Subject: State machine for "empty" agents
From: Rick Reed (Rick.Reed#etsi.fr)
Date: Thu May 31 2001 - 12:16:33 GMT
Dear All,
The following has arisen during ETSI STF188 work:
The handling of a state machine and pid for an agent that has no explicit
state machine and no state machine implied by remote procedures or variables
etc., does not seem to be well-defined by Z.100.
According to the abstract syntax it is possible for a agent to have no state
machine. However, according to the definition of Create Request, the start
node of the agent graph is interpreted and therefore it is assumed that
there is a state machine.
An agent instance, once created, exists until it and all contained agent
instances have interpreted a stop. Any agent that does not contain an
explicit or implicit state machine is therefore persistent (that is, it
never ceases to exist). Any agent that contains such an agent will also be
persistent.
The effect is counter-intuitive to me. A block B that contains two processes
P1 and P2 and no explicit or implicit state machine, does not cease to exist
when the contained processes stop. This means that an attempt to create
another instance of the block B returns NULL if the maximum number of
instances had been reached, even if there all the processes are stopped.
This can be avoided by giving the block an explicit state machine that goes
directly to a stop, but this seems rather heavy handed for what would be the
normal case.
Moreover, if an agent contains such a "persistent" agent, no matter how
deeply nested, the outer agent will itself be persistent.
I suggest that the state machine of block B enters the stopping condition as
soon as it is created.
I suggest to refine the definition of Create Request so that it interprets
the Start Node - if there is one, or enters the stopping condition.
Of course, it may be the case that I have not noticed some text in Z.100
that covers this situation. If this is the case I would be grateful if
someone lets me know where this text is.
Rick Reed (at ETSI)
Tel: +33 4 92 94 42 68
mob: +44 79 70 50 96 50
email: rickreed#tseng.co.uk
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