Saravanan KR daf1e3b7ad Add documentation for role-specific parameters support

With recent update, parameters can be provided to per role basis, this
patch adds the documentation to explain the usage and limitations.

Change-Id: Idf2642be660880b1fdd9cd962c7f84c69d1cf41a

2017-06-12 17:37:49 +05:30

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Role-Specific Parameters

A service can be associated with multiple roles, like nova-compute service can be associated with ComputeRole1 and ComputeRole2. The nova-compute service takes multiple parameters like NovaVcpuPinSet, NovaReservedHostMemory, etc. It is possible to provide separate values specific to a role with the following changes in the user environment file:

parameter_defaults:
  NovaReservedHostMemory: 512
  ComputeRole1Parameters:
    NovaReservedHostMemory: 2048
  ComputeRole2Parameter:
    NovaReservedHostMemory: 1024

The format to provide role-specific parameters is <RoleName>Parameters, where the RoleName is the name of the role as defined in the roles_data.yaml template.

In the above specified example, the value "512" will be applied all the roles which has the nova-compute service, where as the value "2048" will be applied only on the ComputeRole1 role and the value "1024" will be applied only on the ComputeRole2 role.

With this approach, the service implementation has to merge the role-specific parameters with the global parameters in their definition template. The role-specific parameter takes higher precedence than the global parameters.

For any custom service which need to use role-specific parameter, the parameter merging should be done. Here is a sample parameter merging example which will be done by the service implementation:

RoleParametersValue:
  type: OS::Heat::Value
  properties:
    type: json
    value:
      map_replace:
        - map_replace:
          - neutron::agents::ml2::ovs::datapath_type: NeutronDatapathType
            neutron::agents::ml2::ovs::vhostuser_socket_dir: NeutronVhostuserSocketDir
            vswitch::dpdk::driver_type: NeutronDpdkDriverType
            vswitch::dpdk::host_core_list: HostCpusList
            vswitch::dpdk::pmd_core_list: NeutronDpdkCoreList
            vswitch::dpdk::memory_channels: NeutronDpdkMemoryChannels
            vswitch::dpdk::socket_mem: NeutronDpdkSocketMemory
          - values: {get_param: [RoleParameters]}
        - values:
            NeutronDatapathType: {get_param: NeutronDatapathType}
            NeutronVhostuserSocketDir: {get_param: NeutronVhostuserSocketDir}
            NeutronDpdkDriverType: {get_param: NeutronDpdkDriverType}
            HostCpusList: {get_param: HostCpusList}
            NeutronDpdkCoreList: {get_param: NeutronDpdkCoreList}
            NeutronDpdkMemoryChannels: {get_param: NeutronDpdkMemoryChannels}
            NeutronDpdkSocketMemory: {get_param: NeutronDpdkSocketMemory}

Note

As of now, not all parameters can be set per role, it is based on the service or template implementation. Each service should have the implementation to merge the global parameters and role-specific parameters, as explained in the above example. And today, not all services have this merging logic, check the parameter usage before using it as role-specific.

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Role-Specific Parameters

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