Communication Patterns

Wrapyfi supports the publisher-subscriber (PUB/SUB) pattern as well as the request-reply (REQ/REP) pattern. The PUB/SUB pattern assumes message arguments are passed from the publisher-calling script to the publishing method. The publisher executes the method and the subscriber (listener) merely triggers the method call, awaits the publisher to execute the method, and returns the publisher’s method returns. The REQ/REP pattern on the other hand assumes arguments from the client (requester) are sent to the server (responder or replier). Once the server receives the request, it passes the arguments to its own method, executes it, and replies to the client back with its method returns.

Communication patterns in Wrapyfi are set by passing the configuration mode argument to activate_communication method as described in the configuration documentation.

Warning

in REQ/REP, the requester transmits all arguments passed to the method as a dictionary encoded as a string. This is not ideal for predefined services, where the service expects a certain object/message type. A better approach would include the option to pass a single item of a certain value and type planned

Publishers and Listeners|Subscribers (PUB/SUB)

The publishers and listeners of the same message type should have identical constructor signatures. The current Wrapyfi version supports 4 universal message types for all middleware. The extended types such as ROSMessage and ROS2Message are exclusive to the provided middleware.

YARP:

Note

YARP publishers remain persistent. To disable persistence, pass the argument persistent=False to the @MiddlewareCommunicator.register decorator.

All messages are transmitted using the yarp Python bindings

  • Image: Transmits and receives a cv2 or numpy image using either yarp.BufferedPortImageRgb or yarp.BufferedPortImageFloat. When JPG conversion is specified, it uses a yarp.BufferedPortBottle message carrying a JPEG encoded string instead

  • AudioChunk: Transmits and receives a numpy audio chunk with the sound properties using yarp.Port transporting yarp.Sound

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays and other formats using yarp.BufferedPortBottle

  • Properties: Transmits properties planned

ROS:

Warning

ROS requires a custom message to handle audio. This message must be compiled first before using Wrapyfi with ROS Audio. Refer to these instructions for compiling Wrapyfi ROS services and messages.

All messages are transmitted using the rospy Python bindings as topic messages

  • Image: Transmits and receives a cv2 or numpy image using sensor_messages.msg.Image. When JPG conversion is specified, uses the sensor_messages.msg.CompressedImage message instead

  • AudioChunk: Transmits and receives a numpy audio chunk using wrapyfi_ros_interfaces.msg.ROSAudioMessage

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays, and other formats using std_msgs.msg.String

  • Properties: Transmits and receives parameters to/from the parameter server using the methods rospy.set_param and rospy.get_param respectively

  • ROSMessage: Transmits and receives a single ROS message per return decorator. Note that currently, only common ROS interface messages are supported and detected automatically. This means that messages defined in common interfaces such as std_msgs, geometry_msgs, and sensor_msgs can be directly returned by the method do not need to be converted to native types

ROS 2:

Warning

ROS 2 requires a custom message to handle audio. This message must be compiled first before using Wrapyfi with ROS 2 Audio. Refer to these instructions for compiling Wrapyfi ROS 2 services and messages.

All messages are transmitted using the rclpy Python bindings as topic messages

  • Image: Transmits and receives a cv2 or numpy image using sensor_messages.msg.Image. When JPG conversion is specified, uses the sensor_messages.msg.CompressedImage message instead

  • AudioChunk: Transmits and receives a numpy audio chunk using wrapyfi_ros2_interfaces.msg.ROS2AudioMessage

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays, and other formats using std_msgs.msg.String

  • Properties: Transmits properties planned

  • ROS2Message: Transmits and receives a single ROS 2 message per return decorator

ZeroMQ:

Note

ZeroMQ exchanges in REQ/REP rely on a broker with a dedicated socket. By default, Wrapyfi will not spawn a new connection to the socket when multiple threads are created. For multi-threaded applications, this leads to race conditions. We avoid that by detecting whether a new instance of the socket is available in the thread’s local storage. This multi-threading-friendly mode is enabled by passing multi_threaded=True to the @MiddlewareCommunicator.register decorator. This is only recommended when registering methods that are going to be multi-threaded.

All messages are transmitted using the zmq Python bindings. Transmission follows the proxied XPUB/XSUB pattern

  • Image: Transmits and receives a cv2 or numpy image wrapped in the NativeObject construct. Note that all Image types are transmitted as multipart messages, where the first element is the topic name and the second element is the header (e.g., timestamp), and the third element is the image itself

  • AudioChunk: Transmits and receives a numpy audio chunk wrapped in the NativeObject construct

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays and other formats using zmq context.socket(zmq.PUB).send_multipart for publishing and zmq context.socket(zmq.SUB).receive_multipart for receiving messages. The zmq.PUB socket is wrapped in a zmq.proxy to allow multiple subscribers to the same publisher. Note that all NativeObject types are transmitted as multipart messages, where the first element is the topic name and the second element is the message itself (Except for Image)

  • Properties: Transmits properties planned

Websocket:

Note

Websocket assumes a server is running on the specified address and port. The forwarding of messages can only be done manually by the user. An example server can be found here

Note

Unlike the majority of middleware supported by Wrapyfi, websockets are bidirectional, meaning that the publisher can also be a listener. This allows Wrapyfi to support multiple publishers on the same topic (namespaces and rooms)

