprocessing we finally feed everything into a Pandas dataframe, do our little mean and standard deviation calculations and print everything as a tabular separated list. It should also be noted that we set up a logger at the beginning which writes everything to standard error. This really helps a lot with debugging and should be used. For demonstration purposes the vehicle type of the group currently processed is printed.

At this point we would actually be done if it wasn’t for the fact that we are importing external libraries like Pandas. So if we ran this Python script directly as UDAF we would see import errors if Pandas is not installed on all cluster nodes. But in the spirit of David Wheeler’s “All problems in computer science can be solved by another level of indirection.” we just write a little helper script called udaf.sh that does this job for us and calls the Python script afterwards.

#!/bin/bash
set -e
(>&2 echo "Begin of script")
source ./venv.tgz/bin/activate
(>&2 echo "Activated venv")
(>&2 pip list --format=columns --no-cache-dir)
python udaf.py
(>&2 echo "End of script")


Again we use standard error to trace what the script is currently doing. Furthermore, we use pip list to output the content of the virtual environment for debugging reasons. With the help of chmod u+x we make the script executable and now all that’s left is to push both files somewhere on HDFS for the cluster to find:

hdfs dfs -put udaf.py /tmp

hdfs dfs -put udaf.sh /tmp

## 4. Writing the actual HiveQL query

After we are all prepared and set we can write the actual HiveQL query:

DELETE ARCHIVE hdfs:///tmp/venv.tgz;
DELETE FILE hdfs:///tmp/udaf.py;
DELETE FILE hdfs:///tmp/udaf.sh;

USE tmp;
SELECT TRANSFORM(id, vtype, price) USING 'udaf.sh' AS (vtype STRING, mean FLOAT, var FLOAT)
FROM (SELECT * FROM foo CLUSTER BY vtype) AS TEMP_TABLE;


At first we add the zipped virtual environment to the distributed cache that will be automatically unpacked for us due to the ADD ARCHIVE command. Then we upload the Python and helper script. To make sure the current version in the cache is actually the latest, so in case changes are made, we prepended DELETE statements before each ADD.

The actual query now calls TRANSFORM with the three input column we expect in our Python script. After the USING statement our helper script is provided as the actual UDAF seen by HiveQL. This is followed by AS defining the names and types of the output columns.

At this point we need to make sure that the script is executed in a reducer step. We assure this by defining a subselect that reads from our foo table and clusters by the vtype. CLUSTER BY which is a shortcut for DISTRIBUTE BY followed by SORT BY asserts that rows having the same vtype column are also located on the same reducer. Furthermore, the implicit SORT BY orders within a reducer the rows with respect to the vtype column. The overall result are consecutive partitions of a given vehicle type (car and bike in our case) whereas each partition resides on a single reducer. Finally, our script is fed the whole data on a single reducer and needs to figure out itself where one partition ends and another one starts (what we did with itertools.groupby).

## Finally

Since our little task is now accomplished, it should also be noted that there are some more Python libraries one should know when working with Hive. To actually execute the HiveQL query we have written with the help of Python, there is impyla by Cloudera with supports Python 3 in contrast to PyHive by Dropbox. In order to work with HDFS the best library around is hdfs3. That would for instance allow us to push changes in udaf.py automatically with a Python script.

Have fun hacking Hive with the power of Python!