Overview

Applications

• Send commands and receive data to/from external systems (either software or hardware), including:
  • Another concurrent systems (DAQ system etc.)
    • Through a messaging system, such as Redis, AMQP, Kafka, ZMQ, …
    • Through a HTTP Get/Post, or Socket messages
    • By running a shell command
  • Measurement devices that accept commands
    • SCPI measurement devices with ethernet interface
    • Anything that can be used from Python
      • Raspberry-Pi GPIO, I2C, SPI, …
      • USB devices with a vendor library
  • Another SlowDash instance for distributed systems
• Analyze data in real-time, construct histograms etc., issue alarms
• Store raw and analyzed values (including histograms) to database systems

Structure

• Controls are implemented as user task python script.
• User task script is a normal Python script. Scripts are executable independently from the SlowDash server.
• Panels on the Slowdash GUI, built by users, can call functions defined in user task scripts.
• A Python library, slowpy, is provided to be imported by the user task scripts, for:
  • Simple unified methods to control external systems
  • Data classes such as histograms, graphs, time-series, etc.
  • Data storage interface to store the data objects, as well as raw data from the external systems

Simple Examples

Reading data from a voltmeter with SCPI commands over Ethernet

from slowpy.control import ControlSystem
ctrl = ControlSystem()

# make a control node for a SCIP command of "MEAS:V0" on a device at 182.168.1.43
V0 = ctrl.ethernet(host='192.168.1.43', port=17674).scpi().command('MEAS:V0')

while True:
  # read a value from the control node, with a SCPI command "MEAS:V?"
  value = V0.get()
  ...

Writing data to PostgreSQL database (using a default schema)

from slowpy.store import DataStore_PostgreSQL

datastore = DataStore_PostgreSQL('postgresql://postgres:postgres@localhost:5432/SlowTestData', table="SlowData")

while True:
    value = ...
    datastore.append(value, tag="ch00")

Calling a User Task function from SlowDash GUI Panels

If you have a User Task Script like this:

def set_V0(value):
    V0.set(value)

and write a SlowDash HTML panel like this:

<form>
  V0 value: <input name="value">
  <input type="submit" name="test.set_V0()" value="Set">
</form>

Then clicking the Set button will call the function set_V0() with a parameter in the value input field.

Displaying the readout values on the SlowDash panels

For a control node V0, and V1, defining _export() function in the User Task Script will export these node values, making them available in SlowDash GUI in the same way as the values stored in database.

device = ctrl.ethernet(host='192.168.1.43', port=17674).scpi()
V0 = device.command('MEAS:V0')
V1 = device.command('MEAS:V1')
def _export():
    return [
        ('V0', V0),
        ('V1', V1)
    ]

Only the “current” values are available in this way. If you need historical values, store the values in a database.

Demonstration Example Project

In slowdash/ExampleProjects/SlowTask there is a slowdash project to demonstrate some of the features described here.

$ cd slowdash/ExampleProjects/SlowTask
$ slowdash --port=18881

or

$ cd slowdash/ExampleProjects/SlowTask
$ docker compose up

SlowPy: Controls Library

SlowPy is a Python library (module) that provides functions like:

• connecting external concurrent systems and/or measurement hardware
• histograms, graphs, etc.
• storing raw values, time-series and histograms/graphs to database in a way that SlowDash can easily handle.

The SlowPy library is included in the SlowDash package, under slowdash/lib/slowpy. By running source slowdash/bin/slowdash-bashrc, as instructed in the Installation section, this path will be included in the environmental variable PYTHONPAH, so that users can use the library without modifying users system. It is also possible to install the library in a usual way: you can do pip install slowdas/lib/slowpy to install SlowPy into your Python environment. You might want to combine this with venv not to mess up your Python.

Controls

SlowPy provides an unified interface to connect external software systems and hardware devices; everything will be mapped into a single “ControlTree” where each node has set() and get(). The tree represents logical structure of the system, for example, a SCPI command of MEAS:V to a voltmeter connected to an ethernet would be addressed like ControlSystem.ethernet(host, port).scpi().command('MEAS:V'), and set(value) to this node will send a SCPI command of MEAS:V? to the voltmeter. The get() method makes a read access and returns a value.

Plugin modules can dynamically add branches to the control tree. For example, Redis plugin adds the redis() node and a number of sub-branches for functions that Redis provides, such as hash, json and time-series. Plugins are loaded to a node, not (necessarily) to the root ControlSystem; for example, a plugin that uses ethernet is loaded to an ethernet node, creating a sub-branch under the ethernet node, and the plugin can make use of the function provided by the ethernet node such as send() (which is set() of the node) and receive() (which is get()).

