Experimental Physics and
Industrial Control System

Ralph Lange <[email protected]> · Wed, 08 Jul 2009 16:24:21 +0200

Hello Bruno,

For arrays, the server side queue does not contain the values, but a

pointer to the array.

In the case of the server queue filling up, i.e. the server generating

more updates than CA is able to push onto the network...

...atomic data will be kept as copies in the queue, i.e. data is lost

only if the queue gets filled up completely, in which case the server

overwrites the last queue element each time it gets a new update.

...array data will be kept in the application (e.g. IOC database), so

that any new update happening before the queue is empty will lead to

data loss.

Or - in other words: A half-full server queue is a healthy situation for

atomic data (no updates lost), and an unhealthy situation for array data

(CA will always send the current array data).

Usually the server side queue is a good thing to handle short changes in

network and/or client load. For arrays, as soon as you need the queue,

you lose data.

Background: Arrays can be large. If CA would copy arrays onto the queue,

the IOC could easily run out of memory due to a client or network

problem, which would make the IOC far too vulnerable.

Hope this answers your question...
Ralph

On 08.07.2009 15:58 Bruno Coudoin wrote:

Le mercredi 08 juillet 2009 à 11:08 +0200, Ralph Lange a écrit :

Careful: Array data is not put into the server side queue (only

references to the array), so using an array to add the sequence number

would at the same time enable you to detect missing updates and create a

lot of missing updates. (Probably not what you want.)

I don't understand this point. In what would it make things worse, could
you please explain me.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System