Datapackage Pipelines

The Basics

What is it?

datapackage-pipelines is a framework for declarative stream-processing of tabular data. It is built upon the concepts and tooling of the Frictionless Data project.

Pipelines

The basic concept in this framework is the pipeline.

A pipeline has a list of processing steps, and it generates a single data package as its output. Each step is executed in a processor and consists of the following stages:

• Modify the data package descriptor - For example: add metadata, add or remove resources, change resources' data schema etc.
• Process resources - Each row of each resource is processed sequentially. The processor can drop rows, add new ones or modify their contents.
• Return stats - If necessary, the processor can report a dictionary of data which will be returned to the user when the pipeline execution terminates. This can be used, for example, for calculating quality measures for the processed data.

Not every processor needs to do all of these. In fact, you would often find each processing step doing only one of these.

pipeline-spec.yaml file

Pipelines are defined in a declarative way, and not in code. One or more pipelines can be defined in a pipeline-spec.yaml file. This file specifies the list of processors (referenced by name) and the execution parameters for each of the processors.

Here's an example of a pipeline-spec.yaml file:

worldbank-co2-emissions:
  title: CO2 emission data from the World Bank
  description: Data per year, provided in metric tons per capita.
  environment:
    DEBUG: true
  pipeline:
    -
      run: update_package
      parameters:
        name: 'co2-emissions'
        title: 'CO2 emissions (metric tons per capita)'
        homepage: 'http://worldbank.org/'
    -
      run: load
      parameters:
        from: "http://api.worldbank.org/v2/en/indicator/EN.ATM.CO2E.PC?downloadformat=excel"
        name: 'global-data'
        format: xls
        headers: 4
    -
      run: set_types
      parameters:
         resources: global-data
         types:
           "[12][0-9]{3}":
              type: number
    -
      run: dump_to_zip
      parameters:
          out-file: co2-emissions-wb.zip

In this example we see one pipeline called worldbank-co2-emissions. Its pipeline consists of 4 steps:

• update_package: This is a library processor (see below), which modifies the data-package's descriptor (in our case: the initial, empty descriptor) - adding name, title and other properties to the datapackage.
• load: This is another library processor, which loads data into the data-package. This resource has a name and a from property, pointing to the remote location of the data.
• set_types: This processor assigns data types to fields in the data. In this example, field headers looking like years will be assigned the number type.
• dump_to_zip: Create a zipped and validated datapackage with the provided file name.

Also, we have provided some metadata:

• title: Title of a pipeline
• description: Description of a pipeline
• environment: Dictionary of environment variables to be set for all the pipeline's steps. For examples, it can be used to change the behaviour of the underlaying requests library - https://requests.readthedocs.io/en/master/user/advanced/#ssl-cert-verification

Full JSONSchema of the pipeline-spec.yaml file can be found here

Mechanics

An important aspect of how the pipelines are run is the fact that data is passed in streams from one processor to another. If we get "technical" here, then each processor is run in its own dedicated process, where the datapackage is read from its stdin and output to its stdout. The important thing to note here is that no processor holds the entire data set at any point.

This limitation is by design - to keep the memory and disk requirements of each processor limited and independent of the dataset size.

Quick Start

First off, create a pipeline-spec.yaml file in your current directory. You can take the above file if you just want to try it out.

Then, you can either install datapackage-pipelines locally - note that Python 3.6 or higher is required due to use of Type Hinting and advanced asyncio use:

$ pip install datapackage-pipelines

You should now be able to use the dpp command:

$ dpp
Available Pipelines:
- ./worldbank-co2-emissions (*)

$ $ dpp run --verbose ./worldbank-co2-emissions
RUNNING ./worldbank-co2-emissions
Collecting dependencies
Running async task
Waiting for completion
Async task starting
Searching for existing caches
Building process chain:
- update_package
- load
- set_types
- dump_to_zip
- (sink)
DONE /Users/adam/code/dhq/specstore/dpp_repo/datapackage_pipelines/specs/../lib/update_package.py
load: DEBUG   :Starting new HTTP connection (1): api.worldbank.org:80
load: DEBUG   :http://api.worldbank.org:80 "GET /v2/en/indicator/EN.ATM.CO2E.PC?downloadformat=excel HTTP/1.1" 200 308736
load: DEBUG   :http://api.worldbank.org:80 "GET /v2/en/indicator/EN.ATM.CO2E.PC?downloadformat=excel HTTP/1.1" 200 308736
load: DEBUG   :Starting new HTTP connection (1): api.worldbank.org:80
load: DEBUG   :http://api.worldbank.org:80 "GET /v2/en/indicator/EN.ATM.CO2E.PC?downloadformat=excel HTTP/1.1" 200 308736
load: DEBUG   :http://api.worldbank.org:80 "GET /v2/en/indicator/EN.ATM.CO2E.PC?downloadformat=excel HTTP/1.1" 200 308736
set_types: INFO    :(<dataflows.processors.set_type.set_type object at 0x10a5c79b0>,)
load: INFO    :Processed 264 rows
set_types: INFO    :Processed 264 rows
DONE /Users/adam/code/dhq/specstore/dpp_repo/datapackage_pipelines/specs/../lib/load.py
DONE /Users/adam/code/dhq/specstore/dpp_repo/datapackage_pipelines/specs/../lib/set_types.py
dump_to_zip: INFO    :Processed 264 rows
DONE /Users/adam/code/dhq/specstore/dpp_repo/datapackage_pipelines/manager/../lib/internal/sink.py
DONE /Users/adam/code/dhq/specstore/dpp_repo/datapackage_pipelines/specs/../lib/dump_to_zip.py
DONE V ./worldbank-co2-emissions {'bytes': 692741, 'count_of_rows': 264, 'dataset_name': 'co2-emissions', 'hash': '4dd18effcdfbf5fc267221b4ffc28fa4'}
INFO    :RESULTS:
INFO    :SUCCESS: ./worldbank-co2-emissions {'bytes': 692741, 'count_of_rows': 264, 'dataset_name': 'co2-emissions', 'hash': '4dd18effcdfbf5fc267221b4ffc28fa4'}

Alternatively, you could use our Docker image:

$ docker run -it -v `pwd`:/pipelines:rw \
        frictionlessdata/datapackage-pipelines
<available-pipelines>

$ docker run -it -v `pwd`:/pipelines:rw \
       frictionlessdata/datapackage-pipelines run ./worldbank-co2-emissions
<execution-logs>

The Command Line Interface - dpp

Running a pipeline from the command line is done using the dpp tool.

Running dpp without any argument, will show the list of available pipelines. This is done by scanning the current directory and its subdirectories, searching for pipeline-spec.yaml files and extracting the list of pipeline specifications described within.

Each pipeline has an identifier, composed of the path to the pipeline-spec.yaml file and the name of the pipeline, as defined within that description file.

In order to run a pipeline, you use dpp run <pipeline-id>.

