# Recipes¶

This is a collection of recipes on how to solve typical problems using signac.

## How to migrate (change) the data space schema.¶

Oftentimes, one discovers at a later stage that important keys are missing from the metadata schema. For example, in the tutorial we are modeling a gas using the ideal gas law, but we might discover later that important effects are not captured using this overly simplistic model and decide to replace it with the van der Waals equation:

$\left(p + \frac{N^2 a}{V^2}\right) \left(V - Nb \right) = N k_B T$

Since the ideal gas law can be considered a special case of the equation above with $$a=b=0$$, we could migrate all jobs with:

>>> for job in project:
...     job.sp.setdefault('a', 0)
...     job.sp.setdefault('b', 0)
...


The setdefault() function sets the value for $$a$$ and $$b$$ to 0 in case that they are not already present.

• To delete a key use del job.sp['key_to_be_removed'].
• To rename a key, use job.sp.new_name = job.sp.pop('old_name').

Note

The job.sp and job.doc attributes provide all basic functions of a regular Python dict.

### Apply document-wide changes¶

The safest approach to apply multiple document-wide changes is to replace the document in one operation. Here is an example on how we could recursively replace all dot (.)-characters with the underscore-character in all keys [1]:

import signac
from collections.abc import Mapping

def migrate(doc):
if isinstance(doc, Mapping):
return {k.replace('.', '_'): migrate(v) for k, v in doc.items()}
else:
return doc

for job in signac.get_project():
job.sp = migrate(job.sp)
job.doc = migrate(job.doc)


This approach makes it also easy to compare the pre- and post-migration states before actually applying them.

 [1] The use of dots in keys is deprecated. Dots will be exclusively used to denote nested keywords in the future.

## How to define parameter-dependent operations¶

Operations defined as a function as part of a signac-flow workflow can only have one required argument: the job. That is to ensure reproduciblity of these operations. An operation should be a true function of the job’s data without any hidden parameters.

Here we show how to define operations that are a function of one or more additional parameters without violating the above mentioned principle. Assuming that we have an operation called foo, which depends on parameter bar, here is how we could implement multiple operations that depend on that additional parameter without code duplication:

 class Project(FlowProject):
pass

job.doc.setdefault('foo', dict())

def foo_ran(job):
return bar in job.doc.foo

# Make sure to make the operation-name a function of the parameter(s)!
@Project.operation('foo-{})'.format(bar))
@Project.post(foo_ran)
def foo(job):
job.doc.foo[bar] = 'hello world!'

for bar in (4, 8, 15, 16, 23, 42):


Important

The operation and condition functions must be defined in a way such that they are truly independent from eachother! For instance, the example above would fail if all foo-operations had the same name or if they were all writing output to the same location.

## How to integrate signac-flow with MATLAB or other software without Python interface¶

The easiest way to integrate software that has no native Python interface is to implement signac-flow operations in combination with the flow.cmd decorator. Assuming that we have a MATLAB script called prog.m within the project root directory:

% prog.m
function []=prog(arg1, arg2)

display(arg1);
display(arg2);

exitcode = 0;


Then, we could implement a simple operation that passes it some metadata parameters like this:

@FlowProject.operation
@flow.cmd
def compute_volume(job):
return "matlab -r 'prog {job.sp.foo} {job.sp.bar}' > {job.ws}/output.txt"


Executing this operation will store the output of the matlab script within the job’s workspace within a file called output.txt.

## How to implement MPI-parallelized operations¶

There are basically two strategies to implement FlowProject operations that are MPI-parallelized, one for external programs and one for Python scripts.

Tip

Fully functional scripts can be found in the signac-docs repository under examples/MPI.

### MPI-operations with mpi4py or similar¶

Assuming that your operation is using mpi4py or similar, you do not have to change your code:

@FlowProject.operation
def hello_mpi(job):
from mpi4py import MPI
print("Hello from rank", MPI.COMM_WORLD.Get_rank())


You could run this operation directly with: mpiexec -n 2 python project.py run -o hello_mpi.

Note

This strategy might fail in cases where you cannot ensure that the MPI communicator is initialized within the operation function.

Danger

Read and write operations to the job-/ and project-document are not protected against race-conditions and should only be executed on one rank at a time. This can be ensured for example like this:

from mpi4py import MPI
comm = MPI.COMM_WORLD

if comm.Get_rank() == 0:
job.doc.foo = 'abc'
comm.barrier()


### MPI-operations with flow.cmd¶

Alternatively, you can implement an MPI-parallelized operation with the flow.cmd decorator, optionally in combination with the flow.directives decorator. This strategy lets you define the number of ranks directly within the code and is also the only possible strategy when integrating external programs without a Python interface.

Assuming that we have an MPI-parallelized program named my_program, which expects an input file as its first argument and which we want to run on two ranks, we could implement the operation like this:

@FlowProject.operation
@flow.cmd
@flow.directives(np=2)
def hello_mpi(job):
return "mpiexec -n 2 mpi_program {job.ws}/input_file.txt"


The flow.cmd decorator instructs signac-flow to interpret the operation as a command rather than a Python function. The flow.directives decorator provides additional instructions on how to execute this operation and is not strictly necessary for the example above to work. However, some script templates, including those designed for HPC cluster submissions, will use the value provided by the np key to compute the required compute ranks for a specific submission.

Tip

You do not have to hard-code the number of ranks, it may be a function of the job, e.g.: flow.directives(np=lambda job: job.sp.system_size // 1000).

### MPI-operations with custom script templates¶

Finally, instead of modifying the operation implementation, you could use a custom script template, such as this one:

{% extends base_script %}
{% block body %}
{% for operation in operations %}
mpiexec -n {{ operation.directives.np }} operation.cmd
{% endfor %}
{% endblock %}


Storing the above template in a file called templates/script.sh within your project root directory will prepend every operation command with mpiexec and so on.

## How to run in containerized environments¶

Using signac-flow in combination with container systems such as docker or singularity is easily achieved by modifying the executable directive. For example, assuming that we wanted to use a singularity container named software.simg, which is placed within the project root directory, we use the following directive to specify that a given operation is to be executed within then container:

@Project.operation
@flow.directives(executable='singularity exec software.simg python')
def containerized_operation(job):
pass


If you are using the run command for execution, simply execute the whole script in the container:

\$ singularity exec software.simg python project.py run


Attention

Many cluster environments will not allow you to submit jobs to the scheduler using the container image. This means that the actual submission, (e.g. python project.py submit or similar) will need to be executed with a local Python executable.

To avoid issues with dependencies that are only available in the container image, move imports into the operation function. Condition functions will be executed during the submission process to determine what to submit, so depedencies for those must be installed into the local environment as well.

Tip

You can define a decorator that can be reused like this:

def on_container(func):
return flow.directives(executable='singularity exec software.simg python')(func)

@on_container
@Project.operation
def containerized_operation(job):
pass