rework 0.16.0

A database backed distributed task dispatcher

REWORK

rework is a distributed execution system for the execution of tasks that can belong to independant python environments and code bases, even hosted on different computers.

The only constraint is that postgres must be accessible from all nodes of a given rework installation.

Principles

Rework might interest people who:

• want Postgres (and only Postgres) as a Task Queue Manager, Input/Output store and task log store

• have Python long-running tasks to run, with the ability to preemptively kill tasks

• want a tiny, self-contained tool with great functional test abilities.

The most common python library for such things is Celery. If you don't mind depending on RabbitMQ and the Celery API and feature set, Celery is for you. It is mature, probably well-tested, and largely used.

Basic usage

Setting up a database

You need a postgresql database. Rework will install its tables into its own rework namespace schema, so you can use either a dedicated database or an exising one, with little risk of conflict.

If you don't already have a database, create a fresh one with:

createdb jobstore

To install rework inside:

rework init-db postgres://babar:password@localhost:5432/jobstore

Declaring and scheduling a task

All the features are covered in the test suite, which can henceforth be regarded as a reasonnable source of documentation. However here's a simple example:

from time import sleep
from rework import api
from sqlalchemy import create_engine


@api.task
def my_first_task(task):
    with task.capturelogs(std=True):
        print('I am running')
        somevalue = task.input * 2
        task.save_output(somevalue)
        print('I am done')


def main(dburi):
    engine = create_engine(dburi)

    # record the decorated tasks
    api.freeze_operations(engine)

    # now, schedule tasks
    t1 = api.schedule(engine, 'my_first_task', 'hello')
    t2 = api.schedule(engine, 'my_first_task', 100)

    # wait til they are completed
    t1.join()
    t2.join()

    assert t1.output == 'hellohello'
    assert t2.output == 200


if __name__ == '__main__':
    main('postgres://babar:password@localhost:5432/jobstore')

If you put this into a test_rework.py module and type python test_rework.py it should hang forever. Hold on, what's missing ?

On another terminal, one needs to start the workers that will execute the tasks. Do as follows:

rework monitor postgres://babar:password@localhost:5432/jobstore

Then, the script will quickly terminate, as both tasks have been executed.

Specifying inputs

Having a formal declaration of the task input can help validate them and also, in rework_ui it will provide an interactive web form allowing subsequent launch of the task.

from rework import api, io

@api.task(inputs=(
    io.file('myfile.txt', required=True),
    io.string('name', required=True),
    io.string('option', choices=('foo', 'bar')),
    io.number('weight'),
    io.datetime('birthdate'),
    io.moment('horizon')
))
def compute_things(task):
    inp = task.input
    assert 'name' in inp
    ...

... and then, later:

task = api.schedule(
    engine, 'compute_things',
    {'myfile.txt': b'file contents',
     'birthdate': datetime(1973, 5, 20, 9),
     'name': 'Babar',
     'weight': 65,
     'horizon': '(shifted (today) #:days 7)'
     }
)

assert task.input == {
    'myfile.txt': b'file contents',
    'birthdate': datetime(1973, 5, 20, 9),
    'name': 'Babar',
    'weight': 65,
    'horizon': datetime(2021, 1, 7)
}

Specifying outputs

As for the inputs, and for the same reasons, we can provide a spec for the outputs.

from rework import api, io

@api.task(outputs=(
    io.string('name'),
    io.datetime('birthdate')
))
def compute_things(task):
    ...
    task.save_output({
        'name': 'Babar',
        'birthdate': datetime(1931, 1, 1)
    })

And this will of course be fetched from the other side:

t = api.schedule(engine, 'compute_things')
assert t.output == {
    'name': 'Babar',
    'birthdate': datetime(1931, 1, 1)
}

Scheduling

While the base api provides a schedule call that schedules a task for immediate execution, there is also a prepare call that allows to define the exact moment the task ought to be executed, using a crontab like notation.

Example:

api.prepare(
    engine,
    'compute_things',
    {'myfile.txt': b'file contents',
     'birthdate': datetime(1973, 5, 20, 9),
     'name': 'Babar',
     'weight': 65
    },
    rule='0 0 8,12 * * *'
)

This would schedule the task every day at 8:00 and 12:00. The extended crontab notation also features a field for seconds (in first position).

Debugging

If you need to debug some task, the standard advice is:

• write your task content in plain functions and have them unit-tested with e.g. pytest

@api.task
def my_fancy_task(task):
    the_body_of_my_fancy_task(task.input)

• you can also you use print-based logging as shown there:

@api.task
def my_fancy_task(task):
    with task.capturelogs(std=True):
        print('starting')
        # do stuff
        print('done', result)

• finally, it may happen that a task is "stuck" because of a deadlock, and in this case, starting the monitor with --debug-port will help:

$ pip install pystuck
$ rework monitor postgres://babar:password@localhost:5432/jobstore --debug-port=666

Then launching pystuck (possibly from another machine) is done as such:

$ pystuck -h <host> -p 666

Organize tasks in code

A common pattern is to have a project/tasks.py module.

One can manage the tasks using the register-operations and unregister-operation commands.

