Hi Rajesh,

The SubscriptionException informs the server that the item is considered failed, and just as in the case of success the subscribe method will no longer be called until at least one active client keeps a subscription for that Item.
I think that the best practice to cope with such a scenario is that once the Data Adapter determines that the item name is valid, but an external service fails is to return succesfully the subscribe method an start a background task polling the external service at regular intervals.
If an unbsubscribe will be called before you got a reply from the external service the task will be cancelled.

Otherwise, if you want to avoid the background task, you should let communicate the Metadata Adapter with the Data Adapter, and replace the subsequent subscribe calls with the notifyNewTable calls.

Regards,
Giuseppe