Experimental Physics and
Industrial Control System

Ralph Lange <[email protected]> · Thu, 16 Jun 2005 15:26:27 +0200

Benjamin Franksen wrote:

On Thursday 16 June 2005 12:55, Benjamin Franksen wrote:

On Wednesday 15 June 2005 23:12, Andrew Johnson wrote:

To step back a little: I don't understand why your states have
parameters, and what you could do with them - presumably I can't
put a CA monitor on a member of a tagged union (what happens to the
monitor when the state changes and the member no longer exists?).

It must disconnect, indicating that the value in question no longer
exists. Note that a similar question must be answered for the case of
adding/removing views. Here, too, the client will need to receive
notification that a certain value is no longer available. We could
make this a dedicated event type so that clients can react in a
generic way to such conditions.

Let me state this more precisely:

The very idea of a tagged union implies that independent read or write

access to properties subordinate to a certain tag hardly makes sense. I

cannot change, nor query, the scanRate of a record, as long as I am not

certain that the tag is 'Periodic'. However, subscribing to merely a

certain subordinate property might reduce system load.

The problem is that changing the tag independently brings the other

subordinate properties to an undefined state and this is something we

can't handle because we don't have a representation for "no value

here". Thus, at least write access must provide all values every time

(i.e. tag + add. properties), while reading and subscribing to either

tag only or other properties can be allowed but must fail in case of

unavailability.

Hm. I don't see any problem with my original idea to have the write

traversal for a tagged union (exactly like the read traversal) first

provide the tag and then only the properties that are valid for the new

tag value. Valid write operations would succeed, invalid operations

would fail, no meaningless properties being provided in the process.

Ralph

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System