Workflow Interface

Important Note

The OpenFL workflow interface is experimental, subject to change, and is currently limited to single node execution. To setup and launch a real federation, see Run the Federation

What is it?

A new OpenFL interface that gives significantly more flexility to researchers in the construction of federated learning experiments. It is heavily influenced by the interface and design of Metaflow , the popular framework for data scientists originally developed at Netflix. There are several reasons we converged on Metaflow as inspiration for our work:

  1. Clean expression of task sequence. Flows start with a start task, and end with end. The next task in the sequence is called by

  2. Easy selection of what should be sent between tasks using include or exclude

  3. Excellent tooling ecosystem: the metaflow client gives easy access to prior runs, tasks, and data artifacts generated by an experiment.

There are several modifications we make in our reimagined version of this interface that are necessary for federated learning:

  1. Placement: Metaflow’s @step decorator is replaced by placement decorators that specify where a task will run. In horizontal federated learning, there are server (or aggregator) and client (or collaborator) nodes. Tasks decorated by @aggregator will run on the aggregator node, and @collaborator will run on the collaborator node. These placement decorators are interpreted by Runtime implementations: these do the heavy lifting of figuring out how to get the state of the current task to another process or node.

  2. Runtime: Each flow has a .runtime attribute. The runtime encapsulates the details of the infrastucture where the flow will run. In this experimental release, we support only a LocalRuntime single node implementation, but as this work matures, we will extend to a FederatedRuntime that implements distributed operation across remote infrastructure.

  3. Conditional branches: Perform different tasks if a criteria is met

  4. Loops: Internal loops are within a flow; this is necessary to support rounds of training where the same sequence of tasks is performed repeatedly.

How to use it?

Let’s start with the basics. A flow is intended to define the entirety of federated learning experiment. Every flow begins with the start task and concludes with the end task. At each step in the flow, attributes can be defined, modified, or deleted. Attributes get passed forward to the next step in the flow, which is defined by the name of the task passed to the next function. In the line before each task, there is a placement decorator. The placement decorator defines where that task will be run. The OpenFL Workflow Interface adopts the conventions set by Metaflow, that every workflow begins with start and concludes with the end task. In the following example, the aggregator begins with an optionally passed in model and optimizer. The aggregator begins the flow with the start task, where the list of collaborators is extracted from the runtime (self.collaborators = self.runtime.collaborators) and is then used as the list of participants to run the task listed in, aggregated_model_validation. The model, optimizer, and anything that is not explicitly excluded from the next function will be passed from the start function on the aggregator to the aggregated_model_validation task on the collaborator. Where the tasks run is determined by the placement decorator that precedes each task definition (@aggregator or @collaborator). Once each of the collaborators (defined in the runtime) complete the aggregated_model_validation task, they pass their current state onto the train task, from train to local_model_validation, and then finally to join at the aggregator. It is in join that an average is taken of the model weights, and the next round can begin.

class FederatedFlow(FLSpec):

    def __init__(self, model = None, optimizer = None, rounds=3, **kwargs):
        if model is not None:
            self.model = model
            self.optimizer = optimizer
            self.model = Net()
            self.optimizer = optim.SGD(self.model.parameters(), lr=learning_rate,
        self.rounds = rounds

    def start(self):
        print(f'Performing initialization for model')
        self.collaborators = self.runtime.collaborators
        self.private = 10
        self.current_round = 0,foreach='collaborators',exclude=['private'])

    def aggregated_model_validation(self):
        print(f'Performing aggregated model validation for collaborator {self.input}')
        self.agg_validation_score = inference(self.model,self.test_loader)
        print(f'{self.input} value of {self.agg_validation_score}')

    def train(self):
        self.optimizer = optim.SGD(self.model.parameters(), lr=learning_rate,
        train_losses = []
        for batch_idx, (data, target) in enumerate(self.train_loader):
          output = self.model(data)
          loss = F.nll_loss(output, target)
          if batch_idx % log_interval == 0:
            print('Train Epoch: 1 [{}/{} ({:.0f}%)]\tLoss: {:.6f}'.format(
               batch_idx * len(data), len(self.train_loader.dataset),
              100. * batch_idx / len(self.train_loader), loss.item()))
            self.loss = loss.item()
  , 'model.pth')
  , 'optimizer.pth')
        self.training_completed = True

