Introduction
Topical is a library for defining and serving topics. A topic is a value that can be efficiently observed in real-time by connected clients. The server-side implementation takes care of defining how to intialise the state, and then how to keep it updated.
Multiple clients can subscribe to an instance of a topic - instances are identifier by a path, which matches the route defined by a topic. Multiple instances of a topic can exist by using route placeholders.
Topical takes care of starting topic instances as needed, and stopping them once all clients have disconnected.
uses
Uses
Topics are implemented in Elixir, and behave somewhat like GenServers - the main difference being that their state is easily observable by clients. The main benefit comes from using the JavaScript (and React) client (and a WebSocket adapter) to observe the state, but it's also possible to use the Elixir API.
There's a certain amount of flexibility in how topics are implemented. They can be be used to track ephemeral state - for example user presence, or cursor positions. They can be backed by simple mechanisms like a file or file-based database. Or they can sit in front of an RDBMS (e.g., utilising Postgres notification channels), providing real-time cached views. Or they can be used to implement the event sourcing pattern.
Another use case is for implementing an incremental cache: the state of the cache is setup in the topic initalisation and then kept up to date as necessary, without having to refresh from scratch. Clients accessing the cache will be able to read up-to-date state directly from memory.