If you want Broadway to properly drain messages and perform a graceful shutdown, it should be started as a child of a supervision tree. See the bullet point “Graceful shutdown” here. Typically, people start Broadway as a child of the supervision tree of their main application.
Starting a new Broadway pipeline whenever a user joins a channel seems like overkill to me, as it is using a separate SQS queue for each user (do you create them dynamically?). Without knowing the exact problem you’re trying to solve, I would probably attempt to have a constant number of queues (1 is possible) shared by all users and a single pipeline that fetches messages and routes them to the destination channel.
