Information Technology Reference
In-Depth Information
function standard was also adopted by OMG, becoming the CORBA Object
Trading Service [27].
ODP functions can also be combined to implement certain transparencies.
For example, let us look at how system failure can be hidden from computa-
tional objects in two different ways by using a selection of the ODP functions.
One approach is to save the state of the system from time to time, and
restore operations from this saved copy whenever necessary. To do this,
the checkpoint and recovery function is used to checkpoint a key part
of the application state (held, for example, in a cluster), and then later
recover the information when it is needed.
Another approach is to use the replication function; this allows the man-
agement of a behaviourally compatible object group. If one replica fails,
it is replaced by a new replica that is integrated into the group. Mean-
while, the service is maintained by the remaining members of the group.
ODP functions can also be used to provide other transparencies. For ex-
ample, the migration function allows an application (cluster) to migrate to
another process (capsule). This can be done in two ways. One is to use the
replication function to extend a service into a new node by forming a group
of two objects, and then removing the original member, leaving one provider
at the new node.
Alternatively, migration can be achieved by using the deactivation and
reactivation function. A cluster manager first deactivates its cluster, which
is then checkpointed. The checkpoint is moved to a new node where it is
reactivated in a new capsule.
«NV_Node»
PhoneMobNode
«NV_Node»
Stock DataMgr0
«NV_Capsule»
: PhoneMobCapsule
«NV_Capsule»
«NV_Cluster»
: DataCluster
«NV_BEO»
: Stock DataMgr0
«NV_Cluster»
: ApplicationCluster
«use»
...
«NV_Cluster»
: CorporationCluster
«NV_Node»
Stock DataMgr1
«NV_Channel»
Stock Data
Channel
«NV_Cluster»
: StockCluster
«NV_Capsule»
«use»
«NV_Object»
: Stock DataMgr
«use»
«NV_BEO»
: Stock DataMgr1
FIGURE 5.4: A storage object is replicated on separate nodes to increase its
availability.