You can also use dpp run all for running all pipelines and dpp run dirty to run the just the dirty pipelines (more on that later on).

Deeper look into pipelines

Processor Resolution

As previously seen, processors are referenced by name.

This name is, in fact, the name of a Python script containing the processing code (minus the .py extension). When trying to find where is the actual code that needs to be executed, the processor resolver will search in these predefined locations:

• First of all, it will try to find a custom processor with that name in the directory of the pipeline-spec.yaml file. Processor names support the dot notation, so you could write mycode.custom_processor and it will try to find a processor named custom_processor.py in the mycode directory, in the same path as the pipeline spec file. For this specific resolving phase, if you would write ..custom_processor it will try to find that processor in the parent directory of the pipeline spec file. (read on for instructions on how to write custom processors)
• In case the processor name looks like myplugin.somename, it will try to find a processor named somename in the myplugin plugin. That is - it will see if there's an installed plugin which is called myplugin, and if so, whether that plugin publishes a processor called somename (more on plugins below).
• If no processor was found until this point, it will try to search for this processor in the processor search path. The processor search path is taken from the environment variable DPP_PROCESSOR_PATH. Each of the : separated paths in the path is considered as a possible starting point for resolving the processor.
• Finally, it will try to find that processor in the Standard Processor Library which is bundled with this package.

Excluding directories form scanning for pipeline specs

By default .* directories are excluded from scanning, you can add additional directory patterns for exclusion by creating a .dpp_spec_ignore file at the project root. This file has similar syntax to .gitignore and will exclude directories from scanning based on glob pattern matching.

For example, the following file will ignore test* directories including inside subdirectories and /docs directory will only be ignored at the project root directory

test*
/docs

Caching

By setting the cached property on a specific pipeline step to True, this step's output will be stored on disk (in the .cache directory, in the same location as the pipeline-spec.yaml file).

Rerunning the pipeline will make use of that cache, thus avoiding the execution of the cached step and its precursors.

Internally, a hash is calculated for each step in the pipeline - which is based on the processor's code, it parameters and the hash of its predecessor. If a cache file exists with exactly the same hash as a specific step, then we can remove it (and its predecessors) and use that cache file as an input to the pipeline

This way, the cache becomes invalid in case the code or execution parameters changed (either for the cached processor or in any of the preceding processors).

Dirty tasks and keeping state

The cache hash is also used for seeing if a pipeline is "dirty". When a pipeline completes executing successfully, dpp stores the cache hash along with the pipeline id. If the stored hash is different than the currently calculated hash, it means that either the code or the execution parameters were modified, and that the pipeline needs to be re-run.

dpp works with two storage backends. For running locally, it uses a python sqlite DB to store the current state of each running task, including the last result and cache hash. The state DB file is stored in a file named .dpp.db in the same directory that dpp is being run from.

For other installations, especially ones using the task scheduler, it is recommended to work with the Redis backend. In order to enable the Redis connection, simply set the DPP_REDIS_HOST environment variable to point to a running Redis instance.

Pipeline Dependencies

You can declare that a pipeline is dependent on another pipeline or datapackage. This dependency is considered when calculating the cache hashes of a pipeline, which in turn affect the validity of cache files and the "dirty" state:

• For pipeline dependencies, the hash of that pipeline is used in the calculation
• For datapackage dependencies, the hash property in the datapackage is used in the calculation

If the dependency is missing, then the pipeline is marked as 'unable to be executed'.

Declaring dependencies is done by a dependencies property to a pipeline definition in the pipeline-spec.yaml file. This property should contain a list of dependencies, each one is an object with the following formats:

• A single key named pipeline whose value is the pipeline id to depend on
• A single key named datapackage whose value is the identifier (or URL) for the datapackage to depend on

Example:

cat-vs-dog-populations:
  dependencies:
    -
      pipeline: ./geo/region-areal
    -
      datapackage: http://pets.net/data/dogs-per-region/datapackage.json
    -
      datapackage: http://pets.net/data/dogs-per-region
  ...

Validating

Each processor's input is automatically validated for correctness:

• The datapackage is always validated before being passed to a processor, so there's no possibility for a processor to modify a datapackage in a way that renders it invalid.

• Data is not validated against its respective JSON Table Schema, unless explicitly requested by setting the validate flag to True in the step's info. This is done for two main reasons:

  • Performance wise, validating the data in every step is very CPU intensive
  • In some cases you modify the schema in one step and the data in another, so you would only like to validate the data once all the changes were made

  In any case, when using the set_types standard processor, it will validate and transform the input data with the new types..

Dataflows integration

Dataflows is the successor of datapackage-pipelines and provides a more Pythonic interface to running pipelines. You can integrate dataflows within pipeline specs using the flow attribute instead of run. For example, given the following flow file, saved under my-flow.py:

from dataflows import Flow, dump_to_path, load, update_package

def flow(parameters, datapackage, resources, stats):
    stats['multiplied_fields'] = 0

    def multiply(field, n):
        def step(row):
            row[field] = row[field] * n
            stats['multiplied_fields'] += 1
        return step

    return Flow(update_package(name='my-datapackage'),
                multiply('my-field', 2))

And a pipeline-spec.yaml in the same directory:

my-flow:
  pipeline:
  - run: load_resource
    parameters:
      url: http://example.com/my-datapackage/datapackage.json
      resource: my-resource
  - flow: my-flow
  - run: dump_to_path

You can run the pipeline using dpp run my-flow.

The Standard Processor Library

A few built in processors are provided with the library.

update_package

Adds meta-data to the data-package.

Parameters:

Any allowed property (according to the spec) can be provided here.

Example:

- run: update_package
  parameters:
    name: routes-to-mordor
    license: CC-BY-SA-4
    author: Frodo Baggins <frodo@shire.me>
    contributors:
      - samwise gamgee <samwise1992@yahoo.com>

update_resource

Adds meta-data to the resource.

Parameters:

• resources
  • A name of a resource to operate on
  • A regular expression matching resource names
  • A list of resource names
  • The index of the resource in the package
  • if omitted indicates operation should be done on all resources
• metadata - A dictionary containing any allowed property (according to the spec).

Example:

- run: update_resource
  parameters:
    resources: ['resource1']
    metadata:
      path: 'new-path.csv'

load

Loads data into the package, infers the schema and optionally casts values.

Parameters:

• from - location of the data that is to be loaded. This can be either:
  • a local path (e.g. /path/to/the/data.csv)
  • a remote URL (e.g. https://path.to/the/data.csv)
  • Other supported links, based on the current support of schemes and formats in tabulator
  • a local path or remote URL to a datapackage.json file (e.g. https://path.to/data_package/datapackage.json)
  • a reference to an environment variable containing the source location, in the form of env://ENV_VAR
  • a tuple containing (datapackage_descriptor, resources_iterator)
• resources - optional, relevant only if source points to a datapackage.json file or datapackage/resource tuple. Value should be one of the following:
  • Name of a single resource to load
  • A regular expression matching resource names to load
  • A list of resource names to load
  • 'None' indicates to load all resources
  • The index of the resource in the package
• validate - Should data be casted to the inferred data-types or not. Relevant only when not loading data from datapackage.
• other options - based on the loaded file, extra options (e.g. sheet for Excel files etc., see the link to tabulator above)

printer

Just prints whatever it sees. Good for debugging.

