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.. _parallel_task:
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|
==========================
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|
The IPython task interface
|
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|
==========================
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|
The task interface to the cluster presents the engines as a fault tolerant,
|
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|
dynamic load-balanced system of workers. Unlike the multiengine interface, in
|
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|
the task interface the user have no direct access to individual engines. By
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|
allowing the IPython scheduler to assign work, this interface is simultaneously
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|
simpler and more powerful.
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Best of all, the user can use both of these interfaces running at the same time
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|
to take advantage of their respective strengths. When the user can break up
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the user's work into segments that do not depend on previous execution, the
|
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task interface is ideal. But it also has more power and flexibility, allowing
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|
the user to guide the distribution of jobs, without having to assign tasks to
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|
engines explicitly.
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Starting the IPython controller and engines
|
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|
===========================================
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|
To follow along with this tutorial, you will need to start the IPython
|
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|
controller and four IPython engines. The simplest way of doing this is to use
|
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|
the :command:`ipcluster` command::
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|
$ ipcluster start -n 4
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For more detailed information about starting the controller and engines, see
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|
our :ref:`introduction <ip1par>` to using IPython for parallel computing.
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|
Creating a ``Client`` instance
|
|
|
==============================
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|
The first step is to import the IPython :mod:`IPython.parallel`
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module and then create a :class:`.Client` instance, and we will also be using
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a :class:`LoadBalancedView`, here called `lview`:
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|
.. sourcecode:: ipython
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In [1]: from IPython.parallel import Client
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In [2]: rc = Client()
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This form assumes that the controller was started on localhost with default
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|
configuration. If not, the location of the controller must be given as an
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|
argument to the constructor:
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|
.. sourcecode:: ipython
|
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|
# for a visible LAN controller listening on an external port:
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|
In [2]: rc = Client('tcp://192.168.1.16:10101')
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# or to connect with a specific profile you have set up:
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|
In [3]: rc = Client(profile='mpi')
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For load-balanced execution, we will make use of a :class:`LoadBalancedView` object, which can
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|
be constructed via the client's :meth:`load_balanced_view` method:
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|
.. sourcecode:: ipython
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|
In [4]: lview = rc.load_balanced_view() # default load-balanced view
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|
.. seealso::
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|
For more information, see the in-depth explanation of :ref:`Views <parallel_details>`.
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|
Quick and easy parallelism
|
|
|
==========================
|
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|
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|
In many cases, you simply want to apply a Python function to a sequence of
|
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|
objects, but *in parallel*. Like the multiengine interface, these can be
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|
implemented via the task interface. The exact same tools can perform these
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|
actions in load-balanced ways as well as multiplexed ways: a parallel version
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|
of :func:`map` and :func:`@parallel` function decorator. If one specifies the
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|
argument `balanced=True`, then they are dynamically load balanced. Thus, if the
|
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|
execution time per item varies significantly, you should use the versions in
|
|
|
the task interface.
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|
Parallel map
|
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|
------------
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|
To load-balance :meth:`map`,simply use a LoadBalancedView:
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|
.. sourcecode:: ipython
|
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|
In [62]: lview.block = True
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|
In [63]: serial_result = map(lambda x:x**10, range(32))
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|
In [64]: parallel_result = lview.map(lambda x:x**10, range(32))
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|
In [65]: serial_result==parallel_result
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|
Out[65]: True
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|
|
Parallel function decorator
|
|
|
---------------------------
|
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|
Parallel functions are just like normal function, but they can be called on
|
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|
sequences and *in parallel*. The multiengine interface provides a decorator
|
|
|
that turns any Python function into a parallel function:
|
|
|
|
|
|
.. sourcecode:: ipython
|
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|
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|
In [10]: @lview.parallel()
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|
....: def f(x):
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|
....: return 10.0*x**4
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|
....:
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|
In [11]: f.map(range(32)) # this is done in parallel
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|
Out[11]: [0.0,10.0,160.0,...]
|
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|
.. _parallel_dependencies:
|
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|
|
Dependencies
|
|
|
============
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|
Often, pure atomic load-balancing is too primitive for your work. In these cases, you
|
|
|
may want to associate some kind of `Dependency` that describes when, where, or whether
|
|
|
a task can be run. In IPython, we provide two types of dependencies:
|
|
|
`Functional Dependencies`_ and `Graph Dependencies`_
|
|
|
|
|
|
.. note::
|
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|
|
|
It is important to note that the pure ZeroMQ scheduler does not support dependencies,
|
|
|
and you will see errors or warnings if you try to use dependencies with the pure
|
|
|
scheduler.
