Initial design proposal for message routing (#245)

* Add design doc for routing

* rename

* add explanations

* Update 02 - Message routing.md

* add examples, rename to type

* add naming clarification

* add regex

* update spec based on feedback

* Remove open question

docs/design/02 - Topics.md

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+# Topics
+
+This document describes the semantics and components of publishing messages and subscribing to topics.
+
+## Overview
+
+Topics are used as the primitive to manage which agents receive a given published message. Agents subscribe to topics. There is an application defined mapping from topic to agent instance.
+
+These concepts intentionally map to the [CloudEvents](https://cloudevents.io/) specification. This allows for easy integration with existing systems and tools.
+
+### Non-goals
+
+This document does not specify RPC/direct messaging
+
+## Identifiers
+
+A topic is identified by two components (called a `TopicId`):
+
+- [`type`](https://github.com/cloudevents/spec/blob/v1.0.2/cloudevents/spec.md#type) - represents the type of event that occurs, this is static and defined in code
+    - SHOULD use reverse domain name notation to avoid naming conflicts. For example: `com.example.my-topic`.
+- [`source`](https://github.com/cloudevents/spec/blob/v1.0.2/cloudevents/spec.md#source-1) - represents where the event originated from, this is dynamic and based on the message itself
+    - SHOULD be a URI
+
+Agent instances are identified by two components (called an `AgentId`):
+
+- `type` - represents the type of agent, this is static and defined in code
+    - MUST be a valid identifier as defined [here](https://docs.python.org/3/reference/lexical_analysis.html#identifiers) except that only the ASCII range is allowed
+- `key` - represents the instance of the agent type for the key
+    - SHOULD be a URI
+
+For example: `GraphicDesigner:1234`
+
+## Subscriptions
+
+Subscriptions define which agents receive messages published to a topic. Subscriptions are dynamic and can be added or removed at any time.
+
+A subscription defines two things:
+
+- Matcher func of type `TopicId -> bool`, telling us "does this subscription match this topic"
+- Mapper func of type `TopicId -> AgentId`, telling us "given this subscription matches this topic, which agent does it map to"
+
+These functions MUST be be free of side effects such that the evaluation can be cached.
+
+### Agent instance creation
+
+If a message is received on a topic that maps to an agent that does not yet exist the runtime will instantiate an agent to fullfil the request.
+
+## Message types
+
+Agents are able to handle certain types of messages. This is an internal detail of an agent's implementation. All agents in a channel will receive all messages, but will ignore messages that it cannot handle.
+
+> [!NOTE]
+> This might be revisited based on scaling and performance considerations.