Action-aware Software Configuration

https://blueprints.launchpad.net/heat/+spec/action-aware-sw-config

Heat resources have a well-defined lifecycle, handling the lifecycle actions CREATE, DELETE, SUSPEND, RESUME and UPDATE. Software components in a Heat template should follow the same lifecycle-awareness and allow for users to provide configuration hooks for the aforementioned actions.

Problem description

With the current design of Heat software orchestration, “software components” defined through SoftwareConfig resources allow for only one configuration (e.g. one script) to be specified. Typically, however, a software component has a lifecycle that is hard to express in a single script. For example, software must be installed (created), there should be support for suspend/resume handling, and it should be possible to allow for deletion-logic. This is also in line with the general Heat resource lifecycle.

To achieve the desired behavior of having all those lifecycle hooks with the current design, one would have to define several SoftwareConfig resources along with several SoftwareDeployment resources, each addressing one specific lifecycle action. Alternative, one would have to design automation scripts in a way so they can conditionally handle each lifecycle action accordingly. Both of those options lack some intuitiveness or impose complexity on the creation of automation scripts. By making software components action-aware like other Heat resources, thus leveraging more of the orchestration capabilities of the Heat engine, creation of software configuration automation and respective Heat templates can be simplified for users.

Proposed change

It is proposed to make software components (defined through SoftwareComponent and SoftwareDeployment resources) lifecylce-action-aware by allowing users to provide configuration scripts for one software component for all standard Heat lifecycle actions (CREATE, DELETE, SUSPEND, RESUME, UPDATE). Those configurations that collective belong to one software component (e.g. Tomcat web server, MySQL database) can be defined in one place (i.e. one SoftwareComponent resource) and can be associated to a server by means of one single SoftwareDeployment resource.

The new SoftwareComponent resource will - like the SoftwareConfig resource - not gain any new behavior, but it will also be static store of software configuration data. Compared to SoftwareConfig, though, it will be extended to provide several configurations corresponding to Heat lifecyle actions in one place and following a well-defined structure so that SoftwareDeployment resources in combination with in-instance agents can act in a lifecycle-aware manner.

New SoftwareComponent resource

It is proposed to implement a new resource type OS::Heat::SoftwareComponent, which is similar to the existing SoftwareConfig resource, but has a richer structure and semantics. As an alternative, we could choose to extend the existing “SoftwareConfig” resource, but the overloaded semantics could cause confusion with users. Furthermore, extension of the existing resource could raise additional complexity when having to maintain backwards-compatibility with existing uses of SoftwareConfig.

The set of properties for OS::Heat::SoftwareComponent will be as follows:

# HOT representation of new SoftwareComponent resource

sw-config:
  type: OS::Heat::SoftwareComponent
  properties:
    # per action configurations
    configs:
      - actions: [ string, ... ]
        config: string
        tool: string
      - actions: [ string, ... ]
        config: string
        tool: string
      # ...
    # inputs and outputs
    inputs: [ ... ]
    outputs: [ ... ]
    options: { ... }

The configs property is a list of configurations for the various lifecycle operations of a software component. Each entry in that list defines the following properties:

actions

This property defines a list of resource actions when the respective config should be applied. Possible values in that list correspond to lifecycle actions of Heat’s resource model (i.e. CREATE, DELETE, SUSPEND, RESUME, and UPDATE). Making this property a list of actions allows for re-using one configuration for multiple resource actions when desired. For example, Chef recipe for deploying some software (i.e. CREATE action) could also be used for handling updates to software configuration properties (i.e. UPDATE action).

Note: One action like CREATE is only allowed to appear in the actions property of at most one config. Otherwise, the ordering of several configs for one lifecycle action at runtime would be unclear. This constraint will be validated in the validate() method of the SoftwareComponent resource. Allowing an action to appear in more than one config (probably with additional annotation for ordering) is something that could be done as future work.

config
This property defines the actual configuration to be applied, analogous to the config property of OS::Heat::SoftwareConfig.
tool
This property specifies the configuration tool to be used. Note that this is analogous to the SoftwareConfig resource’s group property, but it has been suggested to use a more intuitive name here. Having the tool property for each config entry allows for mixing different configuration tools for one software component. For example, the deployment of software (i.e. CREATE) could be done using Chef or Puppet, but a simple script could be used for SUSPEND or RESUME.

The inputs and outputs properties will be defined global for the complete SoftwareComponent definition instead of being provided per config hook. Otherwise, the corresponding SoftwareDeployment resource at runtime would potentially have different or stale attributes depending on which resource action was last run, which would likely introduce more complexity. Template authors will have to make sure that the defined inputs and outputs cover the superset of inputs and outputs for all operation hooks. Typically, the CREATE hook will require the broadest set of inputs and produce most outputs.

The options property will also be defined globally for the complete SoftwareComponent. This property is meant to provide extra options for the respective configuration tool to be used. It is assumed that the same options will apply to all invocations of a configuration for one SoftwareComponent, so making this a per-config settings does not make sense. Note that in case of multiple configuration tools being used in one SoftwareComponent, options need to be namespaced so they can mapped to the respective tools. For that reason, the options map will have to contain sub-sections for the respective tools. For example, for Chef the options map would contain a ‘chef’ entry the value of which is in turn a map of Chef-specific options.

