gevent mode
Warning
Before using gevent mode please note that at the time of writing this documentation, tango.gevent requires the latest version 1.0 of gevent (which has been released the day before :-P).
Using gevent cooperative mode in PyTango is relatively easy:
>>> from tango.gevent import DeviceProxy
>>> dev = DeviceProxy("sys/tg_test/1")
>>> dev.get_green_mode()
tango.GreenMode.Gevent
>>> print(dev.state())
RUNNING
The tango.gevent.DeviceProxy()
API is exactly the same as the standard
DeviceProxy
. The difference is in the semantics of the methods
that involve synchronous network calls (constructor included) which may block
the execution for a relatively big amount of time.
The list of methods that have been modified to accept gevent semantics are,
on the tango.gevent.DeviceProxy()
:
Constructor
state()
status()
read_attribute()
write_attribute()
write_read_attribute()
read_attributes()
write_attributes()
ping()
So how does this work in fact? I see no difference from using the standard
DeviceProxy
.
Well, this is, in fact, one of the goals: be able to use a gevent cooperation
without changing the API. Behind the scenes the methods mentioned before have
been modified to be able to work cooperatively with other greenlets.
All of the above methods have been boosted with two extra keyword arguments wait and timeout which allow to fine tune the behaviour. The wait parameter is by default set to True meaning wait for the request to finish (the default semantics when not using green mode). If wait is set to True, the timeout determines the maximum time to wait for the method to execute. The default timeout is None which means wait forever. If wait is set to False, the timeout is ignored.
If wait is set to True, the result is the same as executing the
standard method on a DeviceProxy
.
If, wait is set to False, the result will be a
gevent.event.AsyncResult
. In this case, to get the actual value
you will need to do something like:
>>> from tango.gevent import DeviceProxy
>>> dev = DeviceProxy("sys/tg_test/1")
>>> result = dev.state(wait=False)
>>> result
<gevent.event.AsyncResult at 0x1a74050>
>>> # this will be the blocking code
>>> state = result.get()
>>> print(state)
RUNNING
Here is another example using read_attribute()
:
>>> from tango.gevent import DeviceProxy
>>> dev = DeviceProxy("sys/tg_test/1")
>>> result = dev.read_attribute('wave', wait=False)
>>> result
<gevent.event.AsyncResult at 0x1aff54e>
>>> dev_attr = result.get()
>>> print(dev_attr)
DeviceAttribute[
data_format = tango.AttrDataFormat.SPECTRUM
dim_x = 256
dim_y = 0
has_failed = False
is_empty = False
name = 'wave'
nb_read = 256
nb_written = 0
quality = tango.AttrQuality.ATTR_VALID
r_dimension = AttributeDimension(dim_x = 256, dim_y = 0)
time = TimeVal(tv_nsec = 0, tv_sec = 1383923292, tv_usec = 886720)
type = tango.CmdArgType.DevDouble
value = array([ -9.61260664e-01, -9.65924853e-01, -9.70294813e-01,
-9.74369212e-01, -9.78146810e-01, -9.81626455e-01,
-9.84807087e-01, -9.87687739e-01, -9.90267531e-01,
...
5.15044507e-1])
w_dim_x = 0
w_dim_y = 0
w_dimension = AttributeDimension(dim_x = 0, dim_y = 0)
w_value = None]
Note
due to the internal workings of gevent, setting the wait flag to
True (default) doesn’t prevent other greenlets from running in parallel.
This is, in fact, one of the major bonus of working with gevent
when
compared with concurrent.futures