Parameters:

• num_rows - modify the number of rows to preview, printer will print multiple samples of this number of rows from different places in the stream
• last_rows - how many of the last rows in the stream to print. optional, defaults to the value of num_rows
• fields - optional, list of field names to preview
• resources - optional, allows to limit the printed resources, same semantics as load processor resources argument

set_types

Sets data types and type options to fields in streamed resources, and make sure that the data still validates with the new types.

This allows to make modifications to the existing table schema, and usually to the default schema from stream_remote_resources.

Parameters:

• resources - Which resources to modify. Can be:

  • List of strings, interpreted as resource names to stream
  • String, interpreted as a regular expression to be used to match resource names

  If omitted, all resources in datapackage are streamed.

• regex - if set to False field names will be interpreted as strings not as regular expressions (True by default)

• types - A map between field names and field definitions.

  • field name is either simply the name of a field, or a regular expression matching multiple fields.
  • field definition is an object adhering to the JSON Table Schema spec. You can use null instead of an object to remove a field from the schema.

Example:

- run: add_resources
  parameters:
    name: example-resource
    url: http://example.com/my-csv-file.csv
    encoding: "iso-8859-2"
- run: stream_remote_resources
- run: set_types
  parameters:
    resources: example-resource
    types:
      age:
        type: integer
      "yearly_score_[0-9]{4}":
        type: number
      "date of birth":
        type: date
        format: "%d/%m/%Y"
      "social security number": null

load_metadata

Loads metadata from an existing data-package.

Parameters:

Loads the metadata from the data package located at url.

All properties of the loaded datapackage will be copied (except the resources)

Example:

- run: load_metadata
  parameters:
    url: http://example.com/my-datapackage/datapackage.json

load_resource

Loads a tabular resource from an existing data-package.

Parameters:

Loads the resource specified in the resource parameter from the data package located at url. All properties of the loaded resource will be copied - path and schema included.

• url - a URL pointing to the datapackage in which the required resource resides

• resource - can be

  • List of strings, interpreted as resource names to load
  • String, interpreted as a regular expression to be used to match resource names
  • an integer, indicating the index of the resource in the data package (0-based)

• limit-rows - if provided, will limit the number of rows fetched from the source. Takes an integer value which specifies how many rows of the source to stream.

• log-progress-rows - if provided, will log the loading progress. Takes an integer value which specifies the number of rows interval at which to log the progress.

• stream - if provided and is set to false, then the resource will be added to the datapackage but not streamed.

• resources - can be used instead of resource property to support loading resources and modify the output resource metadata

  • Value is a dict containing mapping between source resource name to load and dict containing descriptor updates to apply to the loaded resource

• required - if provided and set to false, will not fail if datapackage is not available or resource is missing

Example:

- run: load_resource
  parameters:
    url: http://example.com/my-datapackage/datapackage.json
    resource: my-resource
- run: load_resource
  parameters:
    url: http://example.com/my-other-datapackage/datapackage.json
    resource: 1
- run: load_resource
  parameters:
    url: http://example.com/my-datapackage/datapackage.json
    resources:
      my-resource:
        name: my-renamed-resource
        path: my-renamed-resource.csv

concatenate

Concatenates a number of streamed resources and converts them to a single resource.

Parameters:

• sources - Which resources to concatenate. Same semantics as resources in stream_remote_resources.

  If omitted, all resources in datapackage are concatenated.

  Resources to concatenate must appear in consecutive order within the data-package.

• target - Target resource to hold the concatenated data. Should define at least the following properties:

  • name - name of the resource
  • path - path in the data-package for this file.

  If omitted, the target resource will receive the name concat and will be saved at data/concat.csv in the datapackage.

• fields - Mapping of fields between the sources and the target, so that the keys are the target field names, and values are lists of source field names.

  This mapping is used to create the target resources schema.

  Note that the target field name is always assumed to be mapped to itself.

Example:

- run: concatenate
  parameters:
    target:
      name: multi-year-report
      path: data/multi-year-report.csv
    sources: 'report-year-20[0-9]{2}'
    fields:
      activity: []
      amount: ['2009_amount', 'Amount', 'AMOUNT [USD]', '$$$']

In this example we concatenate all resources that look like report-year-<year>, and output them to the multi-year-report resource.

The output contains two fields:

• activity , which is called activity in all sources
• amount, which has varying names in different resources (e.g. Amount, 2009_amount, amount etc.)

join

Joins two streamed resources.

"Joining" in our case means taking the target resource, and adding fields to each of its rows by looking up data in the source resource.

A special case for the join operation is when there is no target stream, and all unique rows from the source are used to create it. This mode is called deduplication mode - The target resource will be created and deduplicated rows from the source will be added to it.

Parameters:

• source - information regarding the source resource

  • name - name of the resource
  • key - One of
    • List of field names which should be used as the lookup key
    • String, which would be interpreted as a Python format string used to form the key (e.g. {<field_name_1>}:{field_name_2})
  • delete - delete from data-package after joining (False by default)

• target - Target resource to hold the joined data. Should define at least the following properties:

  • name - as in source
  • key - as in source, or null for creating the target resource and performing deduplication.

• fields - mapping of fields from the source resource to the target resource. Keys should be field names in the target resource. Values can define two attributes:

  • name - field name in the source (by default is the same as the target field name)

  • aggregate - aggregation strategy (how to handle multiple source rows with the same key). Can take the following options:

    • sum - summarise aggregated values. For numeric values it's the arithmetic sum, for strings the concatenation of strings and for other types will error.

    • avg - calculate the average of aggregated values.

      For numeric values it's the arithmetic average and for other types will err.

    • max - calculate the maximum of aggregated values.

      For numeric values it's the arithmetic maximum, for strings the dictionary maximum and for other types will error.

    • min - calculate the minimum of aggregated values.

      For numeric values it's the arithmetic minimum, for strings the dictionary minimum and for other types will error.

    • first - take the first value encountered

    • last - take the last value encountered

    • count - count the number of occurrences of a specific key For this method, specifying name is not required. In case it is specified, count will count the number of non-null values for that source field.

    • counters - count the number of occurrences of distinct values Will return an array of 2-tuples of the form [value, count-of-value].

    • set - collect all distinct values of the aggregated field, unordered

    • array - collect all values of the aggregated field, in order of appearance

    • any - pick any value.

    By default, aggregate takes the any value.

  If neither name or aggregate need to be specified, the mapping can map to the empty object {} or to null.

