replace the CLI arguments with a config file

Rationale
=========

The CLI is getting complex so it is worth loading the configuration
from a file instead.

Implementation details
======================

TOML
----

We decided to use TOML for the following reasons:

  - it is human friendly, ie easy to read and write
  - our configuration has a pretty flat structure which makes TOML
    quite adapted
  - it is well specified and has lots of implementation
  - it is well known

The other options we considered:

  - INI: it is quite frequent in the Python ecosystem to use INI for
    config files, and the standard library even provides support for
    this. However, INI is not as powerful as TOML and does not have a
    specification
  - JSON: it is very popular but is not human friendly. For instance,
    it does not support comments, is very verbose, and breaks
    easily (if a trailing comma is forgotten at the end of a list for
    instance)
  - YAML: another popular choice, but is in my opinion more complex
    than TOML.

Validation
----------

We use the third-party `schema` library to validate the
configuration. It provides a convenient way to:

- declare a schema to validate our config
- leverage third-party libraries to validate some inputs (we use the
  `validators` library to validate IP addresses and URL for instance)
- define our own validators
- transform data after it has been validated: this can be useful to
  turn a relative path into an absolute one for example
- provide user friendly error message when the configuration is
  invalid

The `Config` class
------------------

By default, the `schema` library returns a dictionary containing a
valid configuration, but that is not convenient to manipulate in
Python. Therefore, we dynamically create a `Config` class from the
configuration schema, and instantiate a `Config` object from the data
returned by the `schema` validator.

Package re-organization
-----------------------

We moved the command line and config file logic into its own `config`
sub-package, and moved the former `xain_fl.cli.main` entrypoint into
the `xain_fl.__main__` module.

config.toml

@@ -0,0 +1,40 @@
+[server]
+
+# Address to listen on for incoming gRPC connections
+host = "localhost"
+# Port to listen on for incoming gRPC connections
+port = 50051
+
+
+[ai]
+
+# Path to a file containing a numpy ndarray to use a initial model weights.
+initial_weights = "./test_array.npy"
+
+# Number of global rounds the model is going to be trained for. This
+# must be a positive integer.
+rounds = 1
+
+# Number of local epochs per round
+epochs = 1
+
+# Minimum number of participants to be selected for a round.
+min_participants = 1
+
+# Fraction of total clients that participate in a training round. This
+# must be a float between 0 and 1.
+fraction_participants = 1.0
+
+[storage]
+
+# URL to the storage service to use
+endpoint = "http://localhost:9000"
+
+# Name of the bucket for storing the aggregated models
+bucket = "aggregated_weights"
+
+# AWS secret access to use to authenticate to the storage service
+secret_access_key = "my-secret"
+
+# AWS access key ID to use to authenticate to the storage service
+access_key_id = "my-key-id"

setup.py

@@ -30,6 +30,9 @@
     # TODO: change xain-proto requirement to "xain-proto==0.2.0" once it is released
     "xain-proto @ git+https://github.com/xainag/xain-proto.git@37fc05566da91d263c37d203c0ba70804960be9b#egg=xain_proto-0.1.0&subdirectory=python",  # Apache License 2.0
     "boto3==1.10.48",  # Apache License 2.0
+    "toml==0.10.0",  # MIT
+    "schema==0.6.8",  # MIT
+    "validators==0.14.1",  # MIT
 ]
 
 dev_require = [
@@ -92,5 +95,5 @@
         "docs": docs_require,
         "dev": dev_require + tests_require + docs_require,
     },
-    entry_points={"console_scripts": ["coordinator=xain_fl.cli:main"]},
+    entry_points={"console_scripts": ["coordinator=xain_fl.__main__"]},
 )

tests/store.py

@@ -7,7 +7,8 @@
 
 import numpy as np
 
-from xain_fl.coordinator.store import Store, StoreConfig
+from xain_fl.config import StorageConfig
+from xain_fl.coordinator.store import Store
 
 
 class FakeS3Resource:
@@ -71,7 +72,12 @@ class TestStore(Store):
     #
     # pylint: disable=super-init-not-called
     def __init__(self):
-        self.config = StoreConfig("endpoint_url", "access_key_id", "secret_access_key", "bucket")
+        self.config = StorageConfig(
+            endpoint="endpoint",
+            access_key_id="access_key_id",
+            secret_access_key="secret_access_key",
+            bucket="bucket",
+        )
         self.s3 = FakeS3Resource()
 
     def assert_wrote(self, round: int, weights: np.ndarray):

