CircuitJS supports reading and writing of internal circuit data already, as the demo in jsinterface.html showcases. There is also a shim that connects the CircuitJS JavaScript functionality by exposing it to a WebSocket interface. This is shown in websocket/circuitws.html.

The CircuitJS circuit simulator connects to a given target URI using a WebSocket and then expects to receive JSON-formatted commands; similarly, JSON-formatted responses are generated. There are multiple scenarios in which this can be useful, but generally it allows remote controlling a browser-run simulation to be controlled and read out from any programming language.

You can start by copying circuitws.html and circuitws.js to the war/ directory, then serve it from there. You then need to invoke the URI by adding a ws= query string parameter. The code expects to find a WebSocket end point there and will connect to it (and reconnect if the connection is lost).

Additionally, you can supply a src= query parameter which is the initial src attribute of the iframe. If not supplied, a sensible default is chosen for you so that the page always loads with a simulator.

Lastly, you can specify an autoshutoff query parameter which tells the WebSocket shim that once it has once finished a WebSocket connection, it should automatically shut down. This is useful if your server is short-lived and may crash so that you don't end up a bunch of different (possibly competing) simulators in different browser tabs).

For example, assume you're the war/ subdirectory at https://127.0.0.1:8123, then you could have a WebSocket endpoint listening on port 4444 as ws:https://127.0.0.1:4444/ws. You would then open your web browser with the following URI:

https://127.0.0.1:8123/circuitws.html?ws=ws%3A%2F%2F127.0.0.1%3A4444%2Fws

The simulator will start up and immediately connect to ws:https://127.0.0.1:4444/ws, from which it expects commands.

The command structure is always as follows:

{
	"cmd":		"cmdname",
	"msgid":	1234,
	"foo":		"bar"
}

Every command needs to have the cmd key set in the dictionary. The msgid is optional, but will be reflected in the response so that it is easy to correlate requests with their corresponding responses, even if responses are sent out-of-order.

Additional parameters may be required for some commands. These are commands that are understood, along with examples:

• status: Query the state of the simulator.
• set_running: Start or stop the simulator (depending on the boolean argument in state)
• reload: Reload the iframe. Supports an additional args command, which is a dictionary that contains the query parameters for the new iframe. The source cannot be modified.
• set_timestamp: Sets the timestamp to the double value in the timestep key.
• get_node_voltage: Query the node voltage of nodes. Expects a list of named nodes in the nodes element.
• get_elements: List all elements of the circuit.
• set_ext_voltage: Sets external voltage sources to their values. Expects a voltages dictionary as parameter that contains name/value pairs.
• get_svg: Returns the circuit as a SVG image. The result is returned asynchronously as the "svg_rendered" event.
• circuit_export: Returns the cirucit schematic.
• circuit_import: Loads a circuit, provided a textual representation in the circuit element.

If you have Python3.10+ and the module aiohttp installed, you can easily try the example. It expects the war directory to be served at port 8123 and will open a websocket listening port on 8080:

$ python3 circuitws_server.py
Point your browser to: https://127.0.0.1:8123/circuitws.html?ws=ws%3A%2F%2F127.0.0.1%3A8080%2Fws
======== Running on https://127.0.0.1:8080 ========
(Press CTRL+C to quit)

Then, Python will wait for the user to open the URL. Once that happens, a CLI console starts:

Websocket connected.
Cmd:

You can now enter commands, such as ?, start, stop, svg, etc. If you type help you'll get an overview of what is supported.

Cmd: ?
{'type': 'response', 'status': 'ok', 'cmd': 'status', 'msgid': 1, 'data': {'running': True, 'time': 1.0127750000024012, 'timestep': 5e-06}}
Cmd: stop
{'type': 'response', 'status': 'ok', 'cmd': 'set_running', 'msgid': 2}
Cmd: ?
{'type': 'response', 'status': 'ok', 'cmd': 'status', 'msgid': 3, 'data': {'running': False, 'time': 1.0562150000026858, 'timestep': 5e-06}}
Cmd: ?
{'type': 'response', 'status': 'ok', 'cmd': 'status', 'msgid': 4, 'data': {'running': False, 'time': 1.0562150000026858, 'timestep': 5e-06}}
Cmd: start
{'type': 'response', 'status': 'ok', 'cmd': 'set_running', 'msgid': 5}
Cmd: ?
{'type': 'response', 'status': 'ok', 'cmd': 'status', 'msgid': 6, 'data': {'running': True, 'time': 1.0699000000027754, 'timestep': 5e-06}}
Cmd: sev1
{'type': 'response', 'status': 'ok', 'cmd': 'set_ext_voltage', 'msgid': 7}
Cmd: help
Not understood. Commands: ?, start, stop, setts, gnv, list, sev1, sev2, svg, export, import, q