• full - Boolean,

  • If True (the default), failed lookups in the source will result in "null" values at the source.
  • if False, failed lookups in the source will result in dropping the row from the target.

Important: the "source" resource must appear before the "target" resource in the data-package.

Examples:

- run: join
  parameters:
    source:
      name: world_population
      key: ["country_code"]
      delete: yes
    target:
      name: country_gdp_2015
      key: ["CC"]
    fields:
      population:
        name: "census_2015"
    full: true

The above example aims to create a package containing the GDP and Population of each country in the world.

We have one resource (world_population) with data that looks like:

country_code	country_name	census_2000	census_2015
UK	United Kingdom	58857004	64715810
...

And another resource (country_gdp_2015) with data that looks like:

CC	GDP (£m)	Net Debt (£m)
UK	1832318	1606600
...

The join command will match rows in both datasets based on the country_code / CC fields, and then copying the value in the census_2015 field into a new population field.

The resulting data package will have the world_population resource removed and the country_gdp_2015 resource looking like:

CC	GDP (£m)	Net Debt (£m)	population
UK	1832318	1606600	64715810
...

A more complex example:

- run: join
  parameters:
    source:
      name: screen_actor_salaries
      key: "{production} ({year})"
    target:
      name: mgm_movies
      key: "{title}"
    fields:
      num_actors:
        aggregate: 'count'
      average_salary:
        name: salary
        aggregate: 'avg'
      total_salaries:
        name: salary
        aggregate: 'sum'
    full: false

This example aims to analyse salaries for screen actors in the MGM studios.

Once more, we have one resource (screen_actor_salaries) with data that looks like:

year	production	actor	salary
2016	Vertigo 2	Mr. T	15000000
2016	Vertigo 2	Robert Downey Jr.	7000000
2015	The Fall - Resurrection	Jeniffer Lawrence	18000000
2015	Alf - The Return to Melmack	The Rock	12000000
...

And another resource (mgm_movies) with data that looks like:

title	director	producer
Vertigo 2 (2016)	Lindsay Lohan	Lee Ka Shing
iRobot - The Movie (2018)	Mr. T	Mr. T
...

The join command will match rows in both datasets based on the movie name and production year. Notice how we overcome incompatible fields by using different key patterns.

The resulting dataset could look like:

title	director	producer	num_actors	average_salary	total_salaries
Vertigo 2 (2016)	Lindsay Lohan	Lee Ka Shing	2	11000000	22000000
...

filter

Filter streamed resources.

filter accepts equality and inequality conditions and tests each row in the selected resources. If none of the conditions validate, the row will be discarded.

Parameters:

• resources - Which resources to apply the filter on. Same semantics as resources in stream_remote_resources.
• in - Mapping of keys to values which translate to row[key] == value conditions
• out - Mapping of keys to values which translate to row[key] != value conditions

Both in and out should be a list of objects. However, out should only ever have one element.

Examples:

Filtering just American and European countries, leaving out countries whose main language is English:

- run: filter
  parameters:
    resources: world_population
    in:
      - continent: america
      - continent: europe
- run: filter
  parameters:
    resources: world_population
    out:
      - language: english

To filter out by multiple values, you need multiple filter processors, not multiple out elements. Otherwise some condition will always validate and no rows will be discareded:

- run: filter
  parameters:
    resources: world_population
    out:
      - language: english
- run: filter
  parameters:
    resources: world_population
    out:
      - language: swedish

sort

Sort streamed resources by key.

sort accepts a list of resources and a key (as a Python format string on row fields). It will output the rows for each resource, sorted according to the key (in ascending order by default).

Parameters:

• resources - Which resources to sort. Same semantics as resources in stream_remote_resources.
• sort-by - String, which would be interpreted as a Python format string used to form the key (e.g. {<field_name_1>}:{field_name_2})
• reverse - Optional boolean, if set to true - sorts in reverse order

Examples:

Filtering just American and European countries, leaving out countries whose main language is English:

- run: sort
  parameters:
    resources: world_population
    sort-by: "{country_name}"

deduplicate

Deduplicates rows in resources based on the resources' primary key

deduplicate accepts a resource specifier - for each resource, it will output only unique rows (based on the values in the primary key fields). Rows with duplicate primary keys will be ignored.

Parameters:

• resources - Which resources to sort. Same semantics as resources in stream_remote_resources.

Examples:

Deduplicating rows in the world-population resource.

- run: deduplicate
  parameters:
    resources: world_population

duplicate

Duplicate a resource.

duplicate accepts the name of a single resource in the datapackage. It will then duplicate it in the output datapackage, with a different name and path. The duplicated resource will appear immediately after its original.

Parameters:

• source - Which resources to duplicate. The name of the resource.
• target-name - Name of the new, duplicated resource.
• target-path - Path for the new, duplicated resource.
• duplicate_to_end - Make the duplicate resource appear at the end, rather than immediately after its original.

Examples:

Filtering just American and European countries, leaving out countries whose main language is English:

- run: duplicate
  parameters:
    source: original-resource
    target-name: copy-of-resource
    target-path: data/duplicate.csv

delete_fields

Delete fields (columns) from streamed resources

delete_fields accepts a list of resources and list of fields to remove

Note: if multiple resources provided, all of them should contain all fields to delete

Parameters:

• resources - Which resources to delete columns from. Same semantics as resources in stream_remote_resources.
• fields - List of field (column) names to be removed (exact names or regular expressions for matching field names)
• regex - if set to False field names will be interpreted as strings not as regular expressions (True by default)

Examples:

Deleting country_name and census_2000 columns from world_population resource:

- run: delete_fields
  parameters:
    resources: world_population
    fields:
      - country_name
      - census_2000

add_computed_field

Add field(s) to streamed resources

add_computed_field accepts a list of resources and fields to add to existing resource. It will output the rows for each resource with new field(s) (columns) in it. add_computed_field allows to perform various operations before inserting value into targeted field.

Parameters:

• resources - Resources to add field. Same semantics as resources in stream_remote_resources.
• fields - List of operations to be performed on the targeted fields.
  • operation: operation to perform on values of pre-defined columns of the same row. available operation:
    • constant - add a constant value
    • sum - summed value for given columns in a row.
    • avg - average value from given columns in a row.
    • min - minimum value among given columns in a row.
    • max - maximum value among given columns in a row.
    • multiply - product of given columns in a row.
    • join - joins two or more column values in a row.
    • format - Python format string used to form the value Eg: my name is {first_name}.
  • target - name of the new field.
  • source - list of columns the operations should be performed on (Not required in case of format and constant).
  • with - String passed to constant, format or join operations
    • in constant - used as constant value
    • in format - used as Python format string with existing column values Eg: {first_name} {last_name}
    • in join - used as delimiter

Examples:

Following example adds 4 new field to salaries resource

run: add_computed_field
parameters:
  resources: salaries
  fields:
    -
      operation: sum
      target: total
      source:
        - jan
        - feb
        - may
    -
      operation: avg
      target: average
      source:
        - jan
        - feb
        - may
    -
      operation: format
      target: full_name
      with: '{first_name} {last_name}'
    -
      operation: constant
      target: status
      with: single

We have one resource (salaries) with data that looks like:

first_name	last_name	jan	feb	mar
John	Doe	100	200	300
...

