# How to run multi-step workflows¶

## Writing workflows¶

A workflow in AiiDA is a process that calls other workflows and calculations and optionally returns data and as such can encode the logic of a typical scientific workflow. Currently, there are two ways of implementing a workflow process:

Here we present a brief introduction on how to write both workflow types.

Note

For more details on the concept of a workflow, and the difference between a work function and a work chain, please see the corresponding topics section.

Note

Developing workflows may involve running several lengthy calculations. Consider enabling caching to help avoid repeating long workflow steps.

### Work function¶

A work function is a process function that calls one or more calculation functions and returns data that has been created by the calculation functions it has called. Moreover, work functions can also call other work functions, allowing you to write nested workflows. Writing a work function, whose provenance is automatically stored, is as simple as writing a Python function and decorating it with the workfunction decorator:

"""Basic calcfunction-based workflows for demonstration purposes."""
from aiida.engine import calcfunction, workfunction

@calcfunction
return x + y

@calcfunction
def multiply(x, y):
return x * y

@workfunction
"""Add two numbers and multiply it with a third."""
return product


It is important to reiterate here that the workfunction-decorated add_multiply() function does not create any new data nodes. The add() and multiply() calculation functions create the Int data nodes, all the work function does is return the results of the multiply() calculation function. Moreover, both calculation and workflow functions can only accept and return data nodes, i.e. instances of classes that subclass the Data class.

### Work chain¶

When the workflow you want to run is more complex and takes longer to finish, it is better to write a work chain. Writing a work chain in AiiDA requires creating a class that inherits from the WorkChain class. Below is an example of a work chain that takes three integers as inputs, multiplies the first two and then adds the third to obtain the final result:

"""Implementation of the MultiplyAddWorkChain for testing and demonstration purposes."""
from aiida.orm import Code, Int
from aiida.engine import calcfunction, WorkChain, ToContext
from aiida.plugins.factories import CalculationFactory

@calcfunction
def multiply(x, y):
return x * y

"""WorkChain to multiply two numbers and add a third, for testing and demonstration purposes."""

@classmethod
def define(cls, spec):
"""Specify inputs and outputs."""
super().define(spec)
spec.input('x', valid_type=Int)
spec.input('y', valid_type=Int)
spec.input('z', valid_type=Int)
spec.input('code', valid_type=Code)
spec.outline(
cls.multiply,
cls.validate_result,
cls.result,
)
spec.output('result', valid_type=Int)
spec.exit_code(400, 'ERROR_NEGATIVE_NUMBER', message='The result is a negative number.')

def multiply(self):
"""Multiply two integers."""
self.ctx.product = multiply(self.inputs.x, self.inputs.y)

"""Add two numbers using the ArithmeticAddCalculation calculation job plugin."""
inputs = {'x': self.ctx.product, 'y': self.inputs.z, 'code': self.inputs.code}

def validate_result(self):
"""Make sure the result is not negative."""

if result.value < 0:
return self.exit_codes.ERROR_NEGATIVE_NUMBER

def result(self):
"""Add the result to the outputs."""


You can give the work chain any valid Python class name, but the convention is to have it end in WorkChain so that it is always immediately clear what it references. Let’s go over the methods of the MultiplyAddWorkChain one by one:

@classmethod
def define(cls, spec):
"""Specify inputs and outputs."""
super().define(spec)
spec.input('x', valid_type=Int)
spec.input('y', valid_type=Int)
spec.input('z', valid_type=Int)
spec.input('code', valid_type=Code)
spec.outline(
cls.multiply,
cls.validate_result,
cls.result,
)
spec.output('result', valid_type=Int)
spec.exit_code(400, 'ERROR_NEGATIVE_NUMBER', message='The result is a negative number.')


The most important method to implement for every work chain is the define() method. This class method must always start by calling the define() method of its parent class. Next, the define() method should be used to define the specifications of the work chain, which are contained in the work chain spec:

• the inputs, specified using the spec.input() method. The first argument of the input() method is a string that specifies the label of the input, e.g. 'x'. The valid_type keyword argument allows you to specify the required node type of the input. Other keyword arguments allow the developer to set a default for the input, or indicate that an input should not be stored in the database, see the process topics section for more details.