tests/test_config.py

@@ -0,0 +1,176 @@
+# pylint: disable=missing-docstring,redefined-outer-name
+import re
+
+import pytest
+
+from xain_fl.config import Config, InvalidConfig
+
+
+@pytest.fixture
+def server_sample():
+    """
+    Return a valid "server" section
+    """
+    return {"host": "localhost", "port": 50051}
+
+
+@pytest.fixture
+def ai_sample():
+    """
+    Return a valid "ai" section
+    """
+    return {
+        "initial_weights": "./test_array.npy",
+        "rounds": 1,
+        "epochs": 1,
+        "min_participants": 1,
+        "fraction_participants": 1.0,
+    }
+
+
+@pytest.fixture
+def storage_sample():
+    """
+    Return a valid "storage" section
+    """
+    return {
+        "endpoint": "http://localhost:9000",
+        "bucket": "aggregated_weights",
+        "secret_access_key": "my-secret",
+        "access_key_id": "my-key-id",
+    }
+
+
+@pytest.fixture
+def config_sample(server_sample, ai_sample, storage_sample):
+    """
+    Return a valid config
+    """
+    return {
+        "ai": ai_sample,
+        "server": server_sample,
+        "storage": storage_sample,
+    }
+
+
+def test_load_valid_config(config_sample):
+    """
+    Check that a valid config is loaded correctly
+    """
+    config = Config.from_unchecked_dict(config_sample)
+
+    assert config.server.host == "localhost"
+    assert config.server.port == 50051
+
+    assert config.ai.initial_weights == "./test_array.npy"
+    assert config.ai.rounds == 1
+    assert config.ai.epochs == 1
+    assert config.ai.min_participants == 1
+    assert config.ai.fraction_participants == 1.0
+
+    assert config.storage.endpoint == "http://localhost:9000"
+    assert config.storage.bucket == "aggregated_weights"
+    assert config.storage.secret_access_key == "my-secret"
+    assert config.storage.access_key_id == "my-key-id"
+
+
+def test_server_config_ip_address(config_sample, server_sample):
+    """Check that the config is loaded correctly when the `server.host`
+    key is an IP address
+
+    """
+    # Ipv4 host
+    server_sample["host"] = "1.2.3.4"
+    config_sample["server"] = server_sample
+    config = Config.from_unchecked_dict(config_sample)
+    assert config.server.host == server_sample["host"]
+
+    # Ipv6 host
+    server_sample["host"] = "::1"
+    config_sample["server"] = server_sample
+    config = Config.from_unchecked_dict(config_sample)
+    assert config.server.host == server_sample["host"]
+
+
+def test_server_config_extra_key(config_sample, server_sample):
+    """Check that the config is rejected when the server section contains
+    an extra key
+
+    """
+    server_sample["extra-key"] = "foo"
+    config_sample["server"] = server_sample
+
+    with AssertInvalid() as err:
+        Config.from_unchecked_dict(config_sample)
+
+    err.check_section("server")
+    err.check_extra_key("extra-key")
+
+
+def test_server_config_invalid_host(config_sample, server_sample):
+    """Check that the config is rejected when the `server.host` key is
+    invalid.
+
+    """
+    server_sample["host"] = 1.0
+    config_sample["server"] = server_sample
+
+    with AssertInvalid() as err:
+        Config.from_unchecked_dict(config_sample)
+
+    err.check_other(
+        re.compile("Invalid `server.host`: value must be a valid domain name or IP address")
+    )
+
+
+# FIXME: The library we use for validation rejects valid IPv6
+# addresses
+@pytest.mark.xfail
+def test_server_config_valid_ipv6(config_sample, server_sample):
+    """Check some edge cases with IPv6 `server.host` key"""
+    server_sample["host"] = "::"
+    config_sample["server"] = server_sample
+    config = Config.from_unchecked_dict(config_sample)
+    assert config.server.host == server_sample["host"]
+
+    server_sample["host"] = "fe80::"
+    config_sample["server"] = server_sample
+    config = Config.from_unchecked_dict(config_sample)
+    assert config.server.host == server_sample["host"]
+
+
+# Adapted from unittest's assertRaises
+class AssertInvalid:
+    """A context manager that check that an `xainfl.config.InvalidConfig`
+    exception is raised, and provides helpers to perform checks on the
+    exception.
+
+    """
+
+    def __init__(self):
+        self.message = None
+
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_value, _tb):
+        if exc_type is None:
+            raise Exception("Did not get an exception")
+        if not isinstance(exc_value, InvalidConfig):
+            # let this un-expected exception be re-raised
+            return False
+
+        self.message = str(exc_value)
+
+        return True
+
+    def check_section(self, section):
+        needle = re.compile(f"Key '{section}' error:")
+        assert re.search(needle, self.message)
+
+    def check_extra_key(self, key):
+        needle = re.compile(f"Wrong keys '{key}' in")
+        assert re.search(needle, self.message)
+
+    def check_other(self, needle):
+        assert re.search(needle, self.message)

xain_fl/__main__.py

@@ -0,0 +1,38 @@
+"""This module is the entrypoint to start a new coordinator instance.
+
+"""
+import sys
+
+import numpy as np
+
+from xain_fl.config import Config, InvalidConfig, get_cmd_parameters
+from xain_fl.coordinator.coordinator import Coordinator
+from xain_fl.coordinator.store import Store
+from xain_fl.serve import serve
+
+
+def main():
+    """Start a coordinator instance
+    """
+
+    args = get_cmd_parameters()
+    try:
+        config = Config.load(args.config)
+    except InvalidConfig as err:
+        print(err, file=sys.stderr)
+        sys.exit(1)
+
+    coordinator = Coordinator(
+        weights=list(np.load(config.ai.initial_weights, allow_pickle=True)),
+        num_rounds=config.ai.rounds,
+        epochs=config.ai.epochs,
+        minimum_participants_in_round=config.ai.min_participants,
+        fraction_of_participants=config.ai.fraction_participants,
+    )
+
+    store = Store(config.storage)
+
+    serve(coordinator=coordinator, store=store, host=config.server.host, port=config.server.port)
+
+
+main()