cumulus-message-adapter-python

What is Cumulus?

Cumulus is a cloud-based data ingest, archive, distribution and management prototype for NASA's future Earth science data streams.

Read the Cumulus Documentation

What is the Cumulus Message Adapter?

The Cumulus Message Adapter is a library that adapts incoming messages in the Cumulus protocol to a format more easily consumable by Cumulus tasks, invokes the tasks, and then adapts their response back to the Cumulus message protocol to be sent to the next task.

Installation

$ pip install cumulus-message-adapter-python

Task definition

In order to use the Cumulus Message Adapter, you will need to create two methods in your task module: a handler function and a business logic function.

The handler function is a standard Lambda handler function which takes two parameters (as specified by AWS): event and context.

The business logic function is where the actual work of your task occurs. It should take two parameters: event and context.

The event object contains two keys:

• input - the task's input, typically the payload of the message, produced at runtime
• config - the task's configuration, with any templated variables resolved

The context parameter is the standard Lambda context as passed by AWS.

The return value of the business logic function will be placed in the payload of the resulting Cumulus message.

Expectations for input, config, and return values are all defined by the task, and should be well documented. Tasks should thoughtfully consider their inputs and return values, as breaking changes may have cascading effects on tasks throughout a workflow. Configuration changes are slightly less impactful, but must be communicated to those using the task.

Cumulus Message Adapter interface

The Cumulus Message adapter for python provides one method: run_cumulus_task. It takes four parameters:

• task_function - the function containing your business logic (as described above)
• cumulus_message - the event passed by Lambda, and should be a Cumulus Message, or a CMA parameter encapsulated message (see Cumulus Workflow Documentation):

{
  "cma": {
    "event": "<cumulus message object>"
    "SomeCMAConfigKey": "CMA configuration object>"
  }
}

• context - the Lambda context
• schemas - optional: a dict with input, config, and output properties. Each should be a string set to the filepath of the corresponding JSON schema file. All three properties of this dict are optional. If ommitted, the message adapter will look in /<task_root>/schemas/<schema_type>.json, and if not found there, will be ignored.
• taskargs - Optional. Additional keyword arguments for the task_function

Example

Simple example of using this package's run_cumulus_task function as a wrapper around another function:

from run_cumulus_task import run_cumulus_task

# simple task that returns the event
def task(event, context):
    return event

# handler that is provided to aws lambda
def handler(event, context):
    return run_cumulus_task(task, event, context)

For a full example see the example folder.

Creating a deployment package

Tasks that use this library are just standard AWS Lambda tasks. Information on creating release packages is available here.

Usage in a Cumulus Deployment

For documenation on how to utilize this package in a Cumulus Deployment, view the Cumulus Workflow Documenation.

Development

Dependency Installation

$ pip install -r requirements-dev.txt
$ pip install -r requirements.txt

Logging with CumulusLogger

Included in this package is the cumulus_logger which contains a logging class CumulusLogger that standardizes the log format for Cumulus. Methods are provided to log error, fatal, warning, debug, info, and trace.

Import the CumulusLogger class:

from cumulus_logger import CumulusLogger

Instantiate the logger inside the task definition, name and level are optional:

logger = CumulusLogger(event, context)

Use the logging methods for different levels:

logger.trace('<your message>')
logger.debug('<your message>')
logger.info('<your message>')
logger.warn('<your message>')
logger.error('<your message>')
logger.fatal('<your message>')

It can also take additional non-keyworded and keyworded arguments as in Python Logger.

The msg is the message format string, the args and kwargs are the arguments for string formatting.

If exc_info in kwargs is not false, the exception information in the exc_info or sys.exc_info() is added to the message.

logger.debug(msg, *args, **kwargs)

Example usage:

import os
import sys

from run_cumulus_task import run_cumulus_task
from cumulus_logger import CumulusLogger

# instantiate CumulusLogger
logger = CumulusLogger()

def task(event, context):
    logger.info('task executed')

    # log error when an exception is caught
    logger.error("task formatted message {} exc_info ", "bar", exc_info=True)

    # return the output of the task
    return { "example": "output" }

def handler(event, context):
    # make sure event & context metadata is set in the logger
    logger.setMetadata(event, context)
    return run_cumulus_task(task, event, context)

Running Tests

Running tests requires localstack.

Tests only require localstack running S3, which can be initiated with the following command:

$ SERVICES=s3 localstack start

And then you can check tests pass with the following nosetests command:

$ CUMULUS_ENV=testing nosetests -v -s

Linting

$ pylint run_cumulus_task.py

Why?

This approach has a few major advantages:

1. It explicitly prevents tasks from making assumptions about data structures like meta and cumulus_meta that are owned internally and may therefore be broken in future updates. To gain access to fields in these structures, tasks must be passed the data explicitly in the workflow configuration.
2. It provides clearer ownership of the various data structures. Operators own meta. Cumulus owns cumulus_meta. Tasks define their own config, input, and output formats.
3. The Cumulus Message Adapter greatly simplifies running Lambda functions not explicitly created for Cumulus.
4. The approach greatly simplifies testing for tasks, as tasks don't need to set up cumbersome structures to emulate the message protocol and can just test their business function.

License

Apache 2.0