    def local_model_validation(self):
        self.local_validation_score = inference(self.model,self.test_loader)
        print(f'Doing local model validation for collaborator {self.input}: {self.local_validation_score}'), exclude=['training_completed'])

    def join(self,inputs):
        self.average_loss = sum(input.loss for input in inputs)/len(inputs)
        self.aggregated_model_accuracy = sum(input.agg_validation_score for input in inputs)/len(inputs)
        self.local_model_accuracy = sum(input.local_validation_score for input in inputs)/len(inputs)
        print(f'Average aggregated model validation values = {self.aggregated_model_accuracy}')
        print(f'Average training loss = {self.average_loss}')
        print(f'Average local model validation values = {self.local_model_accuracy}')
        self.model = FedAvg([input.model for input in inputs])
        self.optimizer = [input.optimizer for input in inputs][0]
        self.current_round += 1
        if self.current_round < self.rounds:
  , foreach='collaborators', exclude=['private'])

    def end(self):
        print(f'This is the end of the flow')


Prior interfaces in OpenFL support the standard horizontal FL training workflow:

  1. The collaborator downloads the latest model from the aggregator

  2. The collaborator performs validation with their local validation dataset on the aggregated model, and sends these metrics to the aggregator (aggregated_model_validation task)

  3. The collaborator trains the model on their local training data set, and sends the local model weights and metrics to the aggregator (train task)

  4. The collaborator performs validation with their local validation dataset on their locally trained model, and sends their validation metrics to the aggregator (locally_tuned_model_validation task)

  5. The aggregator applies an aggregation function (weighted average, FedCurv, FedProx, etc.) to the model weights, and reports the aggregate metrics.

The Task Assigner determines the list of collaborator tasks to be performed, and both in the task runner API as well as the interactive API these tasks can be modified (to varying degrees). For example, to perform federated evaluation of a model, only the aggregated_model_validation task would be selected for the assigner’s block of the federated plan. Equivalently for the interactive API, this can be done by only registering a single validation task. But there are many other types of workflows that can’t be easily represented purely by training / validation tasks performed on a collaborator with a single model. An example is training a Federated Generative Adversarial Network (GAN); because this may be represented by separate generative and discriminator models, and could leak information about a collaborator dataset, the interface we provide should allow for better control over what gets sent over the network and how. Another common request we get is for validation with an aggregator’s dataset after training. Prior to OpenFL 1.5, there has not a great way to support this in OpenFL.


  1. Simplify the federated workflow representation

  2. Clean separation of workflow from runtime infrastructure

  3. Help users better understand the steps in federated learning (weight extraction, tensor compression, etc.)

  4. Interface makes it clear what is sent across the network

  5. The placement of tasks and how they connect should be straightforward

  6. Don’t reinvent unless absolutely necessary

Workflow Interface API

The workflow interface formulates the experiment as a series of tasks, or a flow. Every flow begins with the start task and concludes with end.


A Runtime defines where the flow will be executed, who the participants are in the experiment, and the private information that each participant has access to. In this experimental release, single node execution is supported using the LocalRuntime. Let’s see how a LocalRuntime is created:

# Setup participants
aggregator = Aggregator()
aggregator.private_attributes = {}

# Setup collaborators with private attributes
collaborator_names = ['Portland', 'Seattle', 'Chandler','Bangalore']
collaborators = [Collaborator(name=name) for name in collaborator_names]
for idx, collaborator in enumerate(collaborators):
    local_train = deepcopy(mnist_train)
    local_test = deepcopy(mnist_test) =[idx::len(collaborators)]
    local_train.targets = mnist_train.targets[idx::len(collaborators)] =[idx::len(collaborators)]
    local_test.targets = mnist_test.targets[idx::len(collaborators)]
    collaborator.private_attributes = {
            'train_loader':,batch_size=batch_size_train, shuffle=True),
            'test_loader':,batch_size=batch_size_train, shuffle=True)

# This is equivalent to:
# local_runtime = LocalRuntime(aggregator=aggregator, collaborators=collaborators, backend='single_process')
local_runtime = LocalRuntime(aggregator=aggregator, collaborators=collaborators)

Let’s break this down, starting with the Aggregator and Collaborator placeholders. These placeholders represent the nodes where tasks will be executed. Each participant placeholder has its own set of private_attributes; a dictionary where the key is the name of the attribute, and the value is the object. In the above example, each of the four collaborators (‘Portland’, ‘Seattle’, ‘Chandler’, and ‘Bangalore’), have a train_loader and test_loader that they can access. These private attributes can be named anything, and do not necessarily need to be the same across each participant.

