Today in vdsm, every object (StoragePool, StorageDomain, VM, Volume, etc) is
identified by a single UUID.  On the surface, it seems like this is enough info
to properly identify a resource but in practice it's not.  For example, when you
look at the API's dealing with Volumes, almost all of them require an sdUUID,
spUUID, and imgUUID in order to provide proper context for the operation.

Needing to provide these extra UUIDs is a burden on the API user because knowing
which values to pass requires internal knowledge of the API.  For example, the
spUUID parameter is almost always just the connected storage pool.  Since we
know there can currently be only one connected pool, the value is known.

I would like to move away from needing to understand all of these relationships
from the end user perspective by encapsulating the extra context into new object
identifier types as follows:

StoragePoolIdentifier:
    { 'storagepoolID': 'UUID' }
StorageDomainIdentifier:
    { 'storagepoolID*': 'UUID', 'storagedomainID': 'UUID' }
ImageIdentifier:
    { 'storagepoolID*': 'UUID', 'storagedomainID': 'UUID', 'imageID': 'UUID' }
VolumeIdentifier:
    { 'storagepoolID*': 'UUID', 'storagedomainID': 'UUID',
      'imageID': 'UUID', 'volumeID': 'UUID' }
TaskIdentifier:
    { 'taskID': 'UUID' }
VMIdentifier:
    { 'vmID': 'UUID' }

In the new API, anytime a reference to an object is required, one of the above
structures must be passed in place of today's single UUID.  In many cases, this
will allow us to reduce the number of parameters to the function since the
needed contextual parameters (spUUID, etc) will be part of the object's
identifier.  Similarly, any time the API returns an object reference it would
return a *Identifier instead of a bare UUID.

These identifier types are basically opaque blobs to the API users and are only
ever generated by vdsm itself.  Because of this, we can change the internal
structure of the identifier to require new information or (before freezing the
API) remove fields that no longer make sense.

I would greatly appreciate your comments on this proposal.  If it seems
reasonable, I will revamp the current schema to make the necessary changes and
provide the Bridge patch functions to convert between the current implementation
and the new schema.

--- sample schema patch ---

commit 48f6b0f0a111dd0b372d211a4e566ce87f375cee
Author: Adam Litke <a...@us.ibm.com>
Date:   Tue Nov 27 14:14:06 2012 -0600

    schema: Introduce class identifier types
    
    When calling API methods that belong to a particular class, a class instance
    must be indicated by passing a set of identifiers in the request.  The 
location
    of these parameters within the request is: 'params' -> '__obj__'.  Since 
this
    set of identifiers must be used together to correctly instantiate an 
object, it
    makes sense to define these as proper types within the API.  Then, functions
    that return an object (or list of objects) can refer to the correct type.
    
    Signed-off-by: Adam Litke <a...@us.ibm.com>

diff --git a/vdsm_api/vdsmapi-schema.json b/vdsm_api/vdsmapi-schema.json
index 0418e6e..7e2e851 100644
--- a/vdsm_api/vdsmapi-schema.json
+++ b/vdsm_api/vdsmapi-schema.json
@@ -937,7 +937,7 @@
 # Since: 4.10.0
 ##
 {'command': {'class': 'Host', 'name': 'getConnectedStoragePools'},
- 'returns': ['StoragePool']}
+ 'returns': ['StoragePoolIdentifier']}
 
 ##
 # @BlockDeviceType:
@@ -1572,7 +1572,7 @@
 {'command': {'class': 'Host', 'name': 'getStorageDomains'},
  'data': {'*storagepoolID': 'UUID', '*domainClass': 'StorageDomainImageClass',
           '*storageType': 'StorageDomainType', '*remotePath': 'str'},
- 'returns': ['StorageDomain']}
+ 'returns': ['StorageDomainIdentifier']}
 
 ##
 # @Host.getStorageRepoStats:
@@ -2406,7 +2406,7 @@
 ##
 {'command': {'class': 'Host', 'name': 'getVMList'},
  'data': {'*vmList': ['UUID']},
- 'returns': ['VM']}
+ 'returns': ['VMIdentifier']}
 
