Tempo's supervision tree: every process Tempo runs is started under it.

As it starts, it reads the time zone database the application configures (Tempo.time_zone_database/0) and builds the table of every zone the database lists the changes of, once, in :persistent_term: through 2100 for tz, whose rule-made changes are listed with its compiled ones, in about 40 ms. The database is Tempo's for as long as the tree runs: a database configured or updated afterwards is Tempo's once it is restarted.

A question past the end of a zone's table, of a zone the database lists no changes for, or of a database that lists none, is answered by a table built from the database on the first such question and grown where a question reaches past it. Those tables are kept in an ETS table owned by the tree's one process.

Supervision

Tempo's application starts this supervisor, so an application that adds :ex_tempo to its dependencies and does nothing more has Tempo's processes running:

def deps do
  [{:ex_tempo, "~> 2.5"}]
end

An application that supervises Tempo itself sets runtime: false, so that Tempo's application does not start, and lists Tempo.Supervisor in its own supervision tree. Tempo's application would have started Localize, Calendrical and Astro, so the host starts them: listing :calendrical starts Localize and Astro before it, as Calendrical's calendars read the ephemeris Astro loads.

def deps do
  [{:ex_tempo, "~> 2.5", runtime: false}]
end

def application do
  [extra_applications: [:logger, :calendrical]]
end

children = [
  Tempo.Supervisor
]

Without them, formatting fails because Localize's locale loader is not running, and a day that begins at sunset cannot be placed because Astro's ephemeris is not loaded.

The supervisor takes no options. Tempo's own processes depend on no other application having been started. Where they are not running, Tempo still answers every question of a zone, reading the zone's changes from the database each time: correctly, but tens of microseconds a question, and tens of milliseconds for a date past the end of the database's listed changes (2100 for tz).