|
|
|
|
|
|
Functional Dependencies
|
|
|
-----------------------
|
|
|
|
|
|
Functional dependencies are used to determine whether a given engine is capable of running
|
|
|
a particular task. This is implemented via a special :class:`Exception` class,
|
|
|
:class:`UnmetDependency`, found in `IPython.parallel.error`. Its use is very simple:
|
|
|
if a task fails with an UnmetDependency exception, then the scheduler, instead of relaying
|
|
|
the error up to the client like any other error, catches the error, and submits the task
|
|
|
to a different engine. This will repeat indefinitely, and a task will never be submitted
|
|
|
to a given engine a second time.
|
|
|
|
|
|
You can manually raise the :class:`UnmetDependency` yourself, but IPython has provided
|
|
|
some decorators for facilitating this behavior.
|
|
|
|
|
|
There are two decorators and a class used for functional dependencies:
|
|
|
|
|
|
.. sourcecode:: ipython
|
|
|
|
|
|
In [9]: from IPython.parallel import depend, require, dependent
|
|
|
|
|
|
@require
|
|
|
********
|
|
|
|
|
|
The simplest sort of dependency is requiring that a Python module is available. The
|
|
|
``@require`` decorator lets you define a function that will only run on engines where names
|
|
|
you specify are importable:
|
|
|
|
|
|
.. sourcecode:: ipython
|
|
|
|
|
|
In [10]: @require('numpy', 'zmq')
|
|
|
...: def myfunc():
|
|
|
...: return dostuff()
|
|
|
|
|
|
Now, any time you apply :func:`myfunc`, the task will only run on a machine that has
|
|
|
numpy and pyzmq available, and when :func:`myfunc` is called, numpy and zmq will be imported.
|
|
|
|
|
|
@depend
|
|
|
*******
|
|
|
|
|
|
The ``@depend`` decorator lets you decorate any function with any *other* function to
|
|
|
evaluate the dependency. The dependency function will be called at the start of the task,
|
|
|
and if it returns ``False``, then the dependency will be considered unmet, and the task
|
|
|
will be assigned to another engine. If the dependency returns *anything other than
|
|
|
``False``*, the rest of the task will continue.
|
|
|
|
|
|
.. sourcecode:: ipython
|
|
|
|
|
|
In [10]: def platform_specific(plat):
|
|
|
...: import sys
|
|
|
...: return sys.platform == plat
|
|
|
|
|
|
In [11]: @depend(platform_specific, 'darwin')
|
|
|
...: def mactask():
|
|
|
...: do_mac_stuff()
|
|
|
|
|
|
In [12]: @depend(platform_specific, 'nt')
|
|
|
...: def wintask():
|
|
|
...: do_windows_stuff()
|
|
|
|
|
|
In this case, any time you apply ``mytask``, it will only run on an OSX machine.
|
|
|
``@depend`` is just like ``apply``, in that it has a ``@depend(f,*args,**kwargs)``
|
|
|
signature.
|
|
|
|
|
|
dependents
|
|
|
**********
|
|
|
|
|
|
You don't have to use the decorators on your tasks, if for instance you may want
|
|
|
to run tasks with a single function but varying dependencies, you can directly construct
|
|
|
the :class:`dependent` object that the decorators use:
|
|
|
|
|
|
.. sourcecode::ipython
|
|
|
|
|
|
In [13]: def mytask(*args):
|
|
|
...: dostuff()
|
|
|
|
|
|
In [14]: mactask = dependent(mytask, platform_specific, 'darwin')
|
|
|
# this is the same as decorating the declaration of mytask with @depend
|
|
|
# but you can do it again:
|
|
|
|
|
|
In [15]: wintask = dependent(mytask, platform_specific, 'nt')
|
|
|
|
|
|
# in general:
|
|
|
In [16]: t = dependent(f, g, *dargs, **dkwargs)
|
|
|
|
|
|
# is equivalent to:
|
|
|
In [17]: @depend(g, *dargs, **dkwargs)
|
|
|
...: def t(a,b,c):
|
|
|
...: # contents of f
|
|
|
|
|
|
Graph Dependencies
|
|
|
------------------
|
|
|
|
|
|
Sometimes you want to restrict the time and/or location to run a given task as a function
|
|
|
of the time and/or location of other tasks. This is implemented via a subclass of
|
|
|
:class:`set`, called a :class:`Dependency`. A Dependency is just a set of `msg_ids`
|
|
|
corresponding to tasks, and a few attributes to guide how to decide when the Dependency
|
|
|
has been met.
