// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

// Package eventbus provides an in-process event bus.
//
// The event bus connects publishers of typed events with subscribers
// interested in those events.
//
// # Usage
//
// To publish events, use [PublisherOf] to get a typed publisher for
// your event type, then call [Publisher.Publish] as needed. If your
// event is expensive to construct, you can optionally use
// [Publisher.ShouldPublish] to skip the work if nobody is listening
// for the event.
//
// To receive events, first use [Bus.Queue] to create an event
// delivery queue, then use [Subscribe] to get a [Subscriber] for each
// event type you're interested in. Receive the events themselves by
// selecting over all your [Subscriber.Chan] channels, as well as
// [Queue.Done] for shutdown notifications.
//
// # Concurrency properties
//
// The bus serializes all published events, and preserves that
// ordering when delivering to subscribers that are attached to the
// same Queue. In more detail:
//
//   - An event is published to the bus at some instant between the
//     start and end of the call to [Publisher.Publish].
//   - Events cannot be published at the same instant, and so are
//     totally ordered by their publication time. Given two events E1
//     and E2, either E1 happens before E2, or E2 happens before E1.
//   - Queues dispatch events to their Subscribers in publication
//     order: if E1 happens before E2, the queue always delivers E1
//     before E2.
//   - Queues do not synchronize with each other: given queues Q1 and
//     Q2, both subscribed to events E1 and E2, Q1 may deliver both E1
//     and E2 before Q2 delivers E1.
//
// Less formally: there is one true timeline of all published events.
// If you make a Queue and subscribe to events on it, you will receive
// those events one at a time, in the same order as the one true
// timeline. You will "skip over" events you didn't subscribe to, but
// your view of the world always moves forward in time, never
// backwards, and you will observe events in the same order as
// everyone else.
//
// However, you cannot assume that what your subscribers on your queue
// see as "now" is the same as what other subscribers on other
// queues. Their queue may be further behind you in the timeline, or
// running ahead of you. This means you should be careful about
// reaching out to another component directly after receiving an
// event, as its view of the world may not yet (or ever) be exactly
// consistent with yours.
//
// To make your code more testable and understandable, you should try
// to structure it following the actor model: you have some local
// state over which you have authority, but your only way to interact
// with state elsewhere in the program is to receive and process
// events coming from elsewhere, or to emit events of your own.
//
// # Expected subscriber behavior
//
// Subscribers are expected to promptly receive their events on
// [Subscriber.Chan]. The bus has a small, fixed amount of internal
// buffering, meaning that a slow subscriber will eventually cause
// backpressure and block publication of all further events.
//
// In general, you should receive from your subscriber(s) in a loop,
// and only do fast state updates within that loop. Any heavier work
// should be offloaded to another goroutine.
//
// Causing publishers to block from backpressure is considered a bug
// in the slow subscriber causing the backpressure, and should be
// addressed there. Publishers should assume that Publish will not
// block for extended periods of time, and should not make exceptional
// effort to behave gracefully if they do get blocked.
//
// These blocking semantics are provisional and subject to
// change. Please speak up if this causes development pain, so that we
// can adapt the semantics to better suit our needs.
//
// # Debugging facilities
//
// (TODO, not implemented yet, sorry, I promise we're working on it next!)
//
// The bus comes with introspection facilities to help reason about
// the state of the client, and diagnose issues such as slow
// subscribers.
//
// The bus provide a tsweb debugging page that shows the current state
// of the bus, including all publishers, subscribers, and queued
// events.
//
// The bus also has a snooping and tracing facility, which lets you
// observe all events flowing through the bus, along with their
// source, destination(s) and timing information such as the time of
// delivery to each subscriber and end-to-end bus delays.
package eventbus