The resulting dataset could look like:

first_name	last_name	last_name	jan	feb	mar	average	total	status
John	Doe	John Doe	100	200	300	200	600	single
...

find_replace

find and replace string or pattern from field(s) values

Parameters:

• resources - Resources to clean the field values. Same semantics as resources in stream_remote_resources

• fields- list of fields to replace values

  • name - name of the field to replace value
  • patterns - list of patterns to find and replace from field
    • find - String, interpreted as a regular expression to match field value
    • replace - String, interpreted as a regular expression to replace matched pattern

Examples:

Following example replaces field values using regular expression and exact string patterns

run: find_replace
parameters:
  resources: dates
  fields:
    -
      name: year
      patterns:
        -
          find: ([0-9]{4})( \(\w+\))
          replace: \1
    -
      name: quarter
      patterns:
        -
          find: Q1
          replace: '03-31'
        -
          find: Q2
          replace: '06-31'
        -
          find: Q3
          replace: '09-30'
        -
          find: Q4
          replace: '12-31'

We have one resource (dates) with data that looks like:

year	quarter
2000 (1)	2000-Q1
...

The resulting dataset could look like:

year	quarter
2000	2000-03-31
...

unpivot

Unpivots, transposes tabular data so that there's only one record per row.

Parameters:

• resources - Resources to unpivot. Same semantics as resources in stream_remote_resources.
• extraKeyFields - List of target field definitions, each definition is an object containing at least these properties (unpivoted column values will go here)
  • name - Name of the target field
  • type - Type of the target field
• extraValueField - Target field definition - an object containing at least these properties (unpivoted cell values will go here)
  • name - Name of the target field
  • type - Type of the target field
• unpivot - List of source field definitions, each definition is an object containing at least these properties
  • name - Either simply the name, or a regular expression matching the name of original field to unpivot.
  • keys - A Map between target field name and values for original field
    • Keys should be target field names from extraKeyFields
    • Values may be either simply the constant value to insert, or a regular expression matching the name.

Examples:

Following example will unpivot data into 3 new fields: year, direction and amount

parameters:
  resources: balance
  extraKeyFields:
    -
      name: year
      type: integer
    -
      name: direction
      type: string
      constraints:
        enum:
          - In
          - Out
  extraValueField:
      name: amount
      type: number
  unpivot:
    -
      name: 2015 incomes
      keys:
        year: 2015
        direction: In
    -
      name: 2015 expenses
      keys:
        year: 2015
        direction: Out
    -
      name: 2016 incomes
      keys:
        year: 2016
        direction: In
    -
      name: 2016 expenses
      keys:
        year: 2016
        direction: Out

We have one resource (balance) with data that looks like:

company	2015 incomes	2015 expenses	2016 incomes	2016 expenses
Inc	1000	900	2000	1700
Org	2000	800	3000	2000
...

The resulting dataset could look like:

company	year	direction	amount
Inc	2015	In	1000
Inc	2015	Out	900
Inc	2016	In	2000
Inc	2016	Out	1700
Org	2015	In	2000
Org	2015	Out	800
Org	2016	In	3000
Org	2016	Out	2000
...

Similar result can be accomplished by defining regular expressions instead of constant values

parameters:
  resources: balance
  extraKeyFields:
    -
      name: year
      type: integer
    -
      name: direction
      type: string
      constraints:
        enum:
          - In
          - Out
  extraValueField:
      name: amount
      type: number
  unpivot:
    -
      name: ([0-9]{4}) (\\w+)  # regex for original column
      keys:
        year: \\1  # First member of group from above
        direction: \\2  # Second member of group from above

dump_to_sql

Saves the datapackage to an SQL database.

Parameters:

• engine - Connection string for connecting to the SQL Database (URL syntax) Also supports env://<environment-variable>, which indicates that the connection string should be fetched from the indicated environment variable. If not specified, assumes a default of env://DPP_DB_ENGINE
• tables - Mapping between resources and DB tables. Keys are table names, values are objects with the following attributes:
  • resource-name - name of the resource that should be dumped to the table
  • mode - How data should be written to the DB. Possible values:
    • rewrite (the default) - rewrite the table, all previous data (if any) will be deleted.
    • append - write new rows without changing already existing data.
    • update - update the table based on a set of "update keys". For each new row, see if there already an existing row in the DB which can be updated (that is, an existing row with the same values in all of the update keys). If so - update the rest of the columns in the existing row. Otherwise - insert a new row to the DB.
  • update_keys - Only applicable for the update mode. A list of field names that should be used to check for row existence. If left unspecified, will use the schema's primaryKey as default.
  • indexes - TBD
• updated_column - Optional name of a column that will be added to the spewed data with boolean value
  • true - row was updated
  • false - row was inserted
• updated_id_column - Optional name of a column that will be added to the spewed data and contain the id of the updated row in DB.

dump_to_path

Saves the datapackage to a filesystem path.

Parameters:

• out-path - Name of the output path where datapackage.json will be stored.

  This path will be created if it doesn't exist, as well as internal data-package paths.

  If omitted, then . (the current directory) will be assumed.

• force-format - Specifies whether to force all output files to be generated with the same format

  • if True (the default), all resources will use the same format
  • if False, format will be deduced from the file extension. Resources with unknown extensions will be discarded.

• format - Specifies the type of output files to be generated (if force-format is true): csv (the default) or json

• add-filehash-to-path: Specifies whether to include file md5 hash into the resource path. Defaults to False. If True Embeds hash in path like so:

  • If original path is path/to/the/file.ext
  • Modified path will be path/to/the/HASH/file.ext

• counters - Specifies whether to count rows, bytes or md5 hash of the data and where it should be stored. An object with the following properties:

  • datapackage-rowcount: Where should a total row count of the datapackage be stored (default: count_of_rows)
  • datapackage-bytes: Where should a total byte count of the datapackage be stored (default: bytes)
  • datapackage-hash: Where should an md5 hash of the datapackage be stored (default: hash)
  • resource-rowcount: Where should a total row count of each resource be stored (default: count_of_rows)
  • resource-bytes: Where should a total byte count of each resource be stored (default: bytes)
  • resource-hash: Where should an md5 hash of each resource be stored (default: hash) Each of these attributes could be set to null in order to prevent the counting. Each property could be a dot-separated string, for storing the data inside a nested object (e.g. stats.rowcount)

• pretty-descriptor: Specifies how datapackage descriptor (datapackage.json) file will look like:

  • False (default) - descriptor will be written in one line.
  • True - descriptor will have indents and new lines for each key, so it becomes more human-readable.

dump_to_zip

Saves the datapackage to a zipped archive.