Example

Here is an example of using SlowPy Controls. In this example, we use a power supply device that accepts the SCPI commands through ethernet.

from slowpy.control import ControlSystem
ctrl = ControlSystem()

# make a control node for a SCIP command of "MEAS:V0" on a device at 182.168.1.32
V0 = ctrl.ethernet(host='192.168.1.43', port=17674).scpi().command('MEAS:V0', set_format='V0 {};*OPC?')

# write a value to the control node: this will issue a SCPI command "V0 10;*OPC?"
V0.set(10)

while True:
  # read a value from the control node, with a SCPI command "MEAS:V"
  value = V0.get()
  ...

A common start would be importing the slowpy.control, and then creating an instance of the ControlSystem class.

The ControlSystem already includes the Ethernet plugin, but if it were not, the loading plugin code would have been:

ctrl.load_control_module('Ethernet')

This will search for a file named control_Ethernet.py from search directories, load it, and inject the ethernet() Python method to the node class that loaded the plugin (which is ControlSystem in this case). The Ethernet plugin already includes sub-branches for SCPI; for specific protocols not already included, a plugin would be loaded onto the Ethernet node.

Each node constructor takes parameters. In this example, the ethernet node, which sends and receives data to/from the ethernet, takes the host/IP and port parameters, and a SCPI command node, which is bound to a specific SCPI command, takes the SCPI command parameter with optional set_format which overrides the default SCPI command for writing. (The default SCPI command to be written/read .scpi().command(CMD) are CMD and CMD?, respectively. Often write operations should wait for the command completion, and SlowPy expects a return value from each command. A technique to do it, regardless to the actual command behaviors of the device, is to append OPC? to the command, as done in the example.)

Once you get the control node object, you can call node.set(value) to write the value, and call value=node.get() to read from it. As a shortcut, node(value) is equivalent to node.set(value), and value=node() is equivalent to value=node.get(). Also control nodes define Python’s __str__(), __float__(), etc., so print(node) and x = float(node) will implicitly call node.get().

Control Node Threading

If a control node has a threading method of run() or loop(), calling start() of the node will create a dedicated thread and start it. This is useful for:

• The node can perform tasks independent from the set() and get() queries, such as PID loop.
• If fetching data is slow, data can be pre-fetched periodically and stored in a cache.

The run() function is called once, and then loop() is called repeatedly. In the loop() function, sleep() or similar must be inserted to control the frequency.

The thread is stopped by calling the stop() method of the node, or by a global stop request (such as ControlSystem.stop() or by signals). The run() function must watch the stop request (by is_stop_requested()) and terminate itself when a stop request is made. loop() will not be called after stop.

Commonly used nodes

Naming convention: set(), get(), and do_XXX() are usual methods to do something. Methods with a noun name return a sub-node.

Ethernet, SCPI and Telnet

Loading (already included)

ControlSystem.load_control_module('Ethernet')

Nodes

• ethernet(host,port): sends and receives text messages through a TCP socket connection
  • set(value): sends the value as encoded text
  • get(): receives a chunk of text
  • do_get_chunk(timeout=None): receives a chunk of text
  • do_get_line(timeout=None): receives a chunk and returns one line (with reconstruction)
  • do_flush_input(): empties the receiving buffer
  • scpi() bridges the parent ethernet connection and holds SCPI configurations
    • command(cmd) issue a SCPI command via the parent ethernet connection
      • set(value=None): sends cmd value and waits for a reply
      • get(): sends cmd? and waits for a reply, then returns the reply
  • telnet() bridges the parent ethernet connection and holds telnet configurations
    • command(cmd) issue a SCPI command via the parent ethernet connection
      • set(value=None): sends cmd value line_terminator, consumes echo
      • get(): sends cmd line_terminator, consumes echo, and returns a reply until the next prompt

Nodes

• serial(port=‘/dev/ttyUSB0’,baudrate=9600): sends and receives text messages through a serial port
  • set(value): sends the value as encoded text
  • get(): receives a chunk of text
  • do_get_chunk(timeout=None): receives a chunk of text
  • do_get_line(timeout=None): receives a chunk and returns one line (with reconstruction)
  • do_flush_input(): empties the receiving buffer
  • scpi() returns the same object as EthernetNode.scpi()

HTTP (Web API)

Loading (already included)

ControlSystem.load_control_module('HTTP')

Nodes

• http(base_url):
  • path(path): path string to be appended to the base_url
    • set(value): sends a POST request to base_url path with value as its content
    • get(): sends a GET request to base_url path and return the reply content
    • json():
      • set(value): value is a Python object to be converted to JSON
      • get(): receives a JSON document and returns a Python object
    • value(): returns ControlValueNode for ramping() etc.

