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On Tue, Jun 5, 2012 at 10:41 AM, Doug Hellmann
<doug.hellmann at dreamhost.com>wrote:

>
>
> On Tue, Jun 5, 2012 at 9:56 AM, Nick Barcet <nick.barcet at canonical.com>wrote:
>
>> Following up on our last meeting, here is a proposal for centrally
>> hosting configuration of agents in ceilometer.
>>
>> The main idea is that all agents of a given type should be sending
>> similarly formatted information in order for the information to be
>> usable, hence the need to ensure that configuration info is centrally
>> stored and retrieved.  This would rule out, in my mind, the idea that we
>> could use the global flags object, as distribution of the configuration
>> file is left to the cloud implementor and does not lend for easy and
>> synchronized updates of agent config.
>>
>> Configuration format and content is left to the agent's implementation,
>> but it is assumed that each meter covered by an agent can be :
>>  * enabled or disabled
>>  * set to send information at a specified interval.
>>
>
> Right now we only have one interval for all polling. Do you think we need
> to add support for polling different values at different intervals? Do we
> need other per-agent settings, or are all of the settings the same for all
> agents? (I had assumed the latter would be all we needed.)
>
>
>>
>> 1/ Configuration is stored for each agent in the database as follow
>> +-------------------------------------------------------------------+
>> | Field     | Type     | Note                                       |
>> +-------------------------------------------------------------------+
>> | AgentType | String   | Unique agent type                          |
>> | ConfVers  | Integer  | Version of the configuration               |
>> | Config    | Text     | JSON Configuration info (defined by agent) |
>> +-----------+----------+--------------------------------------------+
>>
>> 2/ Config is retreived via the messaging queue upon boot once a day
>> (this should be defined in the global flags object) to check if the
>> config has changed.
>>
>
> Updating the config once a day is not going to be enough in an environment
> with a lot of compute nodes.
>

Two thoughts merged into one sentence there. Need more caffeine.

What I was trying to say, was that updating the config once a day might not
be enough, and in environments with a lot of compute nodes going around to
manually restart the services each time the config changes will be a pain.
See below for more discussion of pushing config settings out.


>
>
>>
>> Request sent by the agent upon boot and :
>>
>>    'reply_to': 'get_config_data',
>>    'correlation_id': xxxxx
>>    'version': '1.0',
>>    'args': {'data': {
>>               'AgentType': agent.type,
>>               'CurrentVersion': agent.version,
>>               'ConfigDefault': agent.default,
>>               },
>>            },
>>
>
> Is this a standard OpenStack RPC call?
>
>
>>
>> Where ConfigDefault are the "sane" default proposed by the agent authors.
>>
>
> Why is the agent proposing default settings?
>
>
>>
>> If no config record is found the collector creates the record, sets
>> ConfVers to 1 and sends back a normal reply.
>>
>> Reply sent by the collector:
>>    'correlation_id': xxxxx
>>    'version': '1.0',
>>
>
> Do we need minor versions for the config settings, or are those simple
> sequence numbers to track which settings are the "most current"?
>
>
>>    'args': {'data': {
>>               'Result': result.code,
>>               'ConfVers': ConfVers,
>>               'Config': Config,
>>               },
>>            },
>>    }
>>
>> Result is set as follow:
>>    200  -> Config was retrieved successfully
>>    201  -> Config was created based on received default (Config is empty)
>>    304  -> Config version is identical to CurrentVersion (Config is empty)
>>
>
> Why does the agent need to know the difference between those? Shouldn't it
> simply use the settings it is given?
>
>
>>
>> This leaves open the question of having some UI to change the config,
>> but I thing we can live with manual updating of the records for the time
>> being.
>>
>
> Since we're using the service and RPC frameworks from nova elsewhere, we
> have the option of issuing commands to all of the agents from a central
> server. That would let us, for example, use a cast() call to push a new
> configuration out to all of the agents at once, on demand (from a command
> line program, for example).
>
> I don't see the need for storing the configuration in the database. It
> seems just as easy to have a configuration file on the central server. The
> collector could read the file each time it is asked for the agent
> configuration, and the command line program that pushes config changes out
> could do the same.
>
> Have you given any thought to distributing the secret value used for
> signing incoming messages? A central configuration authority does not give
> us a secure way to deliver secrets like that. If anyone with access to the
> message queue can retrieve the key by sending RPC requests, we might as
> well not sign the messages.
>
> Doug
>
>
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