Service Object Init References

Question

Has anyone come up with a good work around to allow Service Objects to
reference other service objects in their init methods or during application
startup. Since we can't specify the order in which Service Objects start,
is there a way we can execute some code once all Service Objects have come
online?
Will this idea work?
Start a task in the init method that loops for the referenced service object
to not be nil, then references the needed SO. For example:
while true do
if LogMgrSO = Nil then
task.delay(100);
else
exit;
end if;
end while;
Eric Rasmussen
Project Manager
Online Resources & Communications Corporation
[phone]
To unsubscribe, email '[email protected]' with
'unsubscribe forte-users' as the body of the message.
Searchable thread archive <URL:http://pinehurst.sageit.com/listarchive/>

Answer 1

Hi,
Sorry to answer so late ! I left (one year ago may be) some Tool code on
the Mailing list on that subject.
May be you have to consider some different cases :
1&deg;) Is a Service started because it is only instanciated (<> NIL)?
2&deg;) Forte insures that the first services to be started on a partition
are DBsession and DBResource Managers.
3&deg;) A local Service or a distributed service are not exactly treated the
same way.
4&deg;) The init() method has a specific way to run : the allocations are
made at the end.
1&deg;) When a Service is not NIL it is only that it is instanciated. So
your initialization sequence is not endded may be or
the service is not insured to be started properly. It should be
important if you need to load a cache for instance. I
would recommand to test that a service is ON (for DBsessions for
instance) and to add (if possible) a state to determine
that a service is properly started.
2&deg;) This is only available if you are inside the same partition on the
same machine. So if you have to synchronize with
external ressources from the partition you will need to treat them like
other services.
3&deg;) A local Service will be NIL and then instanciated. The classname
will be the same as in the workshop.
A distrubuted service (exactly a service which is not on the same
partition) will have a different classname (Classname+Proxy).
So the external service proxy may be instanciated but the So May not,
and you will get a DistributedAccessException.
4&deg;) The init() method may not be the best location for a synchronization
if you need to use an array for instance to
store you dependencies. So I would use a start task on an InitService()
method to avoid that problem.
Options :
- A dependency could be optionnal : after a certain amount of tries you
can abort synchronization on the service.
- You can use synchronization on "cold" and "hot" startup of services.
- You can develop a service agent which cold have instruments to see
dependencies and states, and commands to stop/start services.
- The Delay you may play should be different for each service you are
waiting for.
- The order of dependencies should have an importance (first put
mandatory dependancies, and then optional ones).
- A Service is not only a Service Object, but could also be just a
reference to an instance through a container for example.
- Some kind of autoStart : should I start all my services at the
beginning of my application or could I start some services
at the first call ? This should be available if you use your own
application protocole and if your services are inside some
service managers for instance.
Remarks :
Thoses concepts have been tested on a Framework from R2 to R3 of Forte
with success. With those, you can imagine
starting the application without knowing if the database is running, the
application will wait for the database
to be mounted. An other advantage of the synchronization is that you
will resolve the naming of the services at
the begining of the application. Then, you can stop the environment
manager and the application will still work
(for the clients which were already started of course). You can also
imagine transfering your partitions from one
node to an other at run-time.
Hope this helps,
Daniel Nguyen
Freelance Forte Consultant
Stephen McHenry wrote:
>
At 11:04 AM 10/1/98 -0700, John Jamison wrote:
begin
while true do
begin
..attempt "remote" SO reference..
exit; // while true do loop
exception
when e:UsageException do // if in same partition and not yet
initialized,
// you get a NIL object exception
task.errormgr.clear;
when e:DistributedAccessException do (or RemoteAccessException)
// if in a different partition, get this
error
task.errormgr.clear;
end;
event loop
aTimer : Timer = new (tickInterval=5*1000); // 5 seconds -
adjust to taste
aTimer.isActive=true;
when aTimer.Tick do
aTimer.IsActive=false;
exit;
end event;
end while;
end;One of the problems I see with all of these "catch the exception and try
again" schemes is that they fail to take into account that the SO you are
calling may, in fact, never appear (due to some sort of problem, of course)
and then you never exit this loop. It's a "liveness" problem with this
approach. So, be sure to add some alternate way out after 1 minute (or
whatever your particular threshold is) and raise an exception yourself.
Always gotta think about what happens if something goes wrong... ;-)
Stephen
|===========================================================================
===|
|Stephen McHenry | Design Consulting |Training courses
offered: |
|Advanced Software Tech | | -Distributed
Obj-Oriented |
|305 Vineyard Town Ctr #251| [email protected] | Analysis &
Design |
|Morgan Hill, CA 95037 | [phone] x210 | -Intro to Object
Technology|
|USA | http://www.softi.com | -Advanced OO Design
|
|===========================================================================
===|
To unsubscribe, email '[email protected]' with
'unsubscribe forte-users' as the body of the message.
Searchable thread archive <URL:http://pinehurst.sageit.com/listarchive/>-
To unsubscribe, email '[email protected]' with
'unsubscribe forte-users' as the body of the message.
Searchable thread archive <URL:http://pinehurst.sageit.com/listarchive/>