doc: add provider driver development instructions

Change-Id: I31642883f2bfb99e30820ddf72be3feedb66943e

doc/source/devguide.rst

@@ -66,3 +66,84 @@ Drivers
    :members:
 .. autoclass:: nodepool.driver.ProviderConfig
    :members:
+
+
+Writing A New Provider Driver
+-----------------------------
+
+Nodepool drivers are loaded from the nodepool/drivers directory. A driver
+is composed of three main objects:
+
+- A ProviderConfig to manage validation and loading of the provider.
+- A Provider to manage resource allocations.
+- A NodeRequestHandler to manage nodeset (collection of resource) allocations.
+
+Those objects are referenced from the Driver main interface that needs to
+be implemented in the __init__.py file of the driver directory.
+
+
+ProviderConfig
+~~~~~~~~~~~~~~
+
+The ProviderConfig is constructed with the driver object and the provider
+configuration dictionary.
+
+The main procedures of the ProviderConfig are:
+
+- getSchema() exposes a voluptuous schema of the provider configuration.
+- load(config) parses the provider configuration. Note that the config
+  argument is the global Nodepool.yaml configuration. Each provided labels
+  need to be referenced back to the global config.labels dictionary so
+  that the launcher service know which provider provide which labels.
+
+
+Provider
+~~~~~~~~
+
+The Provider is constructed with the ProviderConfig.
+
+The main procedures of the Provider are:
+
+- cleanupNode(external_id) terminates a resource
+
+- listNodes() returns the list of existing resources. This procedure needs to
+  map the nodepool_node_id with each resource. If the provider doesn't support
+  resource metadata, the driver needs to implement a storage facility to
+  associate resource created by Nodepool with the internal nodepool_node_id.
+  The launcher periodically look for non-existent node_id in listNodes() to
+  delete any leaked resources.
+
+- getRequestHandler(pool, request) returns a NodeRequestHandler object to manage
+  the creation of resources. The contract between the handler and the provider is
+  free form. As a rule of thumb, the handler should be in charge of interfacing
+  with Nodepool's database while the provider should provides primitive to create
+  resources. For example the Provider is likely to implement a
+  createResource(pool, label) procedure that will be used by the handler.
+
+
+NodeRequestHandler
+~~~~~~~~~~~~~~~~~~
+
+The NodeRequestHandler is constructed with the assigned pool and the request object.
+Before the handler is used, the following attributes are set:
+
+* self.provider : the provider configuration.
+* self.pool : the pool configuration.
+* self.zk : the database client.
+* self.manager : the Provider object.
+
+The main procedures of the NodeRequestHandler are:
+
+- launch(node) starts the creation of a new resource.
+- launchesComplete() returns True if all the node of the nodesets self
+  attributes are READY.
+
+An Handler may not have to launch each node of the nodesets as Nodepool will
+re-use existing nodes.
+
+The launch procedure usually consists of the following operations:
+
+- Use the provider to create the resources associated with the node label.
+  Once an external_id is obtained, it should be stored to the node.external_id.
+- Once the resource is created, READY should be stored to the node.state.
+  Otherwise raise an exception to restart the launch attempt.