Parameters:

• out-file - Name of the output file where the zipped data will be stored
• force-format and format - Same as in dump_to_path
• add-filehash-to-path - Same as in dump_to_path
• counters - Same as in dump_to_path
• pretty-descriptor - Same as in dump_to_path

Deprecated Processors

These processors will be removed in the next major version.

add_metadata

Alias for update_package, is kept for backward compatibility reasons.

add_resource

Adds a new external tabular resource to the data-package.

Parameters:

You should provide a name and url attributes, and other optional attributes as defined in the spec.

url indicates where the data for this resource resides. Later on, when stream_remote_resources runs, it will use the url (which is stored in the resource in the dpp:streamedFrom property) to read the data rows and push them into the pipeline.

Note that url also supports env://<environment-variable>, which indicates that the resource url should be fetched from the indicated environment variable. This is useful in case you are supplying a string with sensitive information (such as an SQL connection string for streaming from a database table).

Parameters are basically arguments that are passed to a tabulator.Stream instance (see the API). Other than those, you can pass a constants parameter which should be a mapping of headers to string values. When used in conjunction with stream_remote_resources, these constant values will be added to each generated row (as well as to the default schema).

You may also provide a schema here, or use the default schema generated by the stream_remote_resources processor. In case path is specified, it will be used. If not, the stream_remote_resources processor will assign a path for you with a csv extension.

Example:

- run: add_resource
  parameters:
    url: http://example.com/my-excel-file.xlsx
    sheet: 1
    headers: 2
- run: add_resource
  parameters:
    url: http://example.com/my-csv-file.csv
    encoding: "iso-8859-2"

stream_remote_resources

Converts external resources to streamed resources.

External resources are ones that link to a remote data source (url or file path), but are not processed by the pipeline and are kept as-is.

Streamed resources are ones that can be processed by the pipeline, and their output is saved as part of the resulting datapackage.

In case a resource has no schema, a default one is generated automatically here by creating a string field from each column in the data source.

Parameters:

• resources - Which resources to stream. Can be:

  • List of strings, interpreted as resource names to stream
  • String, interpreted as a regular expression to be used to match resource names

  If omitted, all resources in datapackage are streamed.

• ignore-missing - if true, then missing resources won't raise an error but will be treated as 'empty' (i.e. with zero rows). Resources with empty URLs will be treated the same (i.e. will generate an 'empty' resource).

• limit-rows - if provided, will limit the number of rows fetched from the source. Takes an integer value which specifies how many rows of the source to stream.

Example:

- run: stream_remote_resources
  parameters:
    resources: ['2014-data', '2015-data']
- run: stream_remote_resources
  parameters:
    resources: '201[67]-data'

This processor also supports loading plain-text resources (e.g. html pages) and handling them as tabular data - split into rows with a single "data" column. To enable this behavior, add the following attribute to the resource: "format": "txt".

dump.to_sql

Alias for dump_to_sql, is kept for backward compatibility reasons.

dump.to_path

Saves the datapackage to a filesystem path.

Parameters:

• out-path - Name of the output path where datapackage.json will be stored.

  This path will be created if it doesn't exist, as well as internal data-package paths.

  If omitted, then . (the current directory) will be assumed.

• force-format - Specifies whether to force all output files to be generated with the same format

  • if True (the default), all resources will use the same format
  • if False, format will be deduced from the file extension. Resources with unknown extensions will be discarded.

• format - Specifies the type of output files to be generated (if force-format is true): csv (the default) or json

• handle-non-tabular - Specifies whether non tabular resources (i.e. resources without a schema) should be dumped as well to the resulting datapackage. (See note below for more details)

• add-filehash-to-path: Specifies whether to include file md5 hash into the resource path. Defaults to False. If True Embeds hash in path like so:

  • If original path is path/to/the/file.ext
  • Modified path will be path/to/the/HASH/file.ext

• counters - Specifies whether to count rows, bytes or md5 hash of the data and where it should be stored. An object with the following properties:

  • datapackage-rowcount: Where should a total row count of the datapackage be stored (default: count_of_rows)
  • datapackage-bytes: Where should a total byte count of the datapackage be stored (default: bytes)
  • datapackage-hash: Where should an md5 hash of the datapackage be stored (default: hash)
  • resource-rowcount: Where should a total row count of each resource be stored (default: count_of_rows)
  • resource-bytes: Where should a total byte count of each resource be stored (default: bytes)
  • resource-hash: Where should an md5 hash of each resource be stored (default: hash) Each of these attributes could be set to null in order to prevent the counting. Each property could be a dot-separated string, for storing the data inside a nested object (e.g. stats.rowcount)

• pretty-descriptor: Specifies how datapackage descriptor (datapackage.json) file will look like:

  • False (default) - descriptor will be written in one line.
  • True - descriptor will have indents and new lines for each key, so it becomes more human-readable.

• file-formatters: Specifies custom file format handlers. An object with mapping of format name to Python module and class name.

  • Allows to override the existing csv and json format handlers or add support for new formats.
  • Note that such changes may make the resulting datapackage incompatible with the frictionlessdata specs and may cause interoperability problems.
  • Example usage: pipeline-spec.yaml (under the custom-formatters pipeline), XLSXFormat class

dump.to_zip

Saves the datapackage to a zipped archive.

Parameters:

• out-file - Name of the output file where the zipped data will be stored
• force-format and format - Same as in dump_to_path
• handle-non-tabular - Same as in dump_to_path
• add-filehash-to-path - Same as in dump_to_path
• counters - Same as in dump_to_path
• pretty-descriptor - Same as in dump_to_path
• file-formatters - Same as in dump_to_path

Note

dump.to_path and dump.to_zip processors will handle non-tabular resources as well. These resources must have both a url and path properties, and must not contain a schema property. In such cases, the file will be downloaded from the url and placed in the provided path.

Custom Processors

It's quite reasonable that for any non-trivial processing task, you might encounter a problem that cannot be solved using the standard library processors.

For that you might need to write your own processor - here's how it's done.

There are two APIs for writing processors - the high level API and the low level API.

High Level Processor API

The high-level API is quite useful for most processor kinds:

from datapackage_pipelines.wrapper import process

def modify_datapackage(datapackage, parameters, stats):
    # Do something with datapackage
    return datapackage

def process_row(row, row_index,
                resource_descriptor, resource_index,
                parameters, stats):
    # Do something with row
    return row

process(modify_datapackage=modify_datapackage,
        process_row=process_row)

The high level API consists of one method, process which takes two functions:

• modify_datapackage - which makes changes (if necessary) to the data-package descriptor, e.g. adds metadata, adds resources, modifies resources' schema etc.

  Can also be used for initialization code when needed.

