Changes to event subscription¶
New asynchronous subscription and sub_mode parameter¶
cppTango 10.1.0 added a new set of options for subscribing to events. Historically, the
tango.DeviceProxy.subscribe_event() and tango.AttributeProxy.subscribe_event() calls were
synchronous, and included a read of the attribute being subscribed to. In other words, the client had to
wait for both the subscription and the attribute read before continuing. This could delay the client significantly
when subscribing to a large number of attributes, especially if any of those attributes were slow to read. cppTango
has now decoupled the subscription and reading, providing a variety of new options.
The client’s subscribe_event method has a new parameter, sub_mode, which is an enumeration of
EventSubMode. The default value is EventSubMode.SyncRead, which is the standard behaviour
we had in previous versions of Tango. If you want the client to be able to continue sooner after subscription,
the EventSubMode.AsyncRead is better, however, you have no guarantee when the callback with the initial value
will happen. It should be “soon”, if the device is running and the attribute exists.
Event callback changes¶
The event data objects received on event callbacks (or when fetching them from the event queue) now have a new field,
event_reason, of type EventReason. See the event arrived structures.
This field may be of interest for the EventSubMode.Async subscription mode. After initial subscription to an
attribute, we receive an event of type EventData, with EventReason.SubSuccess, but since we aren’t doing a
read, there won’t be a value in the attr_value field (it will be None).
Deprecation of filters and stateless parameters¶
The filters parameter for tango.DeviceProxy.subscribe_event() and tango.AttributeProxy.subscribe_event()
is only useful with old Tango device servers with the notifd event system (rather than ZeroMQ). Support for this
is scheduled for removal in Tango 11. In most clients that are still providing this parameter, they set it to an empty
list, [], since it isn’t applicable to the ZeroMQ event system.
The behaviour of stateless=True is now superseded by providing the sub_mode=EventSubMode.Stateless parameter
instead.
The plan is to remove both of these parameters from PyTango in future. The exact version hasn’t been decided, but the earliest is PyTango 11.0.0. PyTango will emit deprecation warnings if you use these old parameters.
Change code like this:
dp = DeviceProxy("sys/tg_test/1")
callback = tango.utils.EventCallback()
eid = dp.subscribe_event("double_scalar", EventType.CHANGE_EVENT, callback, [], True)
To code like this:
...
eid = dp.subscribe_event("double_scalar", EventType.CHANGE_EVENT, callback, sub_mode=EventSubMode.Stateless)
Or, if you don’t need the callback to be called immediately as part of the subscription, it is better to use the new asynchronous-read subscription mode:
...
eid = dp.subscribe_event("double_scalar", EventType.CHANGE_EVENT, callback, sub_mode=EventSubMode.AsyncRead)
If your old code is setting stateless to False, then the arguments can just be removed.
...
eid = dp.subscribe_event("double_scalar", EventType.CHANGE_EVENT, callback, [], False)
Changes to:
...
eid = dp.subscribe_event("double_scalar", EventType.CHANGE_EVENT, callback)