• the outline or logic of the workflow, specified using the spec.outline() method. The outline of the workflow is constructed from the methods of the WorkChain class. For the MultiplyAddWorkChain, the outline is a simple linear sequence of steps, but it’s possible to include actual logic, directly in the outline, in order to define more complex workflows as well. See the work chain outline section for more details.

• the outputs, specified using the spec.output() method. This method is very similar in its usage to the input() method.

• the exit codes of the work chain, specified using the spec.exit_code() method. Exit codes are used to clearly communicate known failure modes of the work chain to the user. The first and second arguments define the exit_status of the work chain in case of failure (400) and the string that the developer can use to reference the exit code (ERROR_NEGATIVE_NUMBER). A descriptive exit message can be provided using the message keyword argument. For the MultiplyAddWorkChain, we demand that the final result is not a negative number, which is checked in the validate_result step of the outline.

Note

For more information on the define() method and the process spec, see the corresponding section in the topics.

The multiply method is the first step in the outline of the MultiplyAddWorkChain work chain.

def multiply(self):
"""Multiply two integers."""
self.ctx.product = multiply(self.inputs.x, self.inputs.y)


This step simply involves running the calculation function multiply(), on the x and y inputs of the work chain. To store the result of this function and use it in the next step of the outline, it is added to the context of the work chain using self.ctx.

def add(self):
"""Add two numbers using the ArithmeticAddCalculation calculation job plugin."""
inputs = {'x': self.ctx.product, 'y': self.inputs.z, 'code': self.inputs.code}



The add() method is the second step in the outline of the work chain. As this step uses the ArithmeticAddCalculation calculation job, we start by setting up the inputs for this CalcJob in a dictionary. Next, when submitting this calculation job to the daemon, it is important to use the submit method from the work chain instance via self.submit(). Since the result of the addition is only available once the calculation job is finished, the submit() method returns the CalcJobNode of the future ArithmeticAddCalculation process. To tell the work chain to wait for this process to finish before continuing the workflow, we return the ToContext class, where we have passed a dictionary to specify that the future calculation job node should be assigned to the 'addition' context key.

Warning

Never use the global submit() function to submit calculations to the daemon within a WorkChain. Doing so will raise an exception during runtime. See the topics section on work chains for more details.

Note

Instead of passing a dictionary, you can also initialize a ToContext instance by passing the future process as a keyword argument, e.g. ToContext(addition=calcjob_node). More information on the ToContext class can be found in the topics section on submitting sub processes.

def validate_result(self):
"""Make sure the result is not negative."""

if result.value < 0:
return self.exit_codes.ERROR_NEGATIVE_NUMBER


Once the ArithmeticAddCalculation calculation job is finished, the next step in the work chain is to validate the result, i.e. verify that the result is not a negative number. After the addition node has been extracted from the context, we take the sum node from the ArithmeticAddCalculation outputs and store it in the result variable. In case the value of this Int node is negative, the ERROR_NEGATIVE_NUMBER exit code - defined in the define() method - is returned. Note that once an exit code is returned during any step in the outline, the work chain will be terminated and no further steps will be executed.

def result(self):
"""Add the result to the outputs."""


The final step in the outline is to pass the result to the outputs of the work chain using the self.out() method. The first argument ('result') specifies the label of the output, which corresponds to the label provided to the spec in the define() method. The second argument is the result of the work chain, extracted from the Int node stored in the context under the 'addition' key.

For a more complete discussion on workflows and their usage, please read the corresponding topics section.

## Launching a predefined workflow¶

The first step to launching a predefined workflow is loading the work function or work chain class that defines the workflow you want to run. The recommended method for loading a workflow is using the WorkflowFactory, for example:

from aiida.plugins import WorkflowFactory


This is essentially the same as importing the workflow from its respective module, but using the WorkflowFactory has the advantage that the so called entry point (e.g. 'arithmetic.multiply_add') will not change when the packages or plugins are reorganised. This means your code is less likely to break when updating AiiDA or the plugin that supplies the workflow.

