# Channels

The channel classes represent the mechanism for the user to send messages into the network and receive messages from it. A channel needs a context to function; the context represents a node in the network.

## Slots

To be able to route messages to the right channel, channels are associated with a slot (a string name). This slot consists of a user-defined base name and an extension to tell the message type and messaging pattern. Messages send from a channel with slot X, are only received by channels with the same slot X. Slots are case insensitive.

## Messaging patterns

Yoton supports three base messaging patterns. For each messaging pattern there are specific channel classes. All channels derive from yoton.BaseChannel.

publish/subscribe The yoton.PubChannel class is used for sending messages into the network, and the yoton.SubChannel class is used to receiving these messages. Multiple PubChannels and SubChannels can exist in the same network at the same slot; the SubChannels simply collect the messages send by all PubChannels.

request/reply The yoton.ReqChannel class is used to do requests, and the yoton.RepChannel class is used to reply to requests. If multiple ReqChannels are present at the same slot, simple load balancing is performed.

state The yoton.StateChannel class is used to communicate state to other state channels. Each yoton.StateChannel can set and get the state.

## Message types

Messages are of a specific type (text, binary, …), the default being Unicode text. The third (optional) argument to a Channel’s initializer is a MessageType object that specifies how messages should be converted to bytes and the other way around.

This way, the channels classes themself can be agnostic about the message type, while the user can implement its own MessageType class to send whatever messages he/she likes.

class `yoton.``BaseChannel`(context, slot_base, message_type=yoton.TEXT)

  Inherits from object

  Abstract class for all channels.

  Parameters

  contextyoton.Context instance

    The context that this channel uses to send messages in a network.

  slot_basestring

    The base slot name. The channel appends an extension to indicate message type and messaging pattern to create the final slot name. The final slot is used to connect channels at different contexts in a network

  message_typeyoton.MessageType instance

    (default is yoton.TEXT) Object to convert messages to bytes and bytes to messages. Users can create their own message_type class to enable communicating any type of message they want.

  Details

  Messages send via a channel are delivered asynchronically to the corresponding channels.

  All channels are associated with a context and can be used to send messages to other channels in the network. Each channel is also associated with a slot, which is a string that represents a kind of address. A message send by a channel at slot X can only be received by a channel with slot X.

  Note that the channel appends an extension to the user-supplied slot name, that represents the message type and messaging pattern of the channel. In this way, it is prevented that for example a PubChannel can communicate with a RepChannel.

  PROPERTIES

  `closed`

    Get whether the channel is closed.

  `pending`

    Get the number of pending incoming messages.

  `received`

    Signal that is emitted when new data is received. Multiple arrived messages may result in a single call to this method. There is no guarantee that recv() has not been called in the mean time. The signal is emitted with the channel instance as argument.

  `slot_incoming`

    Get the incoming slot name.

  `slot_outgoing`

    Get the outgoing slot name.

  METHODS

  `close`()

    Close the channel, i.e. unregisters this channel at the context. A closed channel cannot be reused.

    Future attempt to send() messages will result in an IOError being raised. Messages currently in the channel’s queue can still be recv()’ed, but no new messages will be delivered at this channel.

class `yoton.``PubChannel`(context, slot_base, message_type=yoton.TEXT)

  Inherits from BaseChannel

  The publish part of the publish/subscribe messaging pattern. Sent messages are received by all yoton.SubChannel instances with the same slot.

  There are no limitations for this channel if events are not processed.

  Parameters

  contextyoton.Context instance

    The context that this channel uses to send messages in a network.

  slot_basestring

    The base slot name. The channel appends an extension to indicate message type and messaging pattern to create the final slot name. The final slot is used to connect channels at different contexts in a network

  message_typeyoton.MessageType instance

    (default is yoton.TEXT) Object to convert messages to bytes and bytes to messages. Users can create their own message_type class to let channels any type of message they want.

  METHODS

  `send`(message)

    Send a message over the channel. What is send as one message will also be received as one message.

    The message is queued and delivered to all corresponding SubChannels (i.e. with the same slot) in the network.

class `yoton.``SubChannel`(context, slot_base, message_type=yoton.TEXT)

  Inherits from BaseChannel

  The subscribe part of the publish/subscribe messaging pattern. Received messages were sent by a yoton.PubChannel instance at the same slot.

  This channel can be used as an iterator, which yields all pending messages. The function yoton.select_sub_channel can be used to synchronize multiple SubChannel instances.

