pubsub

PubSub[A]: a hub that delivers every published message to every subscriber.

A subscriber reads the messages published after it subscribed (plus up to replay earlier ones), in order, through a Subscription that lasts until its scope closes. A message is stored once and dropped when every subscriber that owed it has read it. What happens when capacity messages are unread is fixed at creation:

• bounded: publishers wait until the slowest subscriber reads.
• dropping: the new message is dropped and the publish returns False.
• sliding: the oldest unread message is dropped for everyone.
• unbounded: the hub is never full.

A message published while nobody is subscribed goes only to the replay buffer. end(value) stops publishing: subscribers read what is left, and then every take succeeds with value. shutdown() unsubscribes everyone, interrupting their takes; later publishes return False.

Takes waiting on one subscription are served in the order they started.

Atomic is the store under a hub, with no waiting, for building a hub with PubSub.make. The unsafe_* methods act at once, outside an effect; call them on the scheduler thread of the hub's fibers.

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