To use a DaemonService, one implements the Serviceable interface. Only one
DaemonService will call this at any given time. However, if this Serviceable
object subscribes to or enqueues to the DeamonService multiple times, then
multiple DaemonService threads may call this Serviceable object at the same
time. The Serviceable object must decide what synchronization it needs to
provide depending on what work it needs to do.
The Serviceable interface does not provide a way to pass a work object to
identify what work needs to be done, it is assumed that the Serviceable
object knows that. If a Serviceable object has different work for the
DaemonService to do, it must somehow encapsulate or identify the different
work by an intermediary object which implements the Serviceable interface and
which can an identify the different work and pass it along to the object that
can deal with them.
Do whatever it is that you want the daemon to do for you. There may be
multiple daemon objects on different thread calling performWork at the
The DaemonService will always call performWork with a context manager
set up. the DaemonService will clean up the context if an exception is
thrown. However, it is up to performWork to manage its own
transaction. If you start a transaction in performWork, you
must commit or abort it at the end. You may leave the
transaction open so that other serviceable may use the transaction and
context without starting a new one. On the same token, there may
already be an opened transaction on the context. Serviceable
performWork should always check the state of the context before use.
A Serviceable object should be well behaved while it is performing the
daemon work, i.e., it should not take too many resources or hog the CPU
for too long or deadlock with anyone else.
context - the contextManager set up by the DaemonService. There
may or may not be the necessary context on it, depending on which other
Serviceable object it has done work for.
the return status is only significant if the Serviceable client
was enqueued instead of subscribed. For subscribed client, the return
status is ignored. For enqueue client, it returns DONE or REQUEUE. If
a REQUEUEd is returned, it would be desirable if this should not be
serviceASAP, although no harm is done if this still maintains that this
should be serviced ASAP ...
MT - depends on the work. Be wary of multiple DaemonService thread
calling at the same time if you subscribe or enqueue multiple times.
If this work should be done as soon as possible, then return true.
If it doesn't make any difference if it is done sooner rather than
later, then return false.
The difference is whether or not the daemon service will be notified to
work on this when this work is enqueued or subscribed, in case the
serviceable work is put together but not sent to the daemon service
directly, like in post commit processing
MT - MT safe
If this work should be done immediately on the user thread then return true.
If it doesn't make any difference if this work is done on a the user thread
immediately or if it is performed by another thread asynchronously
later, then return false.