 ##
 # @Host.ping:
@@ -2744,10 +2744,11 @@
  'returns': 'ConnectionRefMap'}
 
 ## Category: @ISCSIConnection 
##################################################
+
 ##
-# @ISCSIConnection:
+# @ISCSIConnectionIdentifier:
 #
-# ISCSIConnection API object.
+# Identifier for an ISCSIConnection object.
 #
 # @host:      A fully-qualified domain name (FQDN) or IP address
 #
@@ -2757,11 +2758,21 @@
 #
 # @password:  #optional The password associated with the given username
 #
-# Since: 4.10.0
+# Since: 4.10.1
+##
+{'type': 'ISCSIConnectionIdentifier',
+ 'data': {'host': 'str', 'port': 'int', '*user': 'str', '*password': 'str'}}
+
+##
+# @ISCSIConnection:
+#
+# ISCSIConnection API object.
+#
+# @ident:  The object identifier
+#
+# Since: 4.10.1
 ##
-{'class': 'ISCSIConnection',
- 'data': {'host': 'str', 'port': 'int', '*user': 'str',
-          '*password': 'str'}}
+{'class': 'ISCSIConnection', 'ident': 'ISCSIConnectionIdentifier'}
 
 ##
 # @ISCSIConnection.discoverSendTargets:
@@ -2777,10 +2788,11 @@
  'returns': ['str']}
 
 ## Category: @Image 
############################################################
+
 ##
-# @Image:
+# @ImageIdentifier:
 #
-# Image API object.
+# Identifier for an Image object.
 #
 # @imageID:          The UUID of the Image
 #
@@ -2788,13 +2800,24 @@
 #
 # @storagedomainID:  The UUID of the Storage Domain associated with the Image
 #
-# Since: 4.10.0
+# Since: 4.10.1
 ##
-{'class': 'Image',
+{'type': 'ImageIdentifier',
  'data': {'imageID': 'UUID', 'storagepoolID': 'UUID',
           'storagedomainID': 'UUID'}}
 
 ##
+# @Image:
+#
+# Image API object.
+#
+# @ident:  The object identifier
+#
+# Since: 4.10.1
+##
+{'class': 'Image', 'ident': 'ImageIdentifier'}
+
+##
 # @Image.delete:
 #
 # Delete the Image and all of its Volumes.
@@ -2843,7 +2866,7 @@
 # Since: 4.10.0
 ##
 {'command': {'class': 'Image', 'name': 'getVolumes'},
- 'returns': ['Volume']}
+ 'returns': ['VolumeIdentifier']}
 
 ##
 # @Image.mergeSnapshots:
@@ -2905,17 +2928,26 @@
 
 ## Category: @LVMVolumeGroup 
###################################################
 ##
+# @LVMVolumeGroupIdentifier:
+#
+# An identifier for a LVMVolumeGroup object.
+#
+# @lvmvolumegroupID:  The volume group UUID
+#
+# Since: 4.10.1
+##
+{'type': 'LVMVolumeGroupIdentifier', 'data': {'lvmvolumegroupID': 'UUID'}}
+
+##
 # @LVMVolumeGroup:
 #
 # LVMVolumeGroup API object.
 #
-# @lvmvolumegroupID:  #optional Associate this object with an existing LVM
-#                     Volume Group
+# @ident:  The object identifier
 #
-# Since: 4.10.0
+# Since: 4.10.1
 ##
-{'class': 'LVMVolumeGroup',
- 'data': {'lvmvolumegroupID': 'UUID'}}
+{'class': 'LVMVolumeGroup', 'ident': 'LVMVolumeGroupIdentifier'}
 
 ##
 # @LVMVolumeGroup.create:
@@ -2964,21 +2996,32 @@
 
 ## Category: @StorageDomain 
####################################################
 ##
-# @StorageDomain:
+# @StorageDomainIdentifier:
 #
-# StorageDomain API object.
+# An identifier for a StorageDomain object.
 #
 # @storagedomainID:  Associate this object with a new or existing Storage 
Domain
 #
 # @storagepoolID:    #optional The Storage Pool UUID if this Storage Domain is
 #                    attached
 #
-# Since: 4.10.0
+# Since: 4.10.1
 ##
-{'class': 'StorageDomain',
+{'type': 'StorageDomainIdentifier',
  'data': {'storagedomainID': 'UUID', 'storagepoolID': 'UUID'}}
 
 ##
+# @StorageDomain:
+#
+# StorageDomain API object.
+#
+# @ident:  The object identifier
+#
+# Since: 4.10.1
+##
+{'class': 'StorageDomain', 'ident': 'StorageDomainIdentifier'}
+
+##
 # @StorageDomain.activate:
 #
 # Activate an attached but inactive Storage Domain.
@@ -3184,7 +3227,7 @@
 # Since: 4.10.0
 ##
 {'command': {'class': 'StorageDomain', 'name': 'getImages'},
- 'returns': ['Image']}
+ 'returns': ['ImageIdentifier']}
 
