Architecture:
The two main components in the J2EE activity service are the
Activity service and the High Level Service (HLS).
The HLS is the implementation of a specific extended unit of a
work model which allows the interfaces and API to interact
with both the application that uses it and the application
server that hosts it. The HLS delegates control to the Activity
service which in turn manages the actual relationships between
this transaction context and other JTS contexts. The
implementation of the HLS is not defined in the J2EE activity
specification. It is defined by the actual implementer of the
specific UOW model.
The Activity service manages the HLS’s service context. It
provides interfaces to the HLS for support context demarcation
and coordination of the HLS specific objects. The application
interacts with the activity service through the HLS.
Features:
Pluggable transaction models:
The J2EE activity service is designed in such a way that the
extended transaction services can be managed by any application
server without needing to know the details of the smaller
transactions. Furthermore it makes it independent of any
application server whereby transaction services can be portable
across any J2EE compliant application server. The portability is
ensured by the portable extended services.
Generic Coordination:
The responsibility of distributing HLS specific messages
between the HLS and the Activity service is shared by both of
them. The Activity service provides the framework for sending
generic messages to activity participants, but the activity
participants and the messages are implemented by the HLS that
uses the Activity service. The HLS provides a SignalSet that is
responsible for producing signals or messages. The Activity
Coordinator is part of the Activity service which in turn
registers all the SignalSets within itself. It also drives the
SignalSets to generate signals at the appropriate time and then
distributes the signals to the right participants.
Grouping and Management of Context:
The HLS can start new activities or can create an activity as a
child activity fully encapsulated within a bigger activity. An
activity can have one or more JTA transactions within it or can
itself be encapsulated within a JTA transaction. The Activity
service context can help propagate a defined context activity in
one application server to another. The Activity service supports
Context Groups which can be used by the HLS to group disparate
activities in a single group and manage them cooperatively.
However activities started by the same HLS on the same thread
may be totally unrelated. The HLS may group activities into one
context group.
Activity Lifecycle:
All activities are started and ended by operations on the
UserActivity interface and is controlled by the
ActivityCoordinator. The Activity service ensures that only the
component that started an activity can complete the activity or
force a premature completion. Attempts by any other component
throws a NotOriginatorException. It also throws a
ActivtyPendingException if a parent activity tries to complete
with an outstanding child activity still pending completion.
However if an application working under an Activity context is
not performing any work for an extended period of time, the
container can hibernate the activity. Hibernation suspends the
activity from the active thread and makes the resources of the
activity passive and removes them from active memory. The
hibernated activity can be revived when further work is
requested on the activity by bringing it back to active memory.
The hibernation process has nothing to do with the persistence
mechanism for activities.
Distributed Service Context:
The behavior of distributed components running under an
Activity context is made location independent by use of an
Activity service context that is propagated implicitly on all
remote calls.
Distributed Application Property Data:
A distributed activity may consist of a number of components
and resources distributed over a variety of remote servers. The
J2EE Activity specification provides for a PropertyGroup context
to enable different components to share application specific
property data.
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