The list of installed plugins can be easily accessed via the verdi CLI:

$verdi plugin list  To see the list of workflow entry points, simply use: $ verdi plugin list aiida.workflows


By further specifying the entry point of the workflow, you can see its description, inputs, outputs and exit codes:

\$ verdi plugin list aiida.workflows arithmetic.multiply_add


### Work functions¶

Running a work function is as simple as calling a typical Python function: simply call it with the required input arguments:

from aiida.plugins import WorkflowFactory, DataFactory
Int = DataFactory('int')



Here, the add_and_multiply work function returns the output Int node and we assign it to the variable result. Note that the input arguments of a work function must be an instance of Data node, or any of its subclasses. Just calling the add_and_multiply function with regular integers will result in a ValueError, as these cannot be stored in the provenance graph.

Note

Although the example above shows the most straightforward way to run the add_and_multiply work function, there are several other ways of running processes that can return more than just the result. For example, the run_get_node function from the AiiDA engine returns both the result of the workflow and the work function node. See the corresponding topics section for more details.

### Work chains¶

To launch a work chain, you can either use the run or submit functions. For either function, you need to provide the class of the work chain as the first argument, followed by the inputs as keyword arguments. Using the run function, or “running”, a work chain means it is executed in the same system process as the interpreter in which it is launched:

from aiida.engine import run
from aiida.plugins import WorkflowFactory, DataFactory
Int = DataFactory('int')



Alternatively, you can first construct a dictionary of the inputs, and pass it to the run function by taking advantage of Python’s automatic keyword expansion:

inputs = {'x': Int(1), 'y': Int(2), 'z': Int(3), 'code': add_code}


This is particularly useful in case you have a workflow with a lot of inputs. In both cases, running the MultiplyAddWorkChain workflow returns the results of the workflow, i.e. a dictionary of the nodes that are produced as outputs, where the keys of the dictionary correspond to the labels of each respective output.

Note

Similar to other processes, there are multiple functions for launching a work chain. See the section on launching processes for more details.

Since running a workflow will block the interpreter, you will have to wait until the workflow is finished before you get back control. Moreover, you won’t be able to turn your computer or even your terminal off until the workflow has fully terminated, and it is difficult to run multiple workflows in parallel. So, it is advisable to submit more complex or longer work chains to the daemon:

from aiida.engine import submit
from aiida.plugins import WorkflowFactory, DataFactory
Int = DataFactory('int')

inputs = {'x': Int(1), 'y': Int(2), 'z': Int(3), 'code': add_code}



Note that when using submit the work chain is not run in the local interpreter but is sent off to the daemon, and you get back control instantly. This allows you to submit multiple work chains at the same time and the daemon will start working on them in parallel. Once the submit call returns, you will not get the result as with run, but you will get the node representing the work chain. Submitting a work chain instead of directly running it not only makes it easier to execute multiple work chains in parallel, but also ensures that the progress of a workchain is not lost when you restart your computer.

Important

In contrast to work chains, work functions cannot be submitted to the daemon, and hence can only be run.

If you are unfamiliar with the inputs of a particular WorkChain, a convenient tool for setting up the work chain is the process builder. This can be obtained by using the get_builder() method, which is implemented for every CalcJob and WorkChain:

In [1]: from aiida.plugins import WorkflowFactory, DataFactory
...: Int = DataFactory('int')


To explore the inputs of the work chain, you can use tab autocompletion by typing builder. and then hitting TAB. If you want to get more details on a specific input, you can simply add a ? and press enter:

In [2]: builder.x?
Type:        property
String form: <property object at 0x119ad2dd0>
Docstring:   {"name": "x", "required": "True", "valid_type": "<class 'aiida.orm.nodes.data.int.Int'>", "non_db": "False"}


Here you can see that the x input is required, needs to be of the Int type and is stored in the database ("non_db": "False").

Using the builder, the inputs of the WorkChain can be provided one by one:

In [3]: builder.code = load_code(label='add')
...: builder.x = Int(2)
...: builder.y = Int(3)
...: builder.z = Int(5)


Once the required inputs of the workflow have been provided to the builder, you can either run the work chain or submit it to the daemon:

In [4]: from aiida.engine import submit
...: workchain_node = submit(builder)


Note

For more detail on the process builder, see the corresponding topics section.