Shell Command

Loading (already included)

ControlSystem.load_control_module('Shell')

Nodes

• shell(cmd): run a shell command.
  • set(value): launches cmd value,
  • get(): launches cmd and returns the result
  • value(): returns ControlValueNode for ramping() etc.
  • arg(*args): append program arguments to the parent “shell” node.
    Example: adc0 = ctrl.shell('read_adc', '--timeout=0').arg('--ch=0')
    • value(): returns ControlValueNode for ramping() etc.

SlowDash server control

Loading (already included)

ControlSystem.load_control_module('HTTP').load_control_module('Slowdash')

Nodes

• http(url).slowdash()
  • ping(): returns pong
  • channels(): gets a JSON object of the channel list
  • data(channels,length=3600): gets a JSON object of data for the channels
  • config(): gets a JSON object of the project configuration
  • config_file(name): gets or posts a configuration file

Redis Interface

Loading

ControlSystem.load_control_module('Redis')

Nodes

• redis(url)
  • string(key): read and write a simple key-value pair
    • set(value): writes a simple key-value pair
    • get(): returns a value of a simple key-value pair
  • hash(key): read and write a map of key-value pairs
    • set(value): writes a map of key-value pairs
    • get(): returns a hash as a map of key-value pairs
    • filed(name):
      • set(value): writes a single element in the parent hash
      • get(): returns a single element value in the parent hash
  • json(key):
    • set(value): writes value (Python dict) to Redis JSON
    • get(): returns a Redis JSON as Python dict
    • node(path): read and write a branch of JSON object
      • set(value): writes value to a branch of Redis JSON
      • get(): returns a value of a branch of Redis JSON
      • node(path): can do this node iteration recursively
  • ts(key): read and write a time-series, where the value is a list of (t,x) tuples
    • current():
      • set(value): adds a data point to the parent time-series using the current time
    • last():
      • get(): returns the last data point as a tuple of (t,x)
      • value():
        • get(): returns the value of the parent time-series point
      • time():
        • get(): returns the UNIX time value of the parent time-series point
      • lapse():
        • get(): returns the number of seconds since the last time-series point

Other Nodes

• Serial (uses pyserial)
• VISA (uses pyvisa)
• LabJack (uses LabJackPython, currently supports only U12)
• DummyDevice

Node Functions

All the control nodes (derived from slowpy.control.ControlNode) have the following methods:

• [ControlNode]
  • has_data(): returns True if a value is available for get()
  • wait(condition_lambda=None, polling_interval=1, timeout=0): blocks until the condition_lambda returns True if the lambda is not None; if condition_lambda is None, wait until has_data() returns True. On timeout, wait() returns None, and on stop-request, returns False. Otherwise, it returns True.
  • sleep(duration_sec): blocks for the duration and returns True unless a stop request is received.
  • is_stop_requested(): returns True if a stop request has been received
  • readonly(): makes the node readonly; it returns a node that has only get() which calls parent_node.get()
  • writeonly(): makes the node write only; it returns a node that has only set() which calls parent_node.set()

Nodes derived from ControlValueNode have the following methods:

• [ControlValueNode] -> [ControlNode]
  • setpoint(limits=[None,None]): holds the setpoint
    • set(value): holds the value as a set-point, and calls set(value) of the parent node.
    • get(): returns the holding set-point value
  • ramping(change_per_sec)
    • set(value): starts ramping to the target value in the parent node. If the value is not numeric or not in the limits, raises an exception
    • get(): returns parent’s get()
    • status()
      • set(value): set(0) will stop the current ramping if it is running
      • get(): returns True if rumping is in progress, otherwise returns False

Nodes derived from ControlThreadNode have the following methods:

• [ControlThreadNode] -> [ControlNode]
  • start(): creates and starts a thread for run() and/or loop(), if the node has one of these methods (or both)
  • stop(): stops the thread