|
|
|
|
|
|
The switches we provide for interpreting whether a given dependency set has been met:
|
|
|
|
|
|
any|all
|
|
|
Whether the dependency is considered met if *any* of the dependencies are done, or
|
|
|
only after *all* of them have finished. This is set by a Dependency's :attr:`all`
|
|
|
boolean attribute, which defaults to ``True``.
|
|
|
|
|
|
success [default: True]
|
|
|
Whether to consider tasks that succeeded as fulfilling dependencies.
|
|
|
|
|
|
failure [default : False]
|
|
|
Whether to consider tasks that failed as fulfilling dependencies.
|
|
|
using `failure=True,success=False` is useful for setting up cleanup tasks, to be run
|
|
|
only when tasks have failed.
|
|
|
|
|
|
Sometimes you want to run a task after another, but only if that task succeeded. In this case,
|
|
|
``success`` should be ``True`` and ``failure`` should be ``False``. However sometimes you may
|
|
|
not care whether the task succeeds, and always want the second task to run, in which case you
|
|
|
should use `success=failure=True`. The default behavior is to only use successes.
|
|
|
|
|
|
There are other switches for interpretation that are made at the *task* level. These are
|
|
|
specified via keyword arguments to the client's :meth:`apply` method.
|
|
|
|
|
|
after,follow
|
|
|
You may want to run a task *after* a given set of dependencies have been run and/or
|
|
|
run it *where* another set of dependencies are met. To support this, every task has an
|
|
|
`after` dependency to restrict time, and a `follow` dependency to restrict
|
|
|
destination.
|
|
|
|
|
|
timeout
|
|
|
You may also want to set a time-limit for how long the scheduler should wait before a
|
|
|
task's dependencies are met. This is done via a `timeout`, which defaults to 0, which
|
|
|
indicates that the task should never timeout. If the timeout is reached, and the
|
|
|
scheduler still hasn't been able to assign the task to an engine, the task will fail
|
|
|
with a :class:`DependencyTimeout`.
|
|
|
|
|
|
.. note::
|
|
|
|
|
|
Dependencies only work within the task scheduler. You cannot instruct a load-balanced
|
|
|
task to run after a job submitted via the MUX interface.
|
|
|
|
|
|
The simplest form of Dependencies is with `all=True,success=True,failure=False`. In these cases,
|
|
|
you can skip using Dependency objects, and just pass msg_ids or AsyncResult objects as the
|
|
|
`follow` and `after` keywords to :meth:`client.apply`:
|
|
|
|
|
|
.. sourcecode:: ipython
|
|
|
|
|
|
In [14]: client.block=False
|
|
|
|
|
|
In [15]: ar = lview.apply(f, args, kwargs)
|
|
|
|
|
|
In [16]: ar2 = lview.apply(f2)
|
|
|
|
|
|
In [17]: ar3 = lview.apply_with_flags(f3, after=[ar,ar2])
|
|
|
|
|
|
In [17]: ar4 = lview.apply_with_flags(f3, follow=[ar], timeout=2.5)
|
|
|
|
|
|
|
|
|
.. seealso::
|
|
|
|
|
|
Some parallel workloads can be described as a `Directed Acyclic Graph
|
|
|
<http://en.wikipedia.org/wiki/Directed_acyclic_graph>`_, or DAG. See :ref:`DAG
|
|
|
Dependencies <dag_dependencies>` for an example demonstrating how to use map a NetworkX DAG
|
|
|
onto task dependencies.
|
|
|
|
|
|
|
|
|
|
|
|
Impossible Dependencies
|
|
|
***********************
|
|
|
|
|
|
The schedulers do perform some analysis on graph dependencies to determine whether they
|
|
|
are not possible to be met. If the scheduler does discover that a dependency cannot be
|
|
|
met, then the task will fail with an :class:`ImpossibleDependency` error. This way, if the
|
|
|
scheduler realized that a task can never be run, it won't sit indefinitely in the
|
|
|
scheduler clogging the pipeline.
|
|
|
|
|
|
The basic cases that are checked:
|
|
|
|
|
|
* depending on nonexistent messages
|
|
|
* `follow` dependencies were run on more than one machine and `all=True`
|
|
|
* any dependencies failed and `all=True,success=True,failures=False`
|
|
|
* all dependencies failed and `all=False,success=True,failure=False`
|
|
|
|
|
|
.. warning::
|
|
|
|
|
|
This analysis has not been proven to be rigorous, so it is likely possible for tasks
|
|
|
to become impossible to run in obscure situations, so a timeout may be a good choice.
