xcube Developer Guide

Version 0.2, draft

IMPORTANT NOTE: Any changes to this doc must be reviewed by dev-team through pull requests.

Preface

Gedacht ist nicht gesagt.
Gesagt ist nicht gehört.
Gehört ist nicht verstanden.
Verstanden ist nicht einverstanden.
Einverstanden ist nicht umgesetzt.
Umgesetzt ist nicht beibehalten.

by Konrad Lorenz (translation is left to the reader)

Table of Contents

• Versioning

• Coding Style

• Main Packages

  • Package xcube.core

  • Package xcube.cli

  • Package xcube.webapi

  • Package xcube.util

• Development Process

Versioning

We adhere to PEP-440. Therefore, the xcube software version uses the format <major>.<minor>.<micro> for released versions and <major>.<minor>.<micro>.dev<n> for versions in development.

• <major> is increased for major enhancements. CLI / API changes may introduce incompatibilities with former version.

• <minor> is increased for new features and and minor enhancements. CLI / API changes are backward compatible with former version.

• <micro> is increased for bug fixes and micro enhancements. CLI / API changes are backward compatible with former version.

• <n> is increased whenever the team (internally) deploys new builds of a development snapshot.

The current software version is in xcube/version.py.

Coding Style

We try adhering to PEP-8.

Main Packages

• xcube.core - Hosts core API functions. Code in here should be maintained w.r.t. backward compatibility. Therefore think twice before adding new or change existing core API.

• xcube.cli - Hosts CLI commands. CLI command implementations should be lightweight. Move implementation code either into core or util.
  CLI commands must be maintained w.r.t. backward compatibility. Therefore think twice before adding new or change existing CLI commands.

• xcube.webapi - Hosts Web API functions. Web API command implementations should be lightweight. Move implementation code either into core or util.
  Web API interface must be maintained w.r.t. backward compatibility. Therefore think twice before adding new or change existing web API.

• xcube.util - Mainly implementation helpers. Comprises classes and functions that are used by cli, core, webapi in order to maximize modularisation and testability but to minimize code duplication.
  The code in here must not be dependent on any of cli, core, webapi. The code in here may change often and in any way as desired by code implementing the cli, core, webapi packages.

The following sections will guide you through extending or changing the main packages that form xcube’s public interface.

Package xcube.cli

Checklist

Make sure your change

1. is covered by unit-tests (package test/cli);

2. is reflected by the CLI’s doc-strings and tools documentation (currently in README.md);

3. follows existing xcube CLI conventions;

4. follows PEP8 conventions;

5. is reflected in API and WebAPI, if desired;

6. is reflected in CHANGES.md.

Hints

Make sure your new CLI command is in line with the others commands regarding command name, option names, as well as metavar arguments names. The CLI command name shall ideally be a verb.

Avoid introducing new option arguments if similar options are already in use for existing commands.

In the following common arguments and options are listed.

Input argument:

@click.argument('input')

If input argument is restricted to an xcube dataset:

@click.argument('cube')

Output (directory) option:

@click.option('--output', '-o', metavar='OUTPUT',
              help='Output directory. If omitted, "INPUT.levels" will be used.')

Output format:

@click.option('--format', '-f', metavar='FORMAT', type=click.Choice(['zarr', 'netcdf']),
              help="Format of the output. If not given, guessed from OUTPUT.")

Output parameters:

@click.option('--param', '-p', metavar='PARAM', multiple=True,
              help="Parameter specific for the output format. Multiple allowed.")

Variable names:

@click.option('--variable',--var', metavar='VARIABLE', multiple=True,
              help="Name of a variable. Multiple allowed.")

For parsing CLI inputs, use helper functions that are already in use. In the CLI command implementation code, raise click.ClickException(message) with a clear message for users.

Common xcube CLI options like -f for FORMAT should be lower case letters and specific xcube CLI options like -S for SIZE in xcube gen are recommended to be uppercase letters.

Extensively validate CLI inputs to avoid that API functions raise ValueError, TypeError, etc. Such errors and their message texts are usually hard to understand by users. They are actually dedicated to to developers, not CLI users.

There is a global option --traceback flag that user can set to dump stack traces. You don’t need to print stack traces from your code.

Package xcube.core

Checklist

Make sure your change

1. is covered by unit-tests (package test/core);

2. is covered by API documentation;

3. follows existing xcube API conventions;

4. follows PEP8 conventions;

5. is reflected in xarray extension class xcube.core.xarray.DatasetAccessor;

6. is reflected in CLI and WebAPI if desired;

7. is reflected in CHANGES.md.

Hints

Create new module in xcube.core and add your functions. For any functions added make sure naming is in line with other API. Add clear doc-string to the new API. Use Sphinx RST format.

Decide if your API methods requires xcube datasets as inputs, if so, name the primary dataset argument cube and add a keyword parameter cube_asserted: bool = False. Otherwise name the primary dataset argument dataset.

Reflect the fact, that a certain API method or function operates only on datasets that conform with the xcube dataset specifications by using cube in its name rather than dataset. For example compute_dataset can operate on any xarray datasets, while get_cube_values_for_points expects a xcube dataset as input or read_cube ensures it will return valid xcube datasets only.

In the implementation, if not cube_asserted, we must assert and verify the cube is a cube. Pass True to cube_asserted argument of other API called later on:

from xcube.core.verify import assert_cube

def frombosify_cube(cube: xr.Dataset, ..., cube_asserted: bool = False):  
    if not cube_asserted:
        assert_cube(cube)
    ...
    result = bibosify_cube(cube, ..., cube_asserted=True)
    ...

If import xcube.core.xarray is imported in client code, any xarray.Dataset object will have an extra property xcube whose interface is defined by the class xcube.core.xarray.DatasetAccessor. This class is an xarray extension that is used to reflect xcube.core functions and make it directly applicable to the xarray.Dataset object.

Therefore any xcube API shall be reflected in this extension class.

Package xcube.webapi

Checklist

Make sure your change

1. is covered by unit-tests (package test/webapi);

2. is covered by Web API specification and documentation (currently in webapi/res/openapi.yml);

3. follows existing xcube Web API conventions;

4. follows PEP8 conventions;

5. is reflected in CLI and API, if desired;

6. is reflected in CHANGES.md.

Hints

• The Web API is defined in webapi.app which defines mapping from resource URLs to handlers

• All handlers are implemented in webapi.handlers. Handler code just delegates to dedicated controllers.

• All controllers are implemented in webapi.controllers.*. They might further delegate into core.*

Development Process

1. Make sure there is an issue ticket for your code change work item

2. Select issue, priorities are as follows

   1. “urgent” and (“important” and “bug”)

   2. “urgent” and (“important” or “bug”)

   3. “urgent”

   4. “important” and “bug”

   5. “important” or “bug”

   6. others

3. Make sure issue is assigned to you, if unclear agree with team first.

4. Add issue label “in progress”.

5. Create development branch named “--