Database Interface

Simple example of writing single values to a long form table:

import time
from slowpy.control import RandomWalkDevice
from slowpy.store import DataStore_PostgreSQL

device = RandomWalkDevice(n=4)
datastore = DataStore_PostgreSQL('postgresql://postgres:postgres@localhost:5432/SlowTestData', table="Test")

while True:
    for ch in range(4):
      value = device.read(ch)
      datastore.append(value, tag='%02d'%ch)

    time.sleep(1)

Example of writing a dict of key-values:

while True:
    record = { 'ch%02d'%ch: device.read(ch) for ch in range(4) }
    datastore.append(record)
    time.sleep(1)

For SQL databases, the “long format” with UNIX timestamps is used by default. To use other table schemata, specify an user defined TableFormat:

class QuickTourTestDataFormat(LongTableFormat):
    schema_numeric = '(datetime DATETIME, timestamp INTEGER, channel STRING, value REAL, PRIMARY KEY(timestamp, channel))'
    schema_text = '(datetime DATETIME, timestamp INTEGER, channel STRING, value REAL, PRIMARY KEY(timestamp, channel))'

    def insert_numeric_data(self, cur, timestamp, channel, value):
        cur.execute(f'INSERT INTO {self.table} VALUES(CURRENT_TIMESTAMP,%d,?,%f)' % (timestamp, value), (channel,))

    def insert_text_data(self, cur, timestamp, channel, value):
        cur.execute(f'INSERT INTO {self.table} VALUES(CURRENT_TIMESTAMP,%d,?,?' % timestamp), (channel, value))

datastore = DataStore_SQLite('sqlite:///QuickTourTestData.db', table="testdata", table_format=QuickTourTestDataFormat())

Analysis Components

SlowPy provides commonly used data objects such as histograms and graphs. These objects can be directly written to the database using the SlowPy Daabase Interface described above.

Histogram

import slowpy as slp
hist = slp.Histogram(100, 0, 10)

device = ...
datastore = ...

while not ControlSystem.is_stop_requested():
  value = device.read(...
  hist.fill(value)
  data_store.append(hist, tag="test_hist")

data_store.append(hist, tag=name) will create a time-series of histograms (one histogram for each time point). To keep only the latest histogram, use data_store.update(hist, tag=name) instead.

Graph

import slowpy as slp
graph = slp.Graph(['channel', 'value'])

while not ControlSystem.is_stop_requested():
  for ch in range(n_ch):
    value = device.read(ch, ...
    graph.fill(ch, value)
  data_store.append(graph, tag="test_graph")

data_store.append(graph, tag=name) will create a time-series of graphs (one graph for each time point). To keep only the latest graph, use data_store.update(graph, tag=name) instead.

Trend

import slowpy as slp
rate_trend = slp.RateTrend(length=300, tick=10)

while not ControlSystem.is_stop_requested():
  value = device.read(...
  rate_trend.fill(time.time())

  data_store.append(rate_trend.time_series('test_rate'))

Scpizing Your Device

If your device has some programming capability, such as Raspberry-Pi GPIO, an easy way to integrate it into a SlowDash system is to implement the SCPI interface on it. This approach is also useful to integrate other non-ethernet devices (such as USB) connected to a remote computer.

Here is an example to wrap a device with the ethernet-SCPI capability:

from slowpy.control import ScpiServer, ScpiAdapter, RandomWalkDevice

class RandomWalkScpiDevice(ScpiAdapter):
    def __init__(self)
        super().__init__(idn='RandomWalk')
        self.device = RandomWalkDevice(n=2)

    def do_command(self, cmd_path, params):
        '''
        parameters:
          cmd_path: array of strings, SCPI command splitted by ':'
          params: array of strings, SCPI command parameters splited by ','
        return value: reply text (even if empty) or None if command is not recognized
        '''
        
        # Implemented Commands: "V0 Value" "V1 Value" "MEASure:V0?" "MEASure:V1?"
        # Common SCPI commands, such as "*IDN?", are implemented in the parent ScpiAdapter class
        if len(params) == 1 and cmd_path[0].startswith('V'):
            try:
                if cmd_path[0] == 'V0':
                    self.device.write(0, float(params[0]))
                elif cmd_path[0] == 'V1':
                    self.device.write(1, float(params[0]))
                else:
                    self.push_error(f'invalid command {cmd_path[0]}')
            except:
                self.push_error(f'bad parameter value: {params[0]}')
            return ''
            
        elif len(cmd_path) == 2 and cmd_path[0].startswith('MEAS'):
            if cmd_path[1] == 'V0?':
                return self.device.read(0)
            elif cmd_path[1] == 'V1?':
                return self.device.read(1)
            else:
                self.push_error(f'invalid command {cmd_path[0]}')
            return ''
            
        return None
        
device = RandomWalkScpiDevice()
server = ScpiServer(device, port=17674)
server.start()

