README.md

Partial allocation

This simple project illustrates a specific usage of allocation with an important characteristic: if one of the two allocations fail for a single service, the service will be able to deallocate the resource that successfully got allocated.

In order to handle the de-allocation, we need to save the allocated value as soon as an error raises during another allocation. To do that, we used the AllocatorWrapper class from the lsm module. This allocator will provide a failsafe mechanism for all the allocators it is feeded in its arguments.

This project has been highly inspired from the example about external allocation. It is in a sense, a simplified version of it. It contains a single module: test_module. This module contains:

• One service, VlanService which needs allocation to populate two of its attributes, north_vlan_id and south_vlan_id.

  entity VlanService extends lsm::ServiceEntity:
      string name
  
      int? north_vlan_id = null
  
      int? south_vlan_id = null
  end
  

  Each of those attributes also have an allocation_key attribute related to it, which will contain the previously mentioned allocation key.
• One resource, VlanDeallocation which we will only instantiate whenever we are deleting a service, during the deallocating state of the service. This entity will always we instantiated with its purged attribute set to true, meaning that we will only need to implement the read and delete operation of its handler.

  entity VlanDeallocation extends std::PurgeableResource:
      int vlan_id
  end
  

  We only care about one attribute in this resource: vlan_id. This is where we pass the vlan id we wish to clean up.
• A mock of an external vlan inventory, VlanPool (python class), which will allow us to reserve and free vlans easily. This is used by the allocator and the resource handler.
• A vlan allocator, ExternalVlanAllocator (python class), which will handle the allocation of vlans. It takes two parameters in its constructor: the name of the attribute where to store the vlan, and the name of the attribute where to store the allocation key.
• A resource handler, VlanDeallocation (python class), which will handles the deallocation.

⚠️ Both the allocator and the handler needs to access the same vlan pool, as this pool is simply saved in a file, the resource agent needs to run on the same machine as the allocation. This is why we use the internal agent for this resource. The path to the file containing the vlan pool has to be specified via an environment variable: VLAN_POOL_FILE. This file must already exist, and should preferably contain a range, conforming to the following format:

{
    "range": {
        "from": 100,
        "to": 200
    }
}

If no range is supplied, the range defaults to [0: 0[. No vlan can then be allocated.