  It has these arguments:

  • datapackage is the current data-package descriptor that needs to be modified. The modified data-package descriptor needs to be returned.
  • parameters is a dict containing the processor's parameters, as provided in the pipeline-spec.yaml file.
  • stats is a dict which should be modified in order to collect metrics and measurements in the process (e.g. validation checks, row count etc.)

• process_row - which modifies a single row in the stream. It receives these arguments:

  • row is a dictionary containing the row to process
  • row_index is the index of the row in the resource
  • resource_descriptor is the descriptor object of the current resource being processed
  • resource_index is the index of the resource in the data-package
  • parameters is a dict containing the processor's parameters, as provided in the pipeline-spec.yaml file.
  • stats is a dict which should be modified in order to collect metrics and measurements in the process (e.g. validation checks, row count etc.)

  and yields zero or more processed rows.

A few examples

# Add license information
from datapackage_pipelines.wrapper import process

def modify_datapackage(datapackage, parameters, stats):
    datapackage['license'] = 'CC-BY-SA'
    return datapackage

process(modify_datapackage=modify_datapackage)

# Add new column with constant value to first resource
# Column name and value are taken from the processor's parameters
from datapackage_pipelines.wrapper import process

def modify_datapackage(datapackage, parameters, stats):
    datapackage['resources'][0]['schema']['fields'].append({
      'name': parameters['column-name'],
      'type': 'string'
    })
    return datapackage

def process_row(row, row_index, resource_descriptor, resource_index, parameters, stats):
    if resource_index == 0:
        row[parameters['column-name']] = parameters['value']
    return row

process(modify_datapackage=modify_datapackage,
        process_row=process_row)

# Row counter
from datapackage_pipelines.wrapper import process

def modify_datapackage(datapackage, parameters, stats):
    stats['row-count'] = 0
    return datapackage

def process_row(row, row_index, resource_descriptor, resource_index, parameters, stats):
    stats['row-count'] += 1
    return row

process(modify_datapackage=modify_datapackage,
        process_row=process_row)

Low Level Processor API

In some cases, the high-level API might be too restricting. In these cases you should consider using the low-level API.

from datapackage_pipelines.wrapper import ingest, spew

if __name__ == '__main__':
  with ingest() as ctx:

    # Initialisation code, if needed

    # Do stuff with datapackage
    # ...

    stats = {}

    # and resources:
    def new_resource_iterator(resource_iterator_):
        def resource_processor(resource_):
            # resource_.spec is the resource descriptor
            for row in resource_:
                # Do something with row
                # Perhaps collect some stats here as well
                yield row
        for resource in resource_iterator_:
            yield resource_processor(resource)

    spew(ctx.datapackage,
         new_resource_iterator(ctx.resource_iterator),
         ctx.stats)

The above code snippet shows the structure of most low-level processors.

We always start with calling ingest() - this method gives us the context, containing the execution parameters, the data-package descriptor (as outputed from the previous step) and an iterator on all streamed resources' rows.

We finish the processing by calling spew(), which sends the processed data to the next processor in the pipeline. spew receives:

• A modified data-package descriptor;
• A (possibly new) iterator on the resources;
• A stats object which will be added to stats from previous steps and returned to the user upon completion of the pipeline, and;
• Optionally, a finalizer function that will be called after it has finished iterating on the resources, but before signalling to other processors that it's finished. You could use it to close any open files, for example.

A more in-depth explanation

spew writes the data it receives in the following order:

• First, the datapackage parameter is written to the stream. This means that all modifications to the data-package descriptor must be done before spew is called. One common pitfall is to modify the data-package descriptor inside the resource iterator - try to avoid that, as the descriptor that the next processor will receive will be wrong.
• Then it starts iterating on the resources. For each resource, it iterates on its rows and writes each row to the stream. This iteration process eventually causes an iteration on the original resource iterator (the one that's returned from ingest). In turn, this causes the process' input stream to be read. Because of the way buffering in operating systems work, "slow" processors will read their input slowly, causing the ones before them to sleep on IO while their more CPU intensive counterparts finish their processing. "quick" processors will not work aimlessly, but instead will either sleep while waiting for incoming data or while waiting for their output buffer to drain. What is achieved here is that all rows in the data are processed more or less at the same time, and that no processor works too "far ahead" on rows that might fail in subsequent processing steps.
• Then the stats are written to the stream. This means that stats can be modified during the iteration, and only the value after the iteration finishes will be used.
• Finally, the finalizer method is called (if we received one).

A few examples

We'll start with the same processors from above, now implemented with the low level API.

# Add license information
from datapackage_pipelines.wrapper import ingest, spew

if __name__ == '__main__':
  with ingest() as ctx:
    ctx.datapackage['license'] = 'MIT'
    spew(ctx.datapackage, ctx.resource_iterator)

# Add new column with constant value to first resource
# Column name and value are taken from the processor's parameters
from datapackage_pipelines.wrapper import ingest, spew

parameters, datapackage, resource_iterator = ingest()

datapackage['resources'][0]['schema']['fields'].append({
   'name': parameters['column-name'],
   'type': 'string'
})

def new_resource_iterator(resource_iterator_):
    def resource_processor(resource_):
        for row in resource_:
            row[parameters['column-name']] = parameters['value']
            yield row

    first_resource = next(resource_iterator_)
    yield(resource_processor(first_resource))

    for resource in resource_iterator_:
        yield resource

spew(datapackage, new_resource_iterator(resource_iterator))

# Row counter
from datapackage_pipelines.wrapper import ingest, spew

_, datapackage, resource_iterator = ingest()

stats = {'row-count': 0}

def new_resource_iterator(resource_iterator_):
    def resource_processor(resource_):
        for row in resource_:
            stats['row-count'] += 1
            yield row

    for resource in resource_iterator_:
        yield resource_processor(resource)

spew(datapackage, new_resource_iterator(resource_iterator), stats)

This next example shows how to implement a simple web scraper. Although not strictly required, web scrapers are usually the first processor in a pipeline. Therefore, they can ignore the incoming data-package and resource iterator, as there's no previous processor generating data:

# Web Scraper
import requests
from datapackage_pipelines.wrapper import ingest, spew
from datapackage_pipelines.utilities.resources import PROP_STREAMING

parameters, _, _ = ingest()

host = parameters['ckan-instance']
package_list_api = 'https://{host}/api/3/action/package_list'
package_show_api = 'https://{host}/api/3/action/package_show'

def scrape_ckan(host_):
    all_packages = requests.get(package_list_api.format(host=host_))\
                           .json()\
                           .get('result', [])
    for package_id in all_packages:
      params = dict(id=package_id)
      package_info = requests.get(package_show_api.format(host=host_),
                                  params=params)\
                             .json()\
                             .get('result')
      if result is not None:
        yield dict(
            package_id=package_id,
            author=package_info.get('author'),
            title=package_info.get('title'),
        )

datapackage = {
  'resources': [
    {
      PROP_STREAMING: True,   # You must set this property for resources being streamed in the pipeline!
      'name': 'package-list',
      'schema': {
        'fields': [
          {'name': 'package_id', 'type': 'string'},
          {'name': 'author',     'type': 'string'},
          {'name': 'title',      'type': 'string'},
        ]
      }
    }
  ]
}

spew(datapackage, [scrape_ckan(host)])

In this example we can see that the initial datapackage is generated from scratch, and the resource iterator is in fact a scraper, yielding rows as they are received from the CKAN instance API.

