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