autogen/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.