Is there a reason you’re using ETS instead of Redis?
You can use Phoenix Presence and push joins/leaves to Redis directly.
Here’s an example of using Redis to track users and user counts.
SADD/SREM handle adding/removing users to a set, while INCRBY/DECRBY update the count.
def handle_metas(topic, %{joins: joins, leaves: leaves}, _presences, state) do
key_id = "#{topic}:users"
key_count = "#{topic}:count"
commands =
Enum.flat_map(joins, fn {id, _} -> [["SADD", key_id, id], ["INCRBY", key_count, 1]] end) ++
Enum.flat_map(leaves, fn {id, _} -> [["SREM", key_id, id], ["DECRBY", key_count, 1]] end)
Task.start(fn -> Redix.pipeline(:redix, commands) end)
{:ok, state}
end
If you want to assign users to room* topics, you could also use a Redis sorted set. Store room IDs as members and their user counts as scores:
ZADD rooms 65 room1
ZADD rooms 54 room2
You can then use ZRANGE or ZPOPMIN to find a room under the threshold (e.g., < 70) and assign users accordingly.