Now let’s see how the runtime for a flow is assigned, and the flow gets run:

flow = FederatedFlow()
flow.runtime = local_runtime

And that’s it! This will run an instance of the FederatedFlow on a single node in a single process.

Runtime Backends

The Runtime defines where code will run, but the Runtime has a Backend - which defines the underlying implementation of how the flow will be executed. 'single_process' is the default in the LocalRuntime: it executes all code sequentially within a single python process, and is well suited to run both on high spec and low spec hardware. For users with large servers or multiple GPUs they wish to take advantage of, we also provide a Ray <> backend. The Ray backend enables parallel task execution for collaborators, and optionally allows users to request dedicated GPUs for collaborator tasks in the placement decorator, as follows:

    # We request one dedicated GPU for this task
    def training(self):
        print(f'CUDA_VISIBLE_DEVICES: {os.environ["CUDA_VISIBLE_DEVICES"]}'))
        self.loss = train_func(self.model, self.train_loader)

 # The Ray Backend will now be used for local execution
 local_runtime = LocalRuntime(aggregator=aggregator, collaborators=collaborators, backend='ray')

Debugging with the Metaflow Client

Federated learning is difficult to debug. A common example of this difficulty comes in the form of mislabeled datasets. Even one mislabeled dataset on a collaborator’s training set in a large federation can result model convergence delay and lower aggregate accuracy. Wouldn’t it be better to pinpoint these problems early instead of after the full experiment has taken place?

To improve debugging of federated learning experiments, we are reusing Metaflow’s interfaces to (optionally) save all of the attributes generated by each participant, every task’s stdout / stderr, and provide a visual representation of the workflow graph.

Capturing this information requires just a one line change to the Flow object initialization by setting checkpoint=True:

flow = FederatedFlow(..., checkpoint=True)

After the flow has started running, you can use the Metaflow Client to get intermediate information from any of the participants tasks:

from metaflow import Flow, Run, Task, Step

# The name of the flow is the name of the class
flow = Flow('FederatedFlow')
run = flow.latest_run
> [Step('FederatedFlow/1671152854447797/end'),
step = Step('FederatedFlow/1671152854447797/aggregated_model_validation')
for task in step:
    if == 'Portland':
        portland_task = task
        model =
> <MetaflowData: train_loader, collaborators, loss, optimizer, model, input, rounds, agg_validation_score, current_round, test_loader, training_completed>
> Net(
   (conv1): Conv2d(1, 10, kernel_size=(5, 5), stride=(1, 1))
   (conv2): Conv2d(10, 20, kernel_size=(5, 5), stride=(1, 1))
   (conv2_drop): Dropout2d(p=0.5, inplace=False)
   (fc1): Linear(in_features=320, out_features=50, bias=True)
   (fc2): Linear(in_features=50, out_features=10, bias=True)

And if we wanted to get log or error message for that task, you can just run:

> Train Epoch: 1 [0/15000 (0%)]      Loss: 2.295608
  Train Epoch: 1 [640/15000 (4%)]    Loss: 2.311402
  Train Epoch: 1 [1280/15000 (9%)]   Loss: 2.281983
  Train Epoch: 1 [1920/15000 (13%)]  Loss: 2.269565
  Train Epoch: 1 [2560/15000 (17%)]  Loss: 2.261440
> [No output]

While this information is useful for debugging, depending on your workflow it may require significant disk space. For this reason, checkpoint is disabled by default.

Runtimes: Future Plans

Our goal is to make it a one line change to configure where and how a flow is executed. While we only support single node execution with the LocalRuntime today, our aim in future releases is to make going from one to multiple nodes as easy as:

flow = FederatedFlow()
# Run on a single node first
local_runtime = LocalRuntime(aggregator=aggregator, collaborators=collaborators)
flow.runtime = local_runtime

# A future example of how the same flow could be run on distributed infrastructure
federated_runtime = FederatedRuntime(...)
flow.runtime = federated_runtime