Sending Different Data To Particular Clients With tff.federated_select

This tutorial demonstrates how to implement custom federated algorithms in TFF that require sending different data to different clients. You may already be familiar with tff.federated_broadcast which sends a single server-placed value to all clients. This tutorial focuses on cases where different parts of a server-based value are sent to different clients. This may be useful for dividing up parts of a model across different clients in order to avoid sending the whole model to any single client.

Let's get started by importing both tensorflow and tensorflow_federated.

pip install --quiet --upgrade tensorflow-federated

import numpy as np
import tensorflow as tf
import tensorflow_federated as tff

Sending Different Values Based On Client Data

Consider the case where we have some server-placed list from which we want to send a few elements to each client based on some client-placed data. For example, a list of strings on the server, and on the clients, a comma-separated list of indices to download. We can implement that as follows:

list_of_strings_type = tff.TensorType(np.str_, [None])
# We only ever send exactly two values to each client. The number of keys per
# client must be a fixed number across all clients.
number_of_keys_per_client = 2
keys_type = tff.TensorType(np.int32, [number_of_keys_per_client])
get_size = tff.tensorflow.computation(lambda x: tf.size(x))
select_fn = tff.tensorflow.computation(lambda val, index: tf.gather(val, index))
client_data_type = np.str_

# A function from our client data to the indices of the values we'd like to
# select from the server.
@tff.tensorflow.computation(client_data_type)
def keys_for_client(client_string):
  # We assume our client data is a single string consisting of exactly three
  # comma-separated integers indicating which values to grab from the server.
  split = tf.strings.split([client_string], sep=',')[0]
  return tf.strings.to_number([split[0], split[1]], tf.int32)

@tff.tensorflow.computation(tff.SequenceType(np.str_))
def concatenate(values):
  def reduce_fn(acc, item):
    return tf.cond(tf.math.equal(acc, ''),
                   lambda: item,
                   lambda: tf.strings.join([acc, item], ','))
  return values.reduce('', reduce_fn)

@tff.federated_computation(tff.FederatedType(list_of_strings_type, tff.SERVER), tff.FederatedType(client_data_type, tff.CLIENTS))
def broadcast_based_on_client_data(list_of_strings_at_server, client_data):
  keys_at_clients = tff.federated_map(keys_for_client, client_data)
  max_key = tff.federated_map(get_size, list_of_strings_at_server)
  values_at_clients = tff.federated_select(keys_at_clients, max_key, list_of_strings_at_server, select_fn)
  value_at_clients = tff.federated_map(concatenate, values_at_clients)
  return value_at_clients

Then we can simulate our computation by providing the server-placed list of strings as well as string data for each client:

client_data = ['0,1', '1,2', '2,0']
broadcast_based_on_client_data(['a', 'b', 'c'], client_data)

[<tf.Tensor: shape=(), dtype=string, numpy=b'a,b'>,
 <tf.Tensor: shape=(), dtype=string, numpy=b'b,c'>,
 <tf.Tensor: shape=(), dtype=string, numpy=b'c,a'>]

Sending A Randomized Element To Each Client

Alternatively, it may be useful to send a random portion of the server data to each client. We can implement that by first generating a random key on each client and then following a similar selection process to the one used above:

@tff.tensorflow.computation(np.int32)
def get_random_key(max_key):
  return tf.random.uniform(shape=[1], minval=0, maxval=max_key, dtype=tf.int32)

list_of_strings_type = tff.TensorType(np.str_, [None])
get_size = tff.tensorflow.computation(lambda x: tf.size(x))
select_fn = tff.tensorflow.computation(lambda val, index: tf.gather(val, index))

@tff.tensorflow.computation(tff.SequenceType(np.str_))
def get_last_element(sequence):
  return sequence.reduce('', lambda _initial_state, val: val)

@tff.federated_computation(tff.FederatedType(list_of_strings_type, tff.SERVER))
def broadcast_random_element(list_of_strings_at_server):
  max_key_at_server = tff.federated_map(get_size, list_of_strings_at_server)
  max_key_at_clients = tff.federated_broadcast(max_key_at_server)
  key_at_clients = tff.federated_map(get_random_key, max_key_at_clients)
  random_string_sequence_at_clients = tff.federated_select(
      key_at_clients, max_key_at_server, list_of_strings_at_server, select_fn)
  # Even though we only passed in a single key, `federated_select` returns a
  # sequence for each client. We only care about the last (and only) element.
  random_string_at_clients = tff.federated_map(get_last_element, random_string_sequence_at_clients)
  return random_string_at_clients

Since our broadcast_random_element function doesn't take in any client-placed data, we have to configure the TFF Simulation Runtime with a default number of clients to use:

tff.backends.native.set_sync_local_cpp_execution_context(default_num_clients=3)

Then we can simulate the selection. We can change default_num_clients above and the list of strings below to generate different results, or simply re-run the computation to generate different random outputs.

broadcast_random_element(tf.convert_to_tensor(['foo', 'bar', 'baz']))