could you please help how to read attribute values directly from the device when polling is turned on?
I’ve tried it in several ways for instance to use read_attribute with the green_mode=GreenMode.Synchronous argument, but didn’t help.
The Tango Manual (8.1) says it is possible to read directly from the device to avoid staled value coming from the polling buffer:
“9.2.3
Reading data from the polling buffer
For a polled command or a polled attribute, a client has three possibilities to get command result or attribute
value (or the thrown exception) :
• From the device itself
• From the polling buffer
• From the polling buffer first and from the device if data in the polling buffer are invalid or if the
polling is badly configured.
The choice is done during the command_inout CORBA operation by positioning one of the operation
parameter.”
Our use case is the following:
we need polling turned on for the archiving
we execute a command which turns the device into operation mode
we try to check whether it is in operation mode reading one of its attributes
but the polling buffer stores the earlier state of the is_…_allowed call and raises an exception: It is currently not allowed to read attribute …
temporarily I changed the is_…_allowed to return always True, in this case we received the staled value
of course if we use a several second waiting before checking the attribute solves the problem, but using a synchronous command we expect a valid attribute value
Tango devices indeed offer 3 possibilities for reading the source of data. The source is selected by setting the source using the appropriate API call :
For example in Python
dev.set_source(DevSource.DEV)
will set the source of your values from this device as the device itself i.e. it will call the read_attribute() method to get the value(s) and ignore what has been stored in the cache as the result from the last polling request. All future calls to using that device proxy will then bypass the cache.
To switch back to reading from the polling cache use:
dev.set_source(DevSource.CACHE_DEV)
Similar calls exist in C++ and Java and are documented in the manuals. Let me know if you need them too.
From what I understood, for CACHE_DEV, it will try to read it from CACHE and if this fails for a reason or another, it will then fallback to read from the device.
The reason it fails could be that the cache is empty or that the value in the cache is too old (older than poll_old_factor * polling_period => API_NotUpdatedAnyMore DevFailed exception).
For CACHE, I think you will get an exception as well if the last attribute value in the cache is too old too.
