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Message Passing Tree

This is a simple python message passing tree.

Overview

We think of the root of the tree as the innermost node and the leaves as the outermost nodes, so towards the root is inwards and towards the leaves is outwards. Each node in the tree is an instance of Agent. An Agent has a parent inward of it (unless it is the root of the tree) and a list of children outward of it. The Agents pass messages to each other. A message consists of a command and a list of arguments. A command is a dot separated string like chart.class.add or error.client.TypeError. When an Agent receives an inbound message from any of its children, by default it passes the message inward to its parent. When an Agent receives an outbound message from its parent, by default it passes the message outward to each of it's children.

Each Agent has a set of command filters that it subscribes to and it will only receive messages from its parent that match one of the command filters they subscribe to. For instance, if an agent is subscribed to the command filters chart and display.background then it will receive chart.class.add, chart.edge.update, and display.background.set_color but not display.set_status.

An Agent also has a set of transformers/consumers for inbound messages and a set of transformers/consumers for outbound messages. A transformer/consumer is a pair a command filter and a handler. When an Agent receives an inbound message, it looks to see whether the command matches any of the command filters that the agent transforms. If the command matches multiple command filters, the Agent always picks the most specific filter (in particular, it will only ever apply at most one transformer/consumer). If the handler applied is a transfomer, then the Agent will pass the transformed message onward. If the handler applied is a consumer, the Agent "consumes" the message and does not pass it onward.

Definitions of Terms

  • A command string is a string consisting of alphanumeric characters and dot "." or underscore "". The command should not start or end with "." and should contain no two consecutive "."'s or ""'s.

  • A message is a triple a command, an argument list, and a key-value pair list.

  • A command filter is a string which is either a command or the string "*".

  • A command filter matches a command if either (1) the command filter is "*" or (2) the command filter is equal to the command or (3) the command filter is a prefix for the command and the next character is a ".". For example, "chart" matches "chart.class.add" but not "chart_class.add".

  • The characters "." and "*" are not allowed in identiers. We map "command filters" to strings that are valid as parts of identifiers as follows: "*" => "_all" "." => "__" Note that if the command filter contains "." or "*" then the output of this transformation is not a valid command filter. We call this the associated command filter subidentifier.

  • A transformer identifier is an identifier which: (1) starts with "transform_" (2) is followed by a command filter subidentifier.

  • A transformer method is a method whose name is a transformer identifier and which takes as a message input a source agent, a command, and a collection of key-value pairs and returns a new command and a new collection of key-value pairs.

  • A consumer identifier is an identifier which: (1) starts with "consume_" (2) is followed by a command filter subidentifier.

  • An Agent class consists of a triple: subscriptions: a list of messages that the Agent subscribes to from it's parent outward_transformers and inward_transformers: maps from command filters to transformers When the agent receives a message pointing inward, it checks if it has any inward transformers with command filters that match the message command. If so, it applies the inward transformer with the most specific match to the message before passing the message to it's parent.