$ rework register-operations <dburi> /path/to/project/tasks.py

and also

``` rework unregister-operation <dburi> <opname>
delete <opname> <domain> /path/to/project/tasks.py <hostid>
really remove those [y/n]? [y/N]: y

This pair of operations can be used also whenever a task input or output specifications have changed.

API

The api module exposes most if what is needed. The task module and task objects provide the rest.

api module

Three functions are provided: the task decorator, the freeze_operations, schedule and prepare functions.

Defining tasks is done using the task decorator:

from rework.api import task

@task
def my_task(task):
    pass

It is also possible to specify a non-default domain:

@task(domain='scrapers')
def my_scraper(task):
    pass

A timeout parameter is also available:

from datetime import timedelta

@task(timeout=timedelta(seconds=30)
def my_time_limited_task(task):
    pass

To make the tasks available for use, they must be recorded within the database referential. We use freeze_operations for this:

from sqlalchemy import create_engine
from rework.api import freeze_operations

engine = create_engine('postgres://babar:password@localhost:5432/jobstore')
api.freeze_operations(engine)

Finally, one can schedule tasks as such:

from sqlalchemy import create_engine
from rework.api import schedule

engine = create_engine('postgres://babar:password@localhost:5432/jobstore')

# immediate executionn
task = api.schedule(engine, 'my_task', 42)

# execution every five minutes
api.prepare(engine, 'my_task', 42, rule='0 */5 * * * *')

The schedule function wants these mandatory parameters:

• engine: sqlalchemy engine

• operation: string

• inputdata: any python picklable object (if no input specification is provided, else the input formalism provides ways for numbers, strings, dates and files)

It also accepts two more options:

• domain: a domain identifier (for cases when the same service is available under several domains and you want to force one)

• hostid: an host identifier (e.g. '192.168.1.1')

• metadata: a json-serializable dictionary (e.g. {'user': 'Babar'})

The prepare function takes the same parameters as schedule plus a rule option using crontab notation with seconds in first position.

Task objects

Task objects can be obtained from the schedule api call (as seen in the previous example) or through the task module.

from task import Task

task = task.byid(engine, 42)

The task object provides:

• .state attribute to describe the task state (amongst: queued, running, aborting, aborted, failed, done)

• .join() method to wait synchronously for the task completion

• .capturelogs(sync=True, level=logging.NOTSET, std=False) method to record matching logs into the db (sync controls whether the logs are written synchronously, level specifies the capture level, std permits to also record prints as logs)

• .input attribute to get the task input (yields any object)

• .save_output(<obj>) method to store any object

• .abort() method to preemptively stop the task

• .log(fromid=None) method to retrieve the task logs (all or from a given log id)

Command line

Operations

If you read the previous chapter, you already know the init-db and monitor commands.

The rework command, if typed without subcommand, shows its usage:

$ rework
Usage: rework [OPTIONS] COMMAND [ARGS]...

Options:
  --help  Show this message and exit.

Commands:
  abort-task            immediately abort the given task This will be done...
  export-scheduled
  import-scheduled
  init-db               initialize the database schema for rework in its
                        own...
  kill-worker           ask to preemptively kill a given worker to its...
  list-monitors
  list-operations
  list-scheduled
  list-tasks
  list-workers
  log-task
  monitor               start a monitor controlling min/max workers
  new-worker            spawn a new worker -- this is a purely *internal*...
  register-operations   register operations from a python module containing...
  shutdown-worker       ask a worker to shut down as soon as it becomes
                        idle...
  unregister-operation  unregister an operation (or several) using its name...
  vacuum                delete non-runing workers or finished tasks

Of those commands, new-worker is for purely internal purposes, and unless you know what you're doing, you should never use it.

One can list the tasks:

rework list-tasks postgres://babar:password@localhost:5432/jobstore
1 my_first_task done [2017-09-13 17:08:48.306970+02]
2 my_first_task done [2017-09-13 17:08:48.416770+02]

It is possible to monitor the output of a given task:

$ rework log-task postgres://babar:password@localhost:5432/jobstore 1
stdout:INFO: 2017-09-13 17:08:49: I am running
stdout:INFO: 2017-09-13 17:08:49: I am done

The last argument 1 is the task identifier as was shown by the list-tasks command.

Notice how we capture the standard output (print calls) using the task.capturelogs context manager. This is completely optional of course but quite handy. The line shown above actually capture standard output, standard error and all logs. It accepts a level parameter, like e.g. capturelogs(level=logging.INFO).

Lastly, list-workers will show the currently running workers:

$ rework list-workers postgres://babar:password@localhost:5432/jobstore
1 4889896@192.168.1.2 30 Mb [running]
2 4889748@192.168.1.2 30 Mb [running]

Extensions

It is possible to augment the rework command with new subcommands (or augment, modify existing commands).

Any program doing so must define a new command and declare a setup tools entry point named rework:subcommand as in e.g.:

    entry_points={'rework.subcommands': [
        'view=rework_ui.cli:view'
    ]}

For instance, the rework_ui python package provides such a view subcommand to launch a monitoring webapp for a given rework job store.