 ##
 # @StorageDomainRole:
@@ -3295,7 +3338,7 @@
 ##
 {'command': {'class': 'StorageDomain', 'name': 'getVolumes'},
  'data': {'imageID': 'UUID'},
- 'returns': ['Volume']}
+ 'returns': ['VolumeIdentifier']}
 
 ##
 # @StorageDomain.setDescription:
@@ -3355,15 +3398,26 @@
 
 ## Category: @StoragePool 
######################################################
 ##
+# @StoragePoolIdentifier:
+#
+# An identifier for a StoragePool object.
+#
+# @storagepoolID:  Associate this object with a new or existing Storage Pool
+#
+# Since: 4.10.1
+##
+{'type': 'StoragePoolIdentifier', 'data': {'storagepoolID': 'UUID'}}
+
+##
 # @StoragePool:
 #
 # StoragePool API object.
 #
-# @storagepoolID:  Associate this object with a new or existing Storage Pool
+# @ident:  The object identifier
 #
-# Since: 4.10.0
+# Since: 4.10.1
 ##
-{'class': 'StoragePool', 'data': {'storagepoolID': 'UUID'}}
+{'class': 'StoragePool', 'ident': 'StoragePoolIdentifier'}
 
 ##
 # @StoragePool.connect:
@@ -3629,7 +3683,7 @@
 ##
 {'command': {'class': 'StoragePool', 'name': 'getDomainsContainingImage'},
  'data': {'imageID': 'UUID', '*onlyDataDomains': 'bool'},
- 'returns': ['StorageDomain']}
+ 'returns': ['StorageDomainIdentifier']}
 
 ##
 # @StoragePool.getIsoList:
@@ -4058,15 +4112,27 @@
 
 ## Category: @Task 
#############################################################
 ##
+# @TaskIdentifier:
+#
+# An identifier for a Task object.
+#
+# @taskID:  The task UUID
+#
+# Since: 4.10.1
+##
+{'type': 'TaskIdentifier', 'data': {'taskID': 'UUID'}}
+
+
+##
 # @Task:
 #
 # Task API object.
 #
-# @taskID:  Associate this object with an existing Task
+# @ident:  The object identifier
 #
-# Since: 4.10.0
+# Since: 4.10.1
 ##
-{'class': 'Task', 'data': {'taskID': 'UUID'}}
+{'class': 'Task', 'ident': 'TaskIdentifier'}
 
 ##
 # @Task.clear:
@@ -4123,15 +4189,26 @@
 
 ## Category: @VM 
###############################################################
 ##
+# @VMIdentifier:
+#
+# An identifier for a VM object.
+#
+# @vmID:  The task UUID
+#
+# Since: 4.10.1
+##
+{'type': 'VMIdentifier', 'data': {'vmID': 'UUID'}}
+
+##
 # @VM:
 #
 # VM API object.
 #
-# @vmID:  Associate this object with an existing VM
+# @ident:  The object identifier
 #
-# Since: 4.10.0
+# Since: 4.10.1
 ##
-{'class': 'VM', 'data': {'vmID': 'UUID'}}
+{'class': 'VM', 'ident': 'VMIdentifier'}
 
 ##
 # @DriveSpecVolume:
@@ -5161,9 +5238,9 @@
 
 ## Category: @Volume 
###########################################################
 ##
-# @Volume:
+# @VolumeIdentifier:
 #
-# Volume API object.
+# An identifier for a Volume object.
 #
 # @volumeID:         The UUID of the Volume
 #
@@ -5173,13 +5250,24 @@
 #
 # @imageID:          The Image associated with @UUID
 #
-# Since: 4.10.0
+# Since: 4.10.1
 ##
-{'class': 'Volume',
+{'type': 'VolumeIdentifier',
  'data': {'volumeID': 'UUID', 'storagepoolID': 'UUID',
           'storagedomainID': 'UUID', 'imageID': 'UUID'}}
 
 ##
+# @Volume:
+#
+# Volume API object.
+#
+# @ident:  The object identifier
+#
+# Since: 4.10.1
+##
+{'class': 'Volume', 'ident': 'VolumeIdentifier'}
+
+##
 # @VolumeRole:
 #
 # An enumeration of Volume Roles.

-- 
Adam Litke <a...@us.ibm.com>
IBM Linux Technology Center

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