Julia Kreger 199e490cbc Add vendor step placement suggestion

As discussed on irc[1], we felt the need to document the stance
on placement of vendor specific steps, in terms of what we as a
project would prefer to do and prefer to avoid.

[1]: http://eavesdrop.openstack.org/irclogs/%23openstack-ironic/%23openstack-ironic.2018-08-30.log.html#t2018-08-30T16:00:57

Change-Id: I795ac06a0c249697ed2f6d066dc46069c7e0f8ae

2018-08-31 10:12:50 +00:00

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System Architecture

High Level description

An Ironic deployment will be composed of the following components:

An admin-only RESTful API service, by which privileged users, such as cloud operators and other services within the cloud control plane, may interact with the managed bare metal servers.
A Conductor service, which does the bulk of the work. Functionality is exposed via the API service. The Conductor and API services communicate via RPC.
A Database and DB API for storing the state of the Conductor and Drivers.
A Deployment Ramdisk or Deployment Agent, which provide control over the hardware which is not available remotely to the Conductor. A ramdisk should be built which contains one of these agents, eg. with diskimage-builder. This ramdisk can be booted on-demand.

Note

The agent is never run inside a tenant instance.

Drivers

The internal driver API provides a consistent interface between the Conductor service and the driver implementations. A driver is defined by a hardware type deriving from the AbstractHardwareType class, defining supported hardware interfaces. See /install/enabling-drivers for a more detailed explanation. See drivers for an explanation on how to write new hardware types and interfaces.

Driver-Specific Periodic Tasks

Drivers may run their own periodic tasks, i.e. actions run repeatedly after a certain amount of time. Such a task is created by using the periodic decorator on an interface method. For example

from futurist import periodics

class FakePower(base.PowerInterface):
    @periodics.periodic(spacing=42)
    def task(self, manager, context):
        pass  # do something

Here the spacing argument is a period in seconds for a given periodic task. For example 'spacing=5' means every 5 seconds.

Driver-Specific Steps

Drivers may have specific steps that may need to be executed or offered to a user to execute in order to perform specific configuration tasks.

These steps should ideally be located on the management interface to enable consistent user experience of the hardware type. What should be avoided is duplication of existing interfaces such as the deploy interface to enable vendor specific cleaning or deployment steps.

Message Routing

Each Conductor registers itself in the database upon start-up, and periodically updates the timestamp of its record. Contained within this registration is a list of the drivers which this Conductor instance supports. This allows all services to maintain a consistent view of which Conductors and which drivers are available at all times.

Based on their respective driver, all nodes are mapped across the set of available Conductors using a consistent hashing algorithm. Node-specific tasks are dispatched from the API tier to the appropriate conductor using conductor-specific RPC channels. As Conductor instances join or leave the cluster, nodes may be remapped to different Conductors, thus triggering various driver actions such as take-over or clean-up.

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