Message Protocol

Note: This is a technical document not optimized for user readability.


A plan is a sequence of atomic operations describing a data acquisition procedure. Each operation is represented by a bluesky.Msg (“message”) object. A plan may be implemented as a simple list of messages:

from bluesky import Msg

# (Behold, the most boring data acquisition ever conducted!)
plan = [Msg('open_run'), Msg('close_run')]

or as a generator the yields messages one at time:

def plan():
    yield Msg('open_run')
    yield Msg('close_run')

The above examples are equivalent. For more sophisticated uses, the second one is more powerful, as it can incorporate loops, conditionals, adaptive logic — generally any Python code.

But, crucially, the plan code itself must not communicate with hardware. (You should never put epics.caput(...) in a plan!) Rather, each operation is represented by a Msg object that describes what should be done. This makes it safe to introspect the plan for error-checking, simulation, and visualization purposes — without touching real hardware. For example, we could print each message in the plan like so:

plan = [Msg('open_run'), Msg('close_run')]

# a very, very simple 'plan simulator'
for msg in plan:

A Msg has five members, accessible as attributes:

  • command

  • obj

  • args

  • kwargs

  • run

where command must be one of a controlled list of commands, obj is the object (i.e. Device) to apply the command to, if applicable, args and kwargs are arguments to the command and run is a user-defined run key. The run key is used by Run Engine to associate each message with one of the open runs, manage the state of each open run, and route run data to a separate set of callbacks (see documentation on Multi-Run Plans).

To execute the plan, the RunEngine consumes it, one message at a time.

def very_simple_run_engine(plan):
    for msg in plan:
        # Process the msg.

The RunEngine has a registry which is used to dispatch the Msg objects based on the value of the Msg.command. For example, if the RunEngine receives the message Msg('set', motor, 5), the RunEngine will:

  1. Identify that the command for this message is 'set'.

  2. Look up 'set' in its command registry and find that it is mapped to RunEngine._set.

  3. Pass Msg('set', motor, 5) to its _set method.

  4. Inside _set, call motor.set(5). (This is where the actual communication with hardware occurs.)

  5. Update some internal caches that will be useful later. For example, it will keep track of that fact that motor may be in motion so that it can stop it safely if an error occurs. This illustrates another important reason that plans must always yield messages to interact with hardware and absolutely never communicate with hardware directly. Calling epics.caput inside a plan prevents the RunEngine from knowing about it and thus circumvents its facilities for putting devices in a safe state in the event of an unexpected exit or error.

A standard set of commands are registered by default. By convention, a Msg with the command 'name' is mapped to a coroutine method on the RunEngine named _name, as in 'set' -> RunEngine._set in the example above. Users can register their own coroutines to add custom commands, though this is very rarely necessary.

Some commands do not involve communication with hardware. For example, Msg('sleep', None, 5) causes the RunEngine to sleep for 5 seconds. None is a placeholder for the “object” (Device) which is not applicable for a 'sleep' command. Just as plans should never communicate with hardware directly, they should also never employ long blocking calls like time.sleep(). Instead, the 'sleep' command, mapped to RunEngine._sleep, integrates with the RunEngine’s event loop to sleep in a non-blocking way that allows for the RunEngine to stay responsive in the meantime — watching for user interruptions and possibility collecting data asynchronously in the background.

Other commands are used to control metadata and I/O. For example, Msg('open_run') and Msg('close_run') delineate the scope of one run. Any keyword arguments passed to the 'open_run' message are interpreted as metadata, encoded into the RunStart document.

The following is a comprehensive overview of the built-in commands.



This section of the documentation is incomplete.

These are the ‘built in’ commands, some of which are deeply tied to the state of the RunEngine instance.


This command tells the run engine that it should start to collect the results of read to create an event. If this is called twice without a save or drop between them it is an exception (as you can not have more than one open event going at a time).

This relies very heavily on the internal state of the run engine and should not be overridden by the user.

This call returns None back to the co-routine.

This ignores all parts of the Msg except the command.


This is the pair to create which bundles and causes Event documents to be emitted. This must be called after a create or a the scan will die and raise IllegalMessageSequence.

This relies very heavily on the internal state of the run engine and should not be messed with.

This call returns None back to the co-routine.

This ignores all parts of the Msg except the command.


This causes read to be called on the obj in the message*msg.args, **msg.kwargs)

Anything that is read between a create and save will be bundled into a single event.

This relies very heavily on the internal state of the run engine and should not be messed with.

Returns the dictionary returned by read to the co-routine.

The args and kwargs parts of the message are passed to the read method.


This is a null message and is ignored by the run engine. This exists to make the algebra work.

Returns None to the co-routine.

Ignores all values in the Msg except the command.


Tells a Mover object to move. Currently this mimics the epics-like logic of immediate motion.

stage and unstage

Instruct the RunEngine to stage/unstage the object. This calls obj.stage()/obj.unstage.

Expected message objects are:

Msg('stage', object)
Msg('unstage', object)

which results in these calls:

staged_devices = object.stage()
unstaged_devices = object.unstage()

where staged_devices/unstaged_devices are a list of the ophyd.Device (s) that were (un)staged, not status objects.