  If no events being processed this channel works as normal, except that the received signal will not be emitted, and sync mode will not work.

  Parameters

  contextyoton.Context instance

    The context that this channel uses to send messages in a network.

  slot_basestring

    The base slot name. The channel appends an extension to indicate message type and messaging pattern to create the final slot name. The final slot is used to connect channels at different contexts in a network

  message_typeyoton.MessageType instance

    (default is yoton.TEXT) Object to convert messages to bytes and bytes to messages. Users can create their own message_type class to let channels any type of message they want.

  METHODS

  `next`()

    Return the next message, or raises StopIteration if non available.

  `recv`(block=True)

    Receive a message from the channel. What was send as one message is also received as one message.

    If block is False, returns empty message if no data is available. If block is True, waits forever until data is available. If block is an int or float, waits that many seconds. If the channel is closed, returns empty message.

  `recv_all`()

    Receive a list of all pending messages. The list can be empty.

  `recv_selected`()

    Receive a list of messages. Use only after calling yoton.select_sub_channel with this channel as one of the arguments.

    The returned messages are all received before the first pending message in the other SUB-channels given to select_sub_channel.

    The combination of this method and the function select_sub_channel enables users to combine multiple SUB-channels in a way that preserves the original order of the messages.

  `set_sync_mode`(value)

    Set or unset the SubChannel in sync mode. When in sync mode, all channels that send messages to this channel are blocked if the queue for this SubChannel reaches a certain size.

    This feature can be used to limit the rate of senders if the consumer (i.e. the one that calls recv()) cannot keep up with processing the data.

    This feature requires the yoton event loop to run at the side of the SubChannel (not necessary for the yoton.PubChannel side).

`yoton.``select_sub_channel`(channel1, channel2, ...)

  Returns the channel that has the oldest pending message of all given yoton.SubCannel instances. Returns None if there are no pending messages.

  This function can be used to read from SubCannels instances in the order that the messages were send.

  After calling this function, use channel.recv_selected() to obtain all messages that are older than any pending messages in the other given channels.

class `yoton.``ReqChannel`(context, slot_base)

  Inherits from BaseChannel

  The request part of the request/reply messaging pattern. A ReqChannel instance sends request and receive the corresponding replies. The requests are replied by a yoton.RepChannel instance.

  This class adopts req/rep in a remote procedure call (RPC) scheme. The handling of the result is done using a yoton.Future object, which follows the approach specified in PEP 3148. Note that for the use of callbacks, the yoton event loop must run.

  Basic load balancing is performed by first asking all potential repliers whether they can handle a request. The actual request is then send to the first replier to respond.

  Parameters

  contextyoton.Context instance

    The context that this channel uses to send messages in a network.

  slot_basestring

    The base slot name. The channel appends an extension to indicate message type and messaging pattern to create the final slot name. The final slot is used to connect channels at different contexts in a network

  Usage

  One performs a call on a virtual method of this object. The actual method is executed by the yoton.RepChannel instance. The method can be called with normal and keyword arguments, which can be (a combination of): None, bool, int, float, string, list, tuple, dict.

  Example

      # Fast, but process is idling when waiting for the response.
      reply = req.add(3,4).result(2.0) # Wait two seconds

      # Asynchronous processing, but no waiting.
      def reply_handler(future):
          ... # Handle reply
      future = req.add(3,4)
      future.add_done_callback(reply_handler)

class `yoton.``RepChannel`(context, slot_base)

  Inherits from BaseChannel

  The reply part of the request/reply messaging pattern. A RepChannel instance receives request and sends the corresponding replies. The requests are send from a yoton.ReqChannel instance.

  This class adopts req/rep in a remote procedure call (RPC) scheme.

  To use a RepChannel, subclass this class and implement the methods that need to be available. The reply should be (a combination of) None, bool, int, float, string, list, tuple, dict.

  This channel needs to be set to event or thread mode to function (in the first case yoton events need to be processed too). To stop handling events again, use set_mode(‘off’).

  Parameters

  contextyoton.Context instance

    The context that this channel uses to send messages in a network.

  slot_basestring

    The base slot name. The channel appends an extension to indicate message type and messaging pattern to create the final slot name. The final slot is used to connect channels at different contexts in a network

  METHODS

  `echo`(arg1, sleep=0.0)

    Default procedure that can be used for testing. It returns a tuple (first_arg, context_id)

  `set_mode`(mode)

    Set the replier to its operating mode, or turn it off.