All messages are transmitted using the python-socketio Python bindings. Transmission follows the socket.io protocol

  • Image: Transmits and receives a cv2 or numpy image wrapped in the NativeObject construct

  • AudioChunk: Transmits and receives a numpy audio chunk wrapped in the NativeObject construct

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays and other formats using socketio.emit for publishing and socketio.on for receiving messages

  • Properties: Transmits properties planned

Zenoh:

All messages are transmitted using the eclipse-zenoh Python bindings. Transmission follows the zenoh protocol

  • Image: Transmits and receives a cv2 or numpy image wrapped in the NativeObject construct as zenoh.Bytes

  • AudioChunk: Transmits and receives a numpy audio chunk wrapped in the NativeObject construct as zenoh.Bytes

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays and other formats as zenoh.Bytes using zenoh.session.key.put for publishing and an asynchronus callback for receiving messages

MQTT:

Note

MQTT runs on a public online broker by default broker.emqx.io for convenience (no setup required), however, it is recommended to use a local broker like Mosquitto for production.

All messages are transmitted using the paho-mqtt Python bindings. Transmission follows the MQTT protocol

  • Image: Transmits and receives a cv2 or numpy image wrapped in the NativeObject construct

  • AudioChunk: Transmits and receives a numpy audio chunk wrapped in the NativeObject construct

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays and other formats using client.publish for publishing and client.on_message for receiving messages

  • Properties: Transmits properties planned

Servers and Clients (REQ/REP)

The servers and clients of the same message type should have identical constructor signatures. The current Wrapyfi version supports 3 universal message types for all middleware. The extended types such as ROSMessage and ROS2Message are exclusive to the provided middleware.

YARP:

All messages are transmitted using the yarp Python bindings for RPC communication. The requester encodes its arguments as a json string supporting all native Python objects, numpy arrays, and other formats using yarp.Bottle. The requester formats its arguments as ([args], {kwargs})

  • Image: Transmits and receives a cv2 or numpy image encoded as a json string using yarp.Bottle. JPG conversion is currently not supported

  • AudioChunk: Transmits and receives a numpy audio chunk encoded as a json string using yarp.Bottle

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays, and other formats using yarp.Bottle

ROS:

Warning

ROS requires a custom service to handle audio. This service must be compiled first before using Wrapyfi with ROS Audio. Refer to these instructions for compiling Wrapyfi ROS services and messages.

All messages are transmitted using the rospy Python bindings as services. The requester encodes its arguments as a json string supporting all native Python objects, numpy arrays, and other formats using std_msgs.msg.String. The requester formats its arguments as ([args], {kwargs})

  • Image: Transmits and receives a cv2 or numpy image using sensor_messages.msg.Image JPG conversion is currently not supported

  • AudioChunk: Transmits and receives a numpy audio chunk using wrapyfi_ros_interfaces.msg.ROSAudioMessage

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays, and other formats using std_msgs.msg.String

ROS 2:

Warning

ROS 2 requires custom services to handle arbitrary messages. These services must be compiled first before using Wrapyfi in this mode. Refer to these instructions for compiling Wrapyfi ROS 2 services.

All messages are transmitted using the rclpy Python bindings as services. The requester encodes its arguments as a json string supporting all native Python objects, numpy arrays, and other formats using std_msgs.msg.String. The requester formats its arguments as ([args], {kwargs})

  • Image: Transmits and receives a cv2 or numpy image using sensor_messages.msg.Image

  • AudioChunk: Transmits and receives a numpy audio chunk using wrapyfi_ros2_interfaces.msg.ROS2AudioMessage

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays, and other formats using std_msgs.msg.String

ZeroMQ:

All messages are transmitted using the zmq Python bindings. Transmission follows the proxied XREP/XREQ pattern The requester encodes its arguments as a json string supporting all native Python objects, numpy arrays, and other formats using zmq context.socket(zmq.REQ).send_multipart. The requester formats its arguments as ([args], {kwargs})

  • Image: Transmits and receives a cv2 or numpy image wrapped in the NativeObject construct

  • AudioChunk: Transmits and receives a numpy audio chunk wrapped in the NativeObject construct

  • NativeObject: Transmits and receives a json string supporting all native Python objects, numpy arrays, and other formats using zmq context.socket(zmq.REP) for replying and zmq context.socket(zmq.REQ) for receiving messages

Publisher- and Listener-specific Arguments

Warning

Differences are expected between the returns of publishers and listeners, sometimes due to compression methods (e.g., setting jpg=True when transmitting an Image compresses the image but the encoding remains the same), intentional setting of different devices for different tensors (refer to device mapping for tensors), and differences in library versions between receiving and transmitting plugins (refer to plugins).

To direct arguments specifically toward the publisher or subscriber without exposing one or the other to the same argument values, the corresponding arguments can be added to the dictionary listener_kwargs to control the listener only, or publisher_kwargs to control the publisher only. Both dictionaries can be passed directly to the Wrapyfi decorator. Since the transmitting and receiving arguments should generally be the same regardless of the communication pattern, publisher_kwargs and listener_kwargs also apply to the servers and clients respectively.