Make this code start automatically on PC boot in your favorite way (/etc/rc.local, Docker, …).

if SlowPy Control Nodes are already available for the device, the nodes can be directly mapped to the SCPI interface:

from slowpy.control import ControlSystem, ScpiServer, ScpiAdapter

ControlSystem.import_control_module('DummyDevice')
device = ControlSystem().randomwalk_device()

adapter = ScpiAdapter(idn='RandomWalk')
adapter.bind_nodes([
    ('CONFIGure:WALK', device.walk().setpoint(limits=(0,None))),
    ('CONFIGure:DECAY', device.decay().setpoint(limits=(0,1))),
    ('V0', device.ch(0).setpoint()),
    ('V1', device.ch(1).setpoint()),
    ('MEASure:V0', device.ch(0).readonly()),
    ('MEASure:V1', device.ch(1).readonly()),
])

server = ScpiServer(adapter, port=17674)
server.start()

SlowTask: GUI-Script Binding

SlowTask is a user Python script placed under the SlowDash config directory with a name like slowtask-XXX.py. SlowDash GUI can start/stop the script, call functions defined in the script, and bind control variables in the script to GUI elements. The SlowPy library was designed to be used in user task scripts, but this is not a requirement.

Starting, Stopping and Reloading the Slow-Task Scripts

SlowTask script files (such as slowtask-test.py) are placed under the project configuration directory. By default, tasks are “listed” in the Task Manager panel of SlowDash web page, but they will not be “loaded” until the start button is clicked. Auto-loading can be set by making an entry in the SlowDash configuration file (SlowdashProject.yaml):

slowdash_project:
  ...

  task:
    name: test
    auto_load: true
    parameters:
      default_ramping_speed: 0.1

  system:
    our_security_is_perfect: false

The parameters are optional, and if given, these will be passed to the _initialize(params) function of the script (described later).

Setting system/our_security_is_peferct to true will enable editing of the Python scripts on the SlowDash web page. While this is convenient, be aware what it means. Python scripts can do anything, such as system("rm *"). Enable this feature only when the access is strictly controlled. Some careful systems run processes only in Docker containers, where the container system runs on a virtual machine (in case the container is cracked) inside a firewall which accepts only VPN or SSH accesses from listed addresses. On top of that, in order to prevent unintended destruction by novice operators, it might be safer to run two instances of SlowDash, one with full features and one with limited operations (without or only with selected tasks); the slowdash command have an option to specify which configuration file to use.

Callback Functions (Command Task)

Functions in SlowTask scripts can be called from SlowDash GUI. If a script (of name test) has a function like this

def destroy_apparatus():
  #... do your work here

This can be called from SlowDash GUI in several ways.

HTML Form Panel

From a HTML form panel in SlowPlot, clicking a <button> with a name of the task module and the function will call the function:

<form>
  <input type="submit" name="test.destroy_apparatus()" value="Finish">
</form>

Here the parenthesis at the and of the button name indicate that this button is bound to a SlowTask function.

SlowTask functions can take parameters:

def set_V0(voltage:float, ramping:float):
  #... do your work here

and these parameter values are taken from the form input values:

<form>
  Voltage: <input type="number" name="voltage" value="0"><br>
  Ramping: <input type="number" name="ramping" value="1"><br>
  <input type="submit" name="test.set_V0()" value="Set">
</form>

By using the type notation in the SlowTask function as shown above, the parameter values are converted to the specified types. If the value literal cannot be converted to the specified type, an error response will be returned to the browser and the SlowTask function will not be called.

The SlowTask function can be either the standard def or async def.

It is possible to place multiple buttons in one form:

<form>
  Ramping: <input type="number" name="ramping" value="1" style="width:5em">/sec
  <p>
  V0: <input type="number" name="V0" value="0"><input type="submit" name="parallel test.set_V0()" value="Set"><br>
  V1: <input type="number" name="V1" value="0"><input type="submit" name="parallel test.set_V1()" value="Set"><br>
  V2: <input type="number" name="V2" value="0"><input type="submit" name="parallel test.set_V2()" value="Set"><br>
  V3: <input type="number" name="V3" value="0"><input type="submit" name="parallel test.set_V3()" value="Set"><br>
  <p>
  <input type="submit" name="test.set_all()" value="Set All">
  <input type="submit" name="parallel test.stop()" value="Stop Ramping"><br>    
</form>

def set_V0(V0:float, ramping:float):
  #...

def set_V1(V1:float, ramping:float):
  #...