    Modes:

      - 0 or ‘off’: do not process requests

      - 1 or ‘event’: use the yoton event loop to process requests

      - 2 or ‘thread’: process requests in a separate thread

class `yoton.``Future`(req_channel, req, request_id)

  Inherits from object

  The Future object represents the future result of a request done at a yoton.ReqChannel.

  It enables:

    - checking whether the request is done.

    - getting the result or the exception raised during handling the request.

    - canceling the request (if it is not yet running)

    - registering callbacks to handle the result when it is available

  METHODS

  `add_done_callback`(fn)

    Attaches the callable fn to the future. fn will be called, with the future as its only argument, when the future is cancelled or finishes running.

    Added callables are called in the order that they were added. If the callable raises a Exception subclass, it will be logged and ignored. If the callable raises a BaseException subclass, the behavior is undefined.

    If the future has already completed or been cancelled, fn will be called immediately.

  `cancel`()

    Attempt to cancel the call. If the call is currently being executed and cannot be cancelled then the method will return False, otherwise the call will be cancelled and the method will return True.

  `cancelled`()

    Return True if the call was successfully cancelled.

  `done`()

    Return True if the call was successfully cancelled or finished running.

  `exception`(timeout)

    Return the exception raised by the call. If the call hasn’t yet completed then this method will wait up to timeout seconds. If the call hasn’t completed in timeout seconds, then a TimeoutError will be raised. timeout can be an int or float. If timeout is not specified or None, there is no limit to the wait time.

    If the future is cancelled before completing then CancelledError will be raised.

    If the call completed without raising, None is returned.

  `result`(timeout=None)

    Return the value returned by the call. If the call hasn’t yet completed then this method will wait up to timeout seconds. If the call hasn’t completed in timeout seconds, then a TimeoutError will be raised. timeout can be an int or float. If timeout is not specified or None, there is no limit to the wait time.

    If the future is cancelled before completing then CancelledError will be raised.

    If the call raised, this method will raise the same exception.

  `result_or_cancel`(timeout=1.0)

    Return the value returned by the call. If the call hasn’t yet completed then this method will wait up to timeout seconds. If the call hasn’t completed in timeout seconds, then the call is cancelled and the method will return None.

  `running`()

    Return True if the call is currently being executed and cannot be cancelled.

  `set_auto_cancel_timeout`(timeout)

    Set the timeout after which the call is automatically cancelled if it is not done yet. By default, this value is 10 seconds.

    If timeout is None, there is no limit to the wait time.

  `set_exception`(exception)

    Sets the result of the work associated with the Future to the Exception exception. This method should only be used by Executor implementations and unit tests.

  `set_result`(result)

    Sets the result of the work associated with the Future to result. This method should only be used by Executor implementations and unit tests.

  `set_running_or_notify_cancel`()

    This method should only be called by Executor implementations before executing the work associated with the Future and by unit tests.

    If the method returns False then the Future was cancelled, i.e. Future.cancel() was called and returned True.

    If the method returns True then the Future was not cancelled and has been put in the running state, i.e. calls to Future.running() will return True.

    This method can only be called once and cannot be called after Future.set_result() or Future.set_exception() have been called.

class `yoton.``StateChannel`(context, slot_base, message_type=yoton.TEXT)

  Inherits from BaseChannel

  Channel class for the state messaging pattern. A state is synchronized over all state channels of the same slot. Each channel can send (i.e. set) the state and recv (i.e. get) the current state. Note however, that if two StateChannel instances set the state around the same time, due to the network delay, it is undefined which one sets the state the last.

  The context will automatically call this channel’s send_last() method when a new context enters the network.

  The recv() call is always non-blocking and always returns the last received message: i.e. the current state.

  There are no limitations for this channel if events are not processed, except that the received signal is not emitted.

  Parameters

  contextyoton.Context instance

    The context that this channel uses to send messages in a network.

  slot_basestring

    The base slot name. The channel appends an extension to indicate message type and messaging pattern to create the final slot name. The final slot is used to connect channels at different contexts in a network

  message_typeyoton.MessageType instance

    (default is yoton.TEXT) Object to convert messages to bytes and bytes to messages. Users can create their own message_type class to let channels any type of message they want.

  METHODS

  `recv`(block=False)

    Get the state of the channel. Always non-blocking. Returns the most up to date state.

  `send`(message)

    Set the state of this channel.

    The state-message is queued and send over the socket by the IO-thread. Zero-length messages are ignored.

  `send_last`()

    Resend the last message.