User guide documentation for backend drivers for oslo.config

New documentation sections explaning how to use the backend
drivers for oslo.config, also include explanation on how to
use the remote_file driver.

Change-Id: I45fc2155f6fe2d8bee058dac880afba8bb9bfd53
Blueprint: oslo-config-drivers

doc/source/reference/drivers.rst

@@ -0,0 +1,12 @@
+
+---------------
+Backend Drivers
+---------------
+
+.. automodule:: oslo_config.sources
+
+
+Known Backend Drivers
+---------------------
+
+.. automodule:: oslo_config.sources._uri

doc/source/reference/index.rst

@@ -21,4 +21,5 @@
    builtins
    sphinxext
    sphinxconfiggen
+   drivers
    faq

oslo_config/sources/__init__.py

@@ -9,6 +9,38 @@
 # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations
 # under the License.
+r"""
+Oslo.config's primary source of configuration data are plaintext, INI-like
+style, configuration files. With the addition of backend drivers support,
+it is possible to store configuration data in different places and with
+different formats, as long as, exists a proper driver to connect to those
+external sources and provides a way to fetch configuration values from them.
+
+A backend driver implementation is divided in two main classes, a driver
+class of type :class:`ConfigurationSourceDriver` and a configuration source
+class of type :class:`ConfigurationSource`.
+
+**IMPORTANT:** At this point, all backend drivers are only able to provide
+unmutable values, this protects applications and services to have options
+from external sources mutaded when they reload configuration files.
+
+Abstract Classes
+----------------
+
+The Driver Class
+================
+
+.. autoclass:: ConfigurationSourceDriver
+   :members:
+
+The Configuration Source Class
+==============================
+
+.. autoclass:: ConfigurationSource
+   :members:
+
+"""
+
 import abc
 import six
 
@@ -20,22 +52,43 @@ _NoValue = object()
 
 @six.add_metaclass(abc.ABCMeta)
 class ConfigurationSourceDriver(object):
-    """A config driver option for Oslo.config.
+    """A backend driver option for oslo.config.
 
-    Represents an oslo.config driver to allow store configuration data in
+    For each group name listed in **config_source** on the **DEFAULT** group,
-    other places, such as secret stores managed by a proper key management
+    a :class:`ConfigurationSourceDriver` is responsible for creating one new
-    store solution.
+    instance of a :class:`ConfigurationSource`. The proper driver class to be
+    used is selected based on the **driver** option inside each one of the
+    groups listed in config_source and loaded through entry points managed by
+    stevedore using the namespace oslo.config.driver::
+
+        [DEFAULT]
+        config_source = source1
+        config_source = source2
+        ...
+
+        [source1]
+        driver = remote_file
+        ...
+
+        [source2]
+        driver = castellan
+        ...
+
+    Each specific driver knows all the available options to properly instatiate
+    a ConfigurationSource using the values comming from the given group in the
+    open_source_from_opt_group() method and is able to generate sample config
+    through the list_options_for_discovery() method.
 
     """
 
     @abc.abstractmethod
     def open_source_from_opt_group(self, conf, group_name):
-        """Return an open option source.
+        """Return an open configuration source.
 
-        Use group_name to find the configuration settings for the new
+        Uses group_name to find the configuration settings for the new
-        source then open the source and return it.
+        source and then open the configuration source and return it.
 
-        If a source cannot be open, raise an appropriate exception.
+        If a source cannot be open, raises an appropriate exception.
 
         :param conf: The active configuration option handler from which
                      to seek configuration values.
@@ -48,13 +101,14 @@ class ConfigurationSourceDriver(object):
 
     @abc.abstractmethod
     def list_options_for_discovery(self):
-        """Return a list of options used by the driver to create the source.
+        """Return the list of options available to configure a new source.
 
-        Drivers should advertise all supported options in this method for the
+        Drivers should advertise all supported options in this method
-        purpose of sample generation by oslo-config-generator.
+        for the purpose of sample generation by oslo-config-generator.
 
-        For an example on how to implement this method you can check the URI
+        For an example on how to implement this method
-        driver at oslo_config.sources._uri.URIConfigurationSourceDriver.
+        you can check the **remote_file** driver at
+        oslo_config.sources._uri.URIConfigurationSourceDriver.
 
         :returns: a list of supported options of a ConfigurationSource.
         """

oslo_config/sources/_uri.py

@@ -9,6 +9,51 @@
 # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 # License for the specific language governing permissions and limitations
 # under the License.
+r"""
+Remote File
+-----------
+
+The **remote_file** backend driver is the first driver implemented by
+oslo.config. It extends the previous limit of only accessing local files
+to a new scenario where it is possible to access configuration data over
+the network. The **remote_file** driver is based on the **requests** module
+and is capable of accessing remote files through **HTTP** or **HTTPS**.
+
+To definition of a remote_file configuration data source can be as minimal as::
+
+    [DEFAULT]
+    config_source = external_config_group
+
+    [external_config_group]
+    driver = remote_file
+    uri = http://mydomain.com/path/to/config/data.conf
+
+Or as complete as::
+
+    [DEFAULT]
+    config_source = external_config_group
+
+    [external_config_group]
+    driver = remote_file
+    uri = https://mydomain.com/path/to/config/data.conf
+    ca_path = /path/to/server/ca.pem
+    client_key = /path/to/my/key.pem
+    client_cert = /path/to/my/cert.pem
+
+On the following sessions, you can find more information about this driver's
+classes and its options.
+
+The Driver Class
+================
+
+.. autoclass:: URIConfigurationSourceDriver
+
+The Configuration Source Class
+==============================
+
+.. autoclass:: URIConfigurationSource
+
+"""
 
 import requests
 import tempfile
@@ -18,7 +63,7 @@ from oslo_config import sources
 
 
 class URIConfigurationSourceDriver(sources.ConfigurationSourceDriver):
-    """A configuration source driver for remote files served through http[s].
+    """A backend driver for remote files served through http[s].
 
     Required options:
       - uri: URI containing the file location.
@@ -80,17 +125,17 @@ class URIConfigurationSourceDriver(sources.ConfigurationSourceDriver):
 class URIConfigurationSource(sources.ConfigurationSource):
     """A configuration source for remote files served through http[s].
 
-    :uri: The Uniform Resource Identifier of the configuration to be
+    :param uri: The Uniform Resource Identifier of the configuration to be
           retrieved.
 
-    :ca_path: The path to a CA_BUNDLE file or directory with
+    :param ca_path: The path to a CA_BUNDLE file or directory with
               certificates of trusted CAs.
 
-    :client_cert: Client side certificate, as a single file path
+    :param client_cert: Client side certificate, as a single file path
                   containing either the certificate only or the
                   private key and the certificate.
 
-    :client_key: Client side private key, in case client_cert is
+    :param client_key: Client side private key, in case client_cert is
                  specified but does not includes the private key.
     """
 
@@ -117,18 +162,6 @@ class URIConfigurationSource(sources.ConfigurationSource):
             return response.text
 
     def get(self, group_name, option_name, opt):
-        """Return the value of the option from the group.
-
-        :param group_name: Name of the group.
-        :type group_name: str
-        :param option_name: Name of the option.
-        :type option_name: str
-        :param opt: The option definition.
-        :type opt: Opt
-        :returns: A tuple (value, location) where value is the option value
-                  or oslo_config.sources._NoValue if the (group, option) is
-                  not present in the source, and location is a LocationInfo.
-        """
         try:
             return self._namespace._get_value(
                 [(group_name, option_name)],