[openstack-dev] [Heat] HOT software configuration refined after design summit discussions

Angus Salkeld asalkeld at redhat.com
Tue Nov 12 23:22:44 UTC 2013

On 12/11/13 10:32 -0800, Clint Byrum wrote:
>Excerpts from Thomas Spatzier's message of 2013-11-11 08:57:58 -0800:
>> Hi all,
>> I have just posted the following wiki page to reflect a refined proposal
>> for HOT software configuration based on discussions at the design summit
>> last week. Angus also put a sample up in an etherpad last week, but we did
>> not have enough time to go thru it in the design session. My write-up is
>> based on Angus' sample, actually a refinement, and on discussions we had in
>> breaks, plus it is trying to reflect all the good input from ML discussions
>> and Steve Baker's initial proposal.
>> https://wiki.openstack.org/wiki/Heat/Blueprints/hot-software-config-WIP
>> Please review and provide feedback.
>Hi Thomas, thanks for spelling this out clearly.
>I am still -1 on anything that specifies the place a configuration is
>hosted inside the configuration definition itself. Because configurations
>are encapsulated by servers, it makes more sense to me that the servers
>(or server groups) would specify their configurations. If changing to a
>more logical model is just too hard for TOSCA to adapt to, then I suggest
>this be an area that TOSCA differs from Heat. We don't need two models
>for communicating configurations to servers, and I'd prefer Heat stay
>focused on making HOT template authors' and users' lives better.
>I have seen an alternative approach which separates a configuration
>definition from a configuration deployer. This at least makes it clear
>that the configuration is a part of a server. In pseudo-HOT:
>  WebConfig:
>    type: OS::Heat::ChefCookbook
>    properties:
>      cookbook_url: https://some.test/foo
>      parameters:
>        endpoint_host:
>          type: string
>  WebServer:
>    type: OS::Nova::Server
>    properties:
>      image: webserver
>      flavor: 100
>  DeployWebConfig:
>    type: OS::Heat::ConfigDeployer
>    properties:
>      configuration: {get_resource: WebConfig}
>      on_server: {get_resource: WebServer}
>      parameters:
>        endpoint_host: {get_attribute: [ WebServer, first_ip]}

This is what Thomas defined, with one optimisation.
- The webconfig is a yaml template.

As you say the component is static - if so why even put it inline in
the template (well that was my thinking, it seems like a template not
really a resource).

>I have implementation questions about both of these approaches though,
>as it appears they'd have to reach backward in the graph to insert
>their configuration, or have a generic bucket for all configuration

Yeah, it does depend on the implementation. If we use Mistral the
agent will need to ask Mistral for the tasks that apply to the server.

$ mistral task-consume \

>to be inserted. IMO that would look a lot like the method I proposed,
>which was to just have a list of components attached directly to the
>server like this:
>  WebConfig:
>    type: Chef::Cookbook
>    properties:
>      cookbook_url: https://some.test/foo
>      parameters:
>        endpoing_host:
>          type: string
>  WebServer:
>    type: OS::Nova::Server
>    properties:
>      image: webserver
>      flavor: 100
>    components:
>      - webconfig:
>        component: {get_component: WebConfig}
>        parameters:
>          endpoint_host: {get_attribute: [ WebServer, first_ip ]}
I'd change this to:

       - webconfig:
         component: {get_file: ./my_configs/webconfig.yaml}
           endpoint_host: {get_attribute: [ WebServer, first_ip ]}

This *could* be a short hand notation like the volumes property on
aws instances.

>Of course, the keen eye will see the circular dependency there with the
>WebServer trying to know its own IP. We've identified quite a few use
>cases for self-referencing attributes, so that is a separate problem we
>should solve independent of the template composition problem.

(aside) I don't like the idea of self ref as it breaks the idea that
         references are resolved top down. Basically we have to put in
         a nasty hack to produce broken behaviour (first resolution is
         bogus and only following resoultions are possibly correct).

In this case just use the deployer to break your circular dep?

>Anyway, I prefer the idea that parse-time things are called components
>and run-time things are resources. I don't need a database entry for
>"WebConfig" above. It is in the template and entirely static, just
>sitting there as a reusable chunk for servers to pull in as-needed.

IMO is should just be a template/formatted file.

>Anyway, I don't feel that we resolved any of these issues in the session
>about configuration at the summit. If we did, we did not record them
>in the etherpad or the blueprint. We barely got through the prepared
>list of requirements and only were able to spell out problems, not
>any solutions. So forgive me if I missed something and want to keep on
>discussing this.
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