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Beacon channels

Channels use the built-in publish/subscribe features of Redis in order to provide a simple way to exchange data between different BLISS processes.

Contrary to Settings, channels data is not persisted. When the last client holding the data disconnects, the channel data is cleared.

Use cases for channels are:

  • to update state between processes
    • for example, for Axis objects, position and state (MOVING, READY…) are shared between listeners
  • caching for performance, e.g. to avoid reloading
    • to memorize last loaded program in a MUSST or Opiom
    • to memorize MCA parameters
  • to prevent unwanted hardware initialization
    • if an object is shared between multiple processes, is in use and is already initialized, there is no need (it can even be harmful) to re-initialize it

Channel object usage

Process A:

>>> from bliss.config.channels import Channel
>>> c = Channel("test", default_value=42)
>>> c.value
42

Process B:

>>> from bliss.config.channels import Channel
>>> c = Channel("test")
>>> c.value
42

It is possible to register a callback to get notified of Channel value change events:

# in process A
>>> def c_value_changed(new_value):
        print(new_value)
>>> c.register_callback(c_value_changed)

# in process B
>>> c.value = "something new"

# in process A, output:
something new

Cache object

A Cache object is a helper to make a Channel attached to an object from the configuration. It guarantees the name of the channel corresponds to the object, by pre-pending the name of the object to the corresponding key in Redis.

>>> from bliss.config.static import get_config
>>> config = get_config()
>>> obj = config.get("my_object")
>>> from bliss.config.channels import Cache
>>> cached_value = Cache(obj, "my_cached_value", default_value="X")
# the cached_value object is a Channel
>>> cached_value.value = "something"

The clear_cache(*devices) function from bliss.config.channels deletes all cached values for the list of devices provided as function arguments.

Note

When the last client holding a channel disconnects, the channel is removed. It is cleared from Redis. In case another channel with the same name is created afterwards, reading it returns the default value.

EventChannel object

EventChannel object is a way to distribute event system wide. All pickable python object can be post The receiver will get the posted event by block.

Usage

Process A:

BLISS [1]: from bliss.config.channels import EventChannel
BLISS [2]: c = EventChannel('test')
BLISS [3]: for i in range(10):
      ...:     c.post(f'event {i}')

Process B:

BLISS [1]: from bliss.config.channels import EventChannel
BLISS [2]: c = EventChannel('test')
BLISS [3]: def f(events_list):
      ...:     print(events_list)
BLISS [4]: c.register_callback(f)
BLISS [5]: ['event 0', 'event 1', 'event 2', 'event 3', 'event 4',
            'event 5', 'event 6', 'event 7', 'event 8', 'event 9']

To disconnect the channel in process B: 

c.unregister_callback(f)