Lesson 3 — Transformations#

In this next installment of the Flowman tutorial we will work with a new data set, namely the weather measurements themselves. These are stored in a proprietary, ASCII based text format. Therefore, we cannot simply use a CSV reader like we did before, instead we need to parse the lines using SQL SUBSTR operations.

1. What to Expect#


  • You will see more non-trivial transformations in action

  • You will learn an elegant way to let Flowman automatically generate the correct schema for outgoing relations

  • You will learn how to use data partitions

You can find the full source code of this lesson on GitHub


This time we will read in the raw measurement data, which contains many weather measurements per weather stations (typically one measurement per weather station and per hour, but there might be more or less). We then will store the extracted measurements in Parquet files again, which are well suited for any downstream analytical workload.

Processing Steps#

So we will perform the following steps:

  1. Read in weather measurements as simple text files from S3

  2. Extract some measurement attributes via SQL functions

  3. Write weather measurements as Parquet files into local file system

2. Implementation#

Again we start with a project definition file project.yml, which looks very similar to the previous ones:

name: "weather"
version: "1.0"

# The following modules simply contain a list of subdirectories containing the specification files
  - model
  - mapping
  - target
  - job
  - config

2.1 Configuration & Environment#

Configuration and environment is still stored in the files config/aws.yml and config/environment.yml. The only difference to the previous lesson is the addition of a new environment variable year, which will be used to process only a single year of measurements. Since each year could possibly contain millions of records, a common approach is to partition the data per year and selectively only process a single year.

  - basedir=file:///tmp/weather
  # Define an environment variable to process only a single year
  - year=2013

We will learn a small but important improvement how to model this in Flowman in the next lesson instead of using a simple environment variable.

2.2 Relations#

Again we define two relations: One source relation containing the raw weather measurements stored in text file and one target relation which contains the extracted and transformed measurements stored as Parquet files. There is one important difference to the last lesson, though: This time both the source and the target relation define a so-called partition column which represents large chunks of data stored in different directories. In our example, data will be stored in different partitions for each year of measurement. This physical organization of data enables to selectively process only individual years. Also query tools like Hive, Impala or Trino can make use of partition columns for pruning while directories when the partition column is used in a SQL WHERE condition.

Source Relation#

As explained above, the source data is stored in simple ASCII files, where each record is stored in a separate line. The records themselves contain a part with fixed locations per attribute and an optional more dynamic part. We solely focus on the fixed locations, which is far simpler to work with.

But before going into details how to extract the attributes, we define the source relation called measurements_raw as a file relation with format text (as opposed to the format csv which we used before). We also add a logical partition column which enables the organization of all data files into separate subdirectories per year. The partition column year is mapped to a subdirectory as specified in the pattern property. The relation is defined in the file model/measurements-raw.yml.

    kind: file
    format: text
    location: "s3a://dimajix-training/data/weather/"
      # Define the pattern to be used for partitions. The pattern uses the "year" partition column defined below
    pattern: "${year}"
    # Define data partitions. Each year is stored in a separate subdirectory
      - name: year
        type: integer
        granularity: 1
        description: "The year when the measurement was made"
      # Specify the (single) column via an embedded schema.
      kind: inline
        - name: raw_data
          type: string
          description: "Raw measurement data"

Target Relation#

The relation is defined in the file model/measurements.yml. The target relation also contains a partition column to store data from each year independently in a different directory. In contrast to the source relation, no partition pattern is specified - Flowman will use a Spark compatible directory layout, which is well understood by many Big Data tools like Hive, Impala, Trino and so on.

We also use a small trick to avoid providing a manually crafted schema. Instead of that we specify a schema of kind mapping and a single property which specifies the name of the mapping whose schema will be used. This means that Flowman will inspect the given mapping (measurements_extracted in this case) and infer the column names and data types from the mapping and use that as a schema. Of course, we should reference the mapping that will also be used in the build target as the source for writing to this relation.

    kind: file
    format: parquet
    location: "$basedir/measurements/"
    # Do NOT define the pattern to be used for partitions. Then Flowman will use a standard pattern, which is
    # well understood by Spark, Hive and many other tools
    # pattern: "${year}"
    # Define data partitions. Each year is stored in a separate subdirectory
      - name: year
        type: integer
        granularity: 1
    # We use the inferred schema of the mapping that is written into the relation
      kind: mapping
      mapping: measurements_extracted

2.3 Mappings#

In order to read the raw data and to extract the measurements we need two different mappings. These are defined in the file mapping/measurements.yml.

Reading Source Data#

The first step is to read in the raw data. We need to specify the logical partition to read. Remember that each partition corresponds to one year of measurement data.

  # This mapping refers to the "raw" relation and reads in data from the source in S3
    kind: relation
    relation: measurements_raw
    # Set the data partition to be read
      year: $year

Extracting Measurements#

The second mapping uses the select kind to extract different attributes from the raw measurements. Since the data is stored at fixed locations within each text record, we can simply use the SQL SUBSTR function to extract snippets. We also perform appropriate data type conversions and scaling as needed.

  # Extract multiple columns from the raw measurements data using SQL SUBSTR functions
    kind: select
    input: measurements_raw
      usaf: "SUBSTR(raw_data,5,6)"
      wban: "SUBSTR(raw_data,11,5)"
      date: "TO_DATE(SUBSTR(raw_data,16,8), 'yyyyMMdd')"
      time: "SUBSTR(raw_data,24,4)"
      report_type: "SUBSTR(raw_data,42,5)"
      wind_direction: "CAST(SUBSTR(raw_data,61,3) AS INT)"
      wind_direction_qual: "SUBSTR(raw_data,64,1)"
      wind_observation: "SUBSTR(raw_data,65,1)"
      wind_speed: "CAST(CAST(SUBSTR(raw_data,66,4) AS FLOAT)/10 AS FLOAT)"
      wind_speed_qual: "SUBSTR(raw_data,70,1)"
      air_temperature: "CAST(CAST(SUBSTR(raw_data,88,5) AS FLOAT)/10 AS FLOAT)"
      air_temperature_qual: "SUBSTR(raw_data,93,1)"

2.4 Targets#

Finally, we also need to slightly adjust the build target. Since we are reading the input data in partitioned chunks per year, we also want to write the data with partitions. We already defined a partition column in the target relation, now we need to specify the value of that partition column for the write operation.

  # Define build target for measurements
    # Again, the target is of type "relation"
    kind: relation
    description: "Write extracted measurements per year"
    # Read records from mapping
    mapping: measurements_extracted
    # ... and write them into the relation "measurements"
    relation: measurements
    # Specify the data partition to be written
      year: $year

2.5 Jobs#

We also need to provide a job definition in job/main.yml, which simply references the single target defined above:

  # Define the 'main' job, which implicitly is used whenever you build the whole project
    # List all targets which should be built as part of the `main` job
      - measurements

3. Execution#

With all pieces in place, we can simply execute the whole project with flowexec:

cd /home/flowman
flowxec -f lessons/03-transformations job build main --force

When we want to process a different year, we can simply override the environment variable year on the command line as follows:

cd /home/flowman
flowxec -f lessons/03-transformations job build main --force -D year=2014

4. Next Lessons#

In the next lesson we will learn how to parametrize Flowman jobs to better fit processing of partitioned data.