Produce a plan that dispatches calls based on a graph of functions, satisfying data dependencies.
About schedula
schedula is a dynamic flow-based programming environment for python, that handles automatically the control flow of the program. The control flow generally is represented by a Directed Acyclic Graph (DAG), where nodes are the operations/functions to be executed and edges are the dependencies between them.
The algorithm of schedula dates back to 2014, when a colleague
asked for a method to automatically populate the missing data of a
database. The imputation method chosen to complete the database was a
system of interdependent physical formulas - i.e., the inputs of a
formula are the outputs of other formulas. The current library has
been developed in 2015 to support the design of the CO:sub:2
MPAS
tool <https://github.com/JRCSTU/CO2MPAS-TA>
_ - a CO:sub:2
vehicle
simulator <https://jrcstu.github.io/co2mpas/model/?url=https://jrcstu.github.io/co2mpas/model/core/CO2MPAS_model/calibrate_with_wltp_h.html>
_.
During the developing phase, the physical formulas (more than 700)
were known on the contrary of the software inputs and outputs.
The design of flow-based programs begins with the definition of the control flow graph, and implicitly of its inputs and outputs. If the program accepts multiple combinations of inputs and outputs, you have to design and code all control flow graphs. With normal schedulers, it can be very demanding.
While with schedula, giving whatever set of inputs, it automatically calculates any of the desired computable outputs, choosing the most appropriate DAG from the dataflow execution model.
Note: The DAG is determined at runtime and it is extracted using the shortest path from the provided inputs. The path is calculated based on a weighted directed graph (dataflow execution model) with a modified Dijkstra algorithm.
schedula makes the code easy to debug, to optimize, and to present it to a non-IT audience through its interactive graphs and charts. It provides the option to run a model asynchronously or in parallel managing automatically the Global Interpreter Lock (GIL), and to convert a model into a web API service.
.. _start-install-core:
Installation
To install it use (with root privileges):
.. code:: console
$ pip install schedula
or download the last git version and use (with root privileges):
.. code:: console
$ python setup.py install
.. _end-install-core:
Some additional functionality is enabled installing the following extras:
io
: enables to read/write functions.
plot
: enables the plot of the Dispatcher model and workflow
(see plot()
).
web
: enables to build a dispatcher Flask app (see web()
).
sphinx
: enables the sphinx extension directives (i.e.,
autosummary and dispatcher).
parallel
: enables the parallel execution of Dispatcher model.
To install schedula and all extras, do:
.. code:: console
$ pip install 'schedula[all]'
Note: plot
extra requires Graphviz. Make sure that the directory
containing the dot
executable is on your systems’ path. If you
have not you can install it from its download page <https://www.graphviz.org/download/>
_.
Tutorial
Let’s assume that we want develop a tool to automatically manage the symmetric cryptography. The base idea is to open a file, read its content, encrypt or decrypt the data and then write them out to a new file. This tutorial shows how to:
define <#model-definition>
_ and execute <#dispatching>
_ a
dataflow execution model,
extract <#sub-model-extraction>
_ a sub-model, and
deploy <#api-server>
_ a web API service.
Note: You can find more examples, on how to use the schedula library,
into the folder examples <https://github.com/vinci1it2000/schedula/tree/master/examples>
_.
First of all we start defining an empty Dispatcher
named
symmetric_cryptography that defines the dataflow execution model:
::
import schedula as sh dsp = sh.Dispatcher(name='symmetric_cryptography')
There are two main ways to get a key, we can either generate a new one
or use one that has previously been generated. Hence, we can define
three functions to simply generate, save, and load the key. To
automatically populate the model inheriting the arguments names, we
can use the decorator add_function()
as follow:
::
import os.path as osp from cryptography.fernet import Fernet @sh.add_function(dsp, outputs=['key'], weight=2) ... def generate_key(): ... return Fernet.generate_key().decode() @sh.add_function(dsp) ... def write_key(key_fpath, key): ... with open(key_fpath, 'w') as f: ... f.write(key) @sh.add_function(dsp, outputs=['key'], input_domain=osp.isfile) ... def read_key(key_fpath): ... with open(key_fpath) as f: ... return f.read()
Note: Since Python does not come with anything that can encrypt/decrypt
files, in this tutorial, we use a third party module named
cryptography
. To install it execute pip install cryptography
.
To encrypt/decrypt a message, you will need a key as previously defined and your data encrypted or decrypted. Therefore, we can define two functions and add them, as before, to the model:
::
@sh.add_function(dsp, outputs=['encrypted']) ... def encrypt_message(key, decrypted): ... return Fernet(key.encode()).encrypt(decrypted.encode()).decode() @sh.add_function(dsp, outputs=['decrypted']) ... def decrypt_message(key, encrypted): ... return Fernet(key.encode()).decrypt(encrypted.encode()).decode()
Finally, to read and write the encrypted or decrypted message,
according to the functional programming philosophy, we can reuse the
previously defined functions read_key
and write_key
changing
the model mapping (i.e., function_id, inputs, and outputs). To
add to the model, we can simply use the add_function
method as
follow:
::
dsp.add_function( ... function_id='read_decrypted', ... function=read_key, ... inputs=['decrypted_fpath'], ... outputs=['decrypted'] ... ) 'read_decrypted' dsp.add_function( ... 'read_encrypted', read_key, ['encrypted_fpath'], ['encrypted'], ... input_domain=osp.isfile ... ) 'read_encrypted' dsp.add_function( ... 'write_decrypted', write_key, ['decrypted_fpath', 'decrypted'], ... input_domain=osp.isfile ... ) 'write_decrypted' dsp.add_function( ... 'write_encrypted', write_key, ['encrypted_fpath', 'encrypted'] ... ) 'write_encrypted'
Note: For more details on how to create a Dispatcher
see:
add_data()
, add_func()
, add_function()
,
add_dispatcher()
, SubDispatch
, MapDispatch
,
SubDispatchFunction
, SubDispatchPipe
, and DispatchPipe
.
To inspect and visualize the dataflow execution model, you can simply plot the graph as follow:
::
dsp.plot()
[graph]
Tip: You can explore the diagram by clicking on it.
To see the dataflow execution model in action and its workflow to
generate a key, to encrypt a message, and to write the encrypt data,
you can simply invoke dispatch()
or __call__()
methods of the
dsp
:
import tempfile tempdir = tempfile.mkdtemp() message = "secret message" sol = dsp(inputs=dict( ... decrypted=message, ... encrypted_fpath=osp.join(tempdir, 'data.secret'), ... key_fpath=osp.join(tempdir,'key.key') ... )) sol.plot(index=True)
[graph]
Note: As you can see from the workflow graph (orange nodes), when some function’s inputs does not respect its domain, the Dispatcher automatically finds an alternative path to estimate all computable outputs. The same logic applies when there is a function failure.
Now to decrypt the data and verify the message without saving the
decrypted message, you just need to execute again the dsp
changing
the inputs and setting the desired outputs. In this way, the
dispatcher automatically selects and executes only a sub-part of the
dataflow execution model.
dsp( ... inputs=sh.selector(('encrypted_fpath', 'key_fpath'), sol), ... outputs=['decrypted'] ... )['decrypted'] == message True
If you want to visualize the latest workflow of the dispatcher, you
can use the plot()
method with the keyword workflow=True
:
dsp.plot(workflow=True, index=True)
[graph]