Example

The following snippet shows an example of a SoftwareComponent definition for an application server. The SoftwareComponent defines dedicated hooks for CREATE, UPDATE and SUSPEND operations.

appserver-config:
  type: OS::Heat::SoftwareComponent
  properties:
    # per action configurations
    configs:
      - actions: [ CREATE ]
        config: { get_file: scripts/install_appserver.sh }
        tool: script
      - actions: [ UPDATE ]
        config: { get_file: scripts/reconfigure_appserver.sh }
        tool: script
      - actions: [ SUSPEND ]
        config: { get_file: scripts/drain_sessions.sh }
        tool: script
    # inputs and outputs
    inputs:
      - name: http_port
      - name: https_port
      - name: default_con_timeout
    outputs:
      - name: admin_url
      - name: root_url

Adaptation of SoftwareDeployment resource

The SoftwareDeployment resource (OS::Heat::SoftwareDeployment) will be adapted to cope with the new SoftwareComponent resource, for example to provide the contents of the configs property to the instance in the appropriate form. Furthermore, the SoftwareDeployment resource’s action and state (e.g. CREATE and IN_PROGRESS) will be passed to the instance so the in-instance configuration hook can select the right configuration to be applied (see also Update to in-instance configuration hooks).

The SoftwareDeployment resource creates transient configuration objects at runtime for providing data to the in-instance tools that actually perform software configuration. When a SoftwareComponent resource is associated to a SoftwareDeployment resource, the complete set of configurations of the software component (i.e. the complete configs property) will be stored in that transient configuration object, and it will therefore be available to in-instance tools.

There will be no change in SoftwareDeployment properties, but there will have to be special handling for the actions property: the actions property will be ignored when a SoftwareComponent resource is associated to a SoftwareDeployment. In that case, the entries defined in the configs property will provide the set of actions on which SoftwareDeployment, or in-instance tools respectively, shall react.

Note: as an alternative to passing the complete set of configurations defined in a SoftwareComponent, along with the SoftwareDeployment’s action and state to the instance, we could make the SoftwareDeployment resource select the right config based on its action and state and only pass this to the instance. This could possibly allow for using the existing in-instance hooks without change. However, at the time of writing this spec, it was decided to implement config select in the in-instance hook since it gives more power to the in-instance implementation for possible future enhancements.

Update to in-instance configuration hooks

The in-instance hooks (55-heat-config) have to be updated to select the appropriate configuration to be applied depending on the action and state indicated by the associated SoftwareDeployment resources.

In case of a SoftwareComponent being deployed, the complete set of configurations will be made available to in-instance hooks via Heat metadata. In addition, SoftwareDeployment resources will add their action and state to the metadata (e.g. CREATE and IN_PROGRESS). Based on that information, the in-instance hook will then be able to select and apply the right configuration at runtime.

As an alternative, we could choose to implement SoftwareDeployment in a way to only pass that configuration to the instance (via Heat metadata) that corresponds to its current action and state. In-instance tools could then potentially remain without changes (see also note in previous section).

Alternatives

Without any change to current implementation, the following alternatives for providing action-specific configuration hooks for a software component would exist:

Use of OS::Heat::StructuredConfig
StructuredConfig allows for defining a map of configurations, i.e it would allow for defining the proposed structure of the configs property to be added to SoftwareConfig. However, StructuredConfig does not define a schema for that map and would thus allow for any free-form data which would make it much harder to enforce well-defined handling. In addition, this would change the semantics of the map structure in StructuredConfig and thus it would be abuse of this resource.
Use of several SoftwareConfigs and SoftwareDeployments:
As already outlined in the problem description, with the current design it would be possible to define separate SoftwareConfigs and SoftwareDeployments, each corresponding to one lifecycle resource action. However, this makes templates much more verbose by having many resources for representing one software component, and the overall structure does not align with the general structure of all other Heat resources.
Use of scripts that conditionally handle actions
It would be possible to provide scripts that get invoked for all of a resource’s lifecycle actions. Those scripts would have to include a lot of conditional logic, which would make them very complicated.

Potential follow-up work

The current specification and implementation will only cover Heat’s basic lifecycle operations CREATE, DELETE, SUSPEND, RESUME and UPDATE. It is recognized that special handling might make sense for scenarios where servers are being quiesced for an upgrade, or where they need to be evacuated for a scaling operation. In addition, users might want to define complete custom actions (see also New SoftwareComponent resource). Handling of those actions are out of scope for now, but can be enabled by follow-up work on-top of the implementation of this specification. For example, an additional property extended_action could be added to SoftwareDeployment which could be set to the extended actions mentioned above. When passing this additional property to in-instance hooks, the hooks could then select and apply the respective config for the specified extended action.

Implementation

Assignee(s)

Primary assignee:
Thomas Spatzier

Milestones

Target Milestone for completion:
Juno-2

Work Items

  • Create new OS::Heat::SoftwareComponent resource
  • Adapt OS::Heat::SoftwareDeployment for new SoftwareComponent
  • Adapt in-instance hook for selecting right configuration to be applied

Dependencies

None