During execution, every component instance is managed independently
by an instance manager that
takes in charge service registration and service dependency management
activities based on the information present in the component
descriptor.
The instance manager is a container that manages the
instance's life-cycle through control methods and interfaces, which are
only
visible to the instance manager, following the Inversion of
Control pattern (see figure 1).
A component instance is in one of two states: valid or invalid. When an instance is valid,
its services are registered in the service registry and it is active
(i.e., executing).
When an instance is invalid, its services are not registered in the
service registry and it is not active. When an instance is created, it
is initially invalid. After creation, the instance manager enters a
configuration phase (see figure 2). At this point, it creates bindings
between the instance it manages and other instances that provide
services required by the managed instance. Configuration can only
succeed if bindings can be created for required service interfaces that
are
categorized as being mandatory.
Figure 2. Instance manager states.
After
configuration, the instance manager activates the component instance,
and registers its services in the service registry. At that point the
instance is valid. If configuration fails, the instance manager waits
for the arrival of services that can be used to configure the instance.
As soon as all mandatory services become present, the instance is
validated.
During execution, the instance manager receives notifications from
the service registry. These notifications can trigger re-configuration
activities, which depend on the dependency properties associated to the
component that is being managed. The behavior of the instance manager
with respect to the service dependency properties is summarized in
table 1.
It must be noted that the instance manager's goal is to constantly
maintain the validity of the instance it manages. This is represented
by the transition between the execution and configuration states.
| Cardinality and policy |
Instance manager's behavior |
| 1..1,
static |
Configuration: The
required service interface corresponds to a single binding that must be
created for the instance to be validated. Execution: The destruction of the binding invalidates the instance |
| 1..1,
dynamic |
Configuration: The required service interface corresponds to a single binding that must be created for the instance to be validated. Execution: If the binding is destroyed because of a change in the instance that provides the service, the instance manager tries to create a replacing binding. If this binding cannot be created, the instance is invalidated. |
| 1..n,
static |
Configuration: The
required service interface corresponds to a set of bindings of which at
least one must be created for the instance to be validated. The
instance manager creates bindings with all the available service
providers at the moment of configuration.
|
| 1..n,
dynamic |
Configuration: The required service interface corresponds to a set of bindings of which at least one must be created for the instance to be validated. The instance manager creates bindings with all the available service providers at the moment of configuration. Execution: New
bindings can be created or existing bindings can be destroyed as long
as there is one binding. If the last binding is destroyed, the instance
is invalidated. |
| 0..1,
static |
Configuration: The required service interface corresponds to a single binding that does not need to be created for the instance to be validated. Execution: If the
binding was created during configuration, the destruction of the
binding invalidates the instance. If the binding was not created during
configuration, it cannot be created during execution. If no binding was
created during configuration, the instance is not invalidated until its
destruction. |
| 0..1,
dynamic |
Configuration: The required service interface corresponds to a single binding that does not need to be created for the instance to be validated. Execution: The binding
can be created or destroyed during execution without the instance being
invalidated. Instance invalidation only occurs when the instance is
destroyed. |
| 0..n,
static |
Configuration: The required service interface corresponds to a set of bindings that do not need to be created for the instance to be validated. The instance manager creates bindings with all the available service providers at the moment of configuration. Execution: The destruction of any binding invalidates the instance. No new bindings can be created. If no bindings were created, the instance is not invalidated until it is destroyed. |
| 0..n,
dynamic |
Configuration: The required service interface corresponds to a set of bindings that do not need to be created for the instance to be validated. The instance manager creates bindings with all the available service providers at the moment of configuration. Execution: New bindings can be created and bindings can be destroyed. Instance invalidation only occurs when the instance is destroyed. |
Table 1. Service dependency properties
and the instance manager's behavior.
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