|
|
|
|
|
|
.. _parallel_schedulers:
|
|
|
|
|
|
Schedulers
|
|
|
==========
|
|
|
|
|
|
There are a variety of valid ways to determine where jobs should be assigned in a
|
|
|
load-balancing situation. In IPython, we support several standard schemes, and
|
|
|
even make it easy to define your own. The scheme can be selected via the ``--scheme``
|
|
|
argument to :command:`ipcontroller`, or in the :attr:`HubFactory.scheme` attribute
|
|
|
of a controller config object.
|
|
|
|
|
|
The built-in routing schemes:
|
|
|
|
|
|
To select one of these schemes, simply do::
|
|
|
|
|
|
$ ipcontroller --scheme <schemename>
|
|
|
for instance:
|
|
|
$ ipcontroller --scheme lru
|
|
|
|
|
|
lru: Least Recently Used
|
|
|
|
|
|
Always assign work to the least-recently-used engine. A close relative of
|
|
|
round-robin, it will be fair with respect to the number of tasks, agnostic
|
|
|
with respect to runtime of each task.
|
|
|
|
|
|
plainrandom: Plain Random
|
|
|
|
|
|
Randomly picks an engine on which to run.
|
|
|
|
|
|
twobin: Two-Bin Random
|
|
|
|
|
|
**Requires numpy**
|
|
|
|
|
|
Pick two engines at random, and use the LRU of the two. This is known to be better
|
|
|
than plain random in many cases, but requires a small amount of computation.
|
|
|
|
|
|
leastload: Least Load
|
|
|
|
|
|
**This is the default scheme**
|
|
|
|
|
|
Always assign tasks to the engine with the fewest outstanding tasks (LRU breaks tie).
|
|
|
|
|
|
weighted: Weighted Two-Bin Random
|
|
|
|
|
|
**Requires numpy**
|
|
|
|
|
|
Pick two engines at random using the number of outstanding tasks as inverse weights,
|
|
|
and use the one with the lower load.
|
|
|
|
|
|
|
|
|
Pure ZMQ Scheduler
|
|
|
------------------
|
|
|
|
|
|
For maximum throughput, the 'pure' scheme is not Python at all, but a C-level
|
|
|
:class:`MonitoredQueue` from PyZMQ, which uses a ZeroMQ ``XREQ`` socket to perform all
|
|
|
load-balancing. This scheduler does not support any of the advanced features of the Python
|
|
|
:class:`.Scheduler`.
|
|
|
|
|
|
Disabled features when using the ZMQ Scheduler:
|
|
|
|
|
|
* Engine unregistration
|
|
|
Task farming will be disabled if an engine unregisters.
|
|
|
Further, if an engine is unregistered during computation, the scheduler may not recover.
|
|
|
* Dependencies
|
|
|
Since there is no Python logic inside the Scheduler, routing decisions cannot be made
|
|
|
based on message content.
|
|
|
* Early destination notification
|
|
|
The Python schedulers know which engine gets which task, and notify the Hub. This
|
|
|
allows graceful handling of Engines coming and going. There is no way to know
|
|
|
where ZeroMQ messages have gone, so there is no way to know what tasks are on which
|
|
|
engine until they *finish*. This makes recovery from engine shutdown very difficult.
|
|
|
|
|
|
|
|
|
.. note::
|
|
|
|
|
|
TODO: performance comparisons
|
|
|
|
|
|
|
|
|
More details
|
|
|
============
|
|
|
|
|
|
The :class:`LoadBalancedView` has many more powerful features that allow quite a bit
|
|
|
of flexibility in how tasks are defined and run. The next places to look are
|
|
|
in the following classes:
|
|
|
|
|
|
* :class:`~IPython.parallel.client.view.LoadBalancedView`
|
|
|
* :class:`~IPython.parallel.client.asyncresult.AsyncResult`
|
|
|
* :meth:`~IPython.parallel.client.view.LoadBalancedView.apply`
|
|
|
* :mod:`~IPython.parallel.controller.dependency`
|
|
|
|
|
|
The following is an overview of how to use these classes together:
|
|
|
|
|
|
1. Create a :class:`Client` and :class:`LoadBalancedView`
|
|
|
2. Define some functions to be run as tasks
|
|
|
3. Submit your tasks to using the :meth:`apply` method of your
|
|
|
:class:`LoadBalancedView` instance.
|
|
|
4. Use :meth:`Client.get_result` to get the results of the
|
|
|
tasks, or use the :meth:`AsyncResult.get` method of the results to wait
|
|
|
for and then receive the results.
|
|
|
|
|
|
.. seealso::
|
|
|
|
|
|
A demo of :ref:`DAG Dependencies <dag_dependencies>` with NetworkX and IPython.
|
|
|
|