Plugins and Source Descriptors

When writing pipelines in a specific problem domain, one might discover that the processing pipelines that are developed follow a certain pattern. Scraping, or fetching source data tends to be similar to one another. Processing, data cleaning, validation are often the same.

In order to ease maintenance and avoid boilerplate, a datapackage-pipelines plugin can be written.

Plugins are Python modules named datapackage_pipelines_<plugin-name>. Plugins can provide two facilities:

• Processor packs - you can pack processors revolving a certain theme or for a specific purpose in a plugin. Any processor foo residing under the datapackage_pipelines_<plugin-name>.processors module can be used from within a pipeline as <plugin-name>.foo.
• Pipeline templates - if the class Generator exists in the datapackage_pipelines_<plugin-name> module, it will be used to generate pipeline based on templates - which we call "source descriptors".

Source Descriptors

A source descriptor is a yaml file containing information which is used to create a full pipeline.

dpp will look for files named <plugin-name>.source-spec.yaml , and will treat them as input for the pipeline generating code - which should be implemented in a class called Generator in the datapackage_pipelines_<plugin-name> module.

This class should inherit from GeneratorBase and should implement two methods:

• generate_pipeline - which receives the source description and returns an iterator of tuples of the form (id, details). id might be a pipeline id, in which case details would be an object containing the pipeline definition. If id is of the form :module:, then the details are treated as a source spec from the specified module. This way a generator might generate other source specs.
• get_schema - which should return a JSON Schema for validating the source description's structure

Example

Let's assume we write a datapackage_pipelines_ckan plugin, used to pull data out of CKAN instances.

Here's how such a hypothetical generator would look like:

import os
import json

from datapackage_pipelines.generators import \
    GeneratorBase, slugify, steps, SCHEDULE_MONTHLY

SCHEMA_FILE = os.path.join(os.path.dirname(__file__), 'schema.json')


class Generator(GeneratorBase):

    @classmethod
    def get_schema(cls):
        return json.load(open(SCHEMA_FILE))

    @classmethod
    def generate_pipeline(cls, source):
        pipeline_id = dataset_name = slugify(source['name'])
        host = source['ckan-instance']
        action = source['data-kind']

        if action == 'package-list':
            schedule = SCHEDULE_MONTHLY
            pipeline_steps = steps(*[
                ('ckan.scraper', {
                   'ckan-instance': host
                }),
                ('metadata', {
                  'name': dataset_name
                }),
                ('dump_to_zip', {
                   'out-file': 'ckan-datapackage.zip'
                })])
            pipeline_details = {
                'pipeline': pipeline_steps,
                'schedule': {'crontab': schedule}
            }
            yield pipeline_id, pipeline_details

In this case, if we store a ckan.source-spec.yaml file looking like this:

ckan-instance: example.com
name: example-com-list-of-packages
data-kind: package-list

Then when running dpp we will see an available pipeline named ./example-com-list-of-packages

This pipeline would internally be composed of 3 steps: ckan.scraper, metadata and dump_to_zip.

Validating Source Descriptors

Source descriptors can have any structure that best matches the parameter domain of the output pipelines. However, it must have a consistent structure, backed by a JSON Schema file. In our case, the Schema might look like this:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "properties": {
    "name":          { "type": "string" },
    "ckan-instance": { "type": "string" },
    "data-kind":     { "type": "string" }
  },
  "required": [ "name", "ckan-instance", "data-kind" ]
}

dpp will ensure that source descriptor files conform to that schema before attempting to convert them into pipelines using the Generator class.

Providing Processor Code

In some cases, a generator would prefer to provide the processor code as well (alongside the pipeline definition). In order to to that, the generator can add a code attribute to any step containing the processor's code. When executed, this step won't try to resolve the processor as usual but will the provided code instead.

Running on a schedule

datapackage-pipelines comes with a celery integration, allowing for pipelines to be run at specific times via a crontab like syntax.

In order to enable that, you simply add a schedule section to your pipeline-spec.yaml file (or return a schedule from the generator class, see above), like so:

co2-information-cdiac:
  pipeline:
    -
        ...
  schedule:
    # minute hour day_of_week day_of_month month_of_year
    crontab: '0 * * * *'

In this example, this pipeline is set to run every hour, on the hour.

To run the celery daemon, use celery's command line interface to run datapackage_pipelines.app. Here's one way to do it:

$ python -m celery worker -B -A datapackage_pipelines.app

Running this server will start by executing all "dirty" tasks, and continue by executing tasks based on their schedules.

As a shortcut for starting the scheduler and the dashboard (see below), you can use a prebuilt Docker image:

$ docker run -v `pwd`:/pipelines:rw -p 5000:5000 \
        frictionlessdata/datapackage-pipelines server

And then browse to http://<docker machine's IP address>:5000/ to see the current execution status dashboard.

Pipeline Dashboard & Status Badges

When installed on a server or running using the task scheduler, it's often very hard to know exactly what's running and what the status is of each pipeline.

To make things easier, you can spin up the web dashboard to provide an overview of each pipeline's status, its basic info and the result of it latest execution.

To start the web server run dpp serve from the command line and browse to http://localhost:5000

The environment variable DPP_BASE_PATH will determine whether dashboard will be served from root or from another base path (example value: /pipelines/).

The dashboard endpoints can be made to require authentication by adding a username and password with the environment variables DPP_BASIC_AUTH_USERNAME and DPP_BASIC_AUTH_PASSWORD.

Even simpler pipeline status is available with a status badge, both for individual pipelines, and for pipeline collections. For a single pipeline, add the full pipeline id to the badge endpoint:

http://localhost:5000/badge/path_to/pipelines/my-pipeline-id

Or for a collection of pipelines:

http://localhost:5000/badge/collection/path_to/pipelines/

Note that these badge endpoints will always be exposed regardless of DPP_BASIC_AUTH_PASSWORD and DPP_BASIC_AUTH_USERNAME settings.

Integrating with other services

Datapackage-pipelines can call a predefined webhook on any pipeline event. This might allow for potential integrations with other applications.

In order to add a webhook in a specific pipeline, add a hooks property in the pipeline definition, which should be a list of URLs. Whenever that pipeline is queued, starts running or finishes running, all the urls will be POSTed with this payload:

{
  "pipeline": "<pipeline-id>",
  "event": "queue/start/progress/finish",
  "success": true/false (when applicable),
  "errors": [list-of-errors, when applicable]
}

Known Issues

• loading a resource which has a lot of data in a single cell raises an exception (#112)