#...

The input values on the Web form will be bound to the function parameters based on the names (<input name="V0"> will be bound to the V0 parameter).

In the example above, some functions have the parallel qualifier: by default, if a previous function call is in execution, a next action cannot be accepted to avoid multi-threading issues in the user code. The parallel qualifier indicates that this function can run in parallel to others. Another common qualifier is await, which instructs the web browser to wait for completion of the function execution before doing any other things (therefore the browser will look frozen).

Canvas Panel

On a canvas panel, a button to call a task can be placed by:

{
    ...
    "items": [
        {
            "type": "button",
            "attr": { "x":400, "y": 630, "width": 130, "label": "Finish Experiment" },
            "action": { "submit": { "name": "test.destroy_apparatus()" } }
        },
        ...

For functions with parameters, a form can be used:

{
    ...
    "items": [
        {
            "type": "valve", 
            "attr": { ... },
            "metric": { "channel": "sccm.GasInlet", ... },
            "action": { "form": "FlowControl" }
        },
        ...
    ],
    "forms": {
        "FlowControl": {
            "title": " Injection Flow",
            "inputs": [
                { "name": "flow", "label": "Set-point (sccm)", "type": "number", "initial": 15, "step": 0.1 }
            ],
            "buttons": [
                { "name": "ATDS.set_flow()", "label": "Apply Set-point" },
                { "name": "ATDS.stop_flow()", "label": "Close Valve" }
            ]
        }
    }
    ...

For more details, see the UI Panels section.

Thread Functions (Routine Task)

If a SlowTask script has functions of _initialize(params), _finalize(), _run(), and/or _loop(), one dedicated thread will be created and these function are called in a sequence:

These functions can be either the standard def or async def.

Control Variable Exporting

If _export() is defined in the SlowTask script, control variables listed in its return value become accessible by other SlowDash components (typically web browsers), in a very simlar way as the data stored in data sources (typically databases), except that only the “curent” values are available.

def _export():
    return [
      ('V0', V0),
      ('V1', V1),
      ('V2', V2),
      ('V3', V3)
    ]

To export a variable that is not a control node, make a temporary control node variable to wrap it:

class RampingStatusNode(ControlNode):
    def get(self):
        return {
            'columns': [ 'Channel', 'Value', 'Ramping' ],
            'table': [
                [ 'Ch0', float(V0), 'Yes' if V0.ramping().status().get() else 'No' ],
                [ 'Ch1', float(V1), 'Yes' if V1.ramping().status().get() else 'No' ],
                [ 'Ch2', float(V2), 'Yes' if V2.ramping().status().get() else 'No' ],
                [ 'Ch3', float(V3), 'Yes' if V3.ramping().status().get() else 'No' ]
            ]
        }
    
def _export():
    return [
        ('V0', V0),
        ('V1', V1),
        ('V2', V2),
        ('V3', V3),
        ('Status', StatusNode())
    ]

Control Variable Binding (Streaming / Live Updating)

If an instance of ValueNode (which typically holds a value but not associated to any external device or object) is exported, it can push the value to receivers (typically web browsers).

import asyncio
from slowpy.control import ValueNode

x = ValueNode(initial_value=0)

async def _loop():
    x = V0.get()
    await x.deliver()
    await asyncio.sleep(1)

The deliver() method calls .get() and publish the value to receivers. As deliver() is an async function, it must be used in an async def and the return value must be awaited.

The exported variables can be “bound” in browser. If a SlowTask control variable is bound in SlowDash HTML, changes to the variable on the browser calls the .set() method of the SlowTask variable.

<form>
  <input type="range" min="0.1" max="9.9" step="0.1" sd-value="scope.fx" sd-live="true">
  <span sd-value="scope.fx" style="font-size:150%"></span>
  ...
</form>

# slowtask-scope.py

fx = ValueNode(initial_value=0)

def _export():
  return [ ('fx', fx) ]

...

In HTML, sd-live="true" indicates that changes of the value on the browser will trigger calling the .set() method of the bound variable.

An example can be found in ExampleProjects/Streaming.

Distributed System / Network Deployment

SlowDash Interconnect

SlowTask Scpization