LivePet is a demo app for my talk at THAT Conference 2023 on Elixir and LiveView. The app aims to demonstrate OTP and LiveView by simulating virtual pets that users can interact with through a LiveView page.
If you want to give it a try, visit https://that-pet.fly.dev/.
LivePet is a standard Phoenix app. To start your Phoenix server:
- Run
mix setupto install and setup dependencies - Start Phoenix endpoint with
mix phx.serveror inside IEx withiex -S mix phx.server
Now you can visit localhost:4000 from your browser.
On startup, the app starts LivePet.PetInitSupervisor to start up every pet and a Registry for what processes are currently viewing a pet. Pets run under a DynamicSupervisor. DynamicSupervisors cannot be given a list of children on startup, so a separate task is started to start all the pet processes.
A lot of this is very naive and not optimized. The goal is to show off some of the power of Elixir, but not going overboard with optimization. Starting every pet is done in a single process. This becomes a bottleneck when simulating large numbers of pets. I started 120K pets and it took over a half hour to start all of them.
Each pet's state updates every five seconds. Currently, they just get older and hungrier. Persistence is handled by a separate LivePet.Pets.Persister process that persists every pet to the DB every minute. Moving the persistence out of the each pet process is a naive strategy to limit the number of DB connections. Allowing each pet to persist every X ticks can lead to DB connections being a bottleneck.
Navigating to the live view for a live pet starts registers the live view process with the pet viewers registry. On every tick, the pet process dispatches a message to all of it's viewers so that the UI can be updated with the new state.
You can visit dead pets, but the live view will not connect to a running process.
You can interact with your pets by feeding them. You can interact with live pets that other users are viewing by giving them treats. There is a GenServer that tops off each user's treats to five when the app starts and every thirty minutes while running.
The list of active pets is done using the Phoenix.Presence module.