One may wonder why the return is a list of Devices as opposed to Status objects, such as in set and similar Msg s. This was debated for awhile. Operations performed during staging are supposed to involve twiddling configuration, and should happen fast. Staging should not involve lengthy set calls.

Why a list of the objects staged? Staging a Device causes that Device’s component Devices (if any) to also be staged. All of these children are added to a list, along with [self], and returned by Device.stage(), so that the plan can keep track of what has been staged, like so:

devices_staged = yield Msg('stage', device)

Why would the plan want to know that? It needs to avoid accidentally trying to stage something twice, such as a staging a parent and then trying to also stage its child. It’s important to avoid that because staging something redundantly raises an error.


This will call the obj.trigger method and cache the returned status object and caches the returned status object.


Sleep the event loop.


Block progress until every object that was triggered or set the keyword argument group=<GROUP> is done.

Expected message object is:

Msg(‘wait’, group=<GROUP>)

where <GROUP> is any hashable key.


Instruct the RunEngine to wait for this asyncio.Future object to be done. This allows for external arbitrary control of the RunEngine. Ex

from asyncio.futures import Future
future = Future()
future.done() # will give false
RE(Msg('wait_for', [lambda : future ,]))
# this sets the future to done
future.done() # will give True


Process an input. Allows for user input during a run.


Msg('input', None)
Msg('input', None, prompt='>')  # customize prompt


Instruct the RunEngine to create a checkpoint so that we can rewind to this point if necessary.


Clear a set checkpoint.



Request the run engine to pause

Expected message object is:

Msg('pause', defer=False, name=None, callback=None)


Start a flyscan object.


Collect data cached by a flyer and emit descriptor and event documents. This calls the obj.collect() method.


Tell a flyer, ‘stop collecting, whenever you are ready’.

This calls the method obj.complete() of the given object. The flyer returns a status object. Some flyers respond to this command by stopping collection and returning a finished status object immediately. Other flyers finish their given course and finish whenever they finish, irrespective of when this command is issued.


Configure an object.

Expected message object is:

Msg('configure', object, *args, **kwargs)

which results in this call:

object.configure(*args, **kwargs)


Add a subscription after the run has started.

This, like subscriptions passed to __call__, will be removed at the end by the RunEngine.

Expected message object is:

Msg(‘subscribe’, None, callback_function, document_name)

where document_name is one of:

{‘start’, ‘descriptor’, ‘event’, ‘stop’, ‘all’}

and callback_function is expected to have a signature of:

f(name, document)

where name is one of the document_name options and document is one of the document dictionaries in the event model.

See the docstring of bluesky.run_engine.Dispatcher.subscribe() for more information.


Remove a subscription during a call – useful for a multi-run call where subscriptions are wanted for some runs but not others.

Expected message object is:

Msg('unsubscribe', None, TOKEN)
Msg('unsubscribe', token=TOKEN)

where TOKEN is the return value from RunEngine._subscribe()


Instruct the RunEngine to start a new “run”

Expected message object is:

Msg('open_run', None, **kwargs)

where **kwargs are any additional metadata that should go into the RunStart document


Instruct the RunEngine to write the RunStop document

Expected message object is:

Msg('close_run', None, exit_status=None, reason=None)

if exit_stats and reason are not provided, use the values stashed on the RE.


Drop a bundle of readings without emitting a completed Event document.

This is a command that abandons previous create and read commands without emitting an event. This can be used to drop known bad events (e.g. no beam) and keep the event document stream clean. It is safe to start another create, read, save sequence after a drop.

This must be called after a create or a the scan will die and raise IllegalMessageSequence.

This call returns None back to the co-routine.

This ignores all parts of the Msg except the command.


Monitor a signal. Emit event documents asynchronously.

A descriptor document is emitted immediately. Then, a closure is defined that emits Event documents associated with that descriptor from a separate thread. This process is not related to the main bundling process (create/read/save).

Expected message object is:

Msg('monitor', obj, **kwargs)
Msg('monitor', obj, name='event-stream-name', **kwargs)

where kwargs are passed through to obj.subscribe()


Stop monitoring; i.e., remove the callback emitting event documents.

Expected message object is:

Msg('unmonitor', obj)


Stop a device.

Expected message object is:

Msg('stop', obj)

This amounts to calling obj.stop().

Registering Custom Commands

The RunEngine can be taught any new commands. They can be registered using the following methods.

RunEngine.register_command(name, func)[source]

Register a new Message command.


This can be a function or a method. The signature is f(msg).


Unregister a Message command.


The list of commands available to Msg.


>>> from bluesky import RunEngine
>>> RE = RunEngine()
>>> # to list commands
>>> RE.commands

This conveniently prints the command registry of available commands.

Verbosebool, optional
verbose print. Default is False


>>> from bluesky import RunEngine
>>> RE = RunEngine()
>>> # Print a very verbose list of currently registered commands
>>> RE.print_command_registry(verbose=True)