A source that fetches each tile from a URL.

use StaticMaps.Source.HTTP and implement url/2 and StaticMaps.Source.info/1; this module supplies fetch/2 and fetch_cacheable/3:

defmodule MyApp.Tiles do
  use StaticMaps.Source.HTTP

  @impl true
  def url(%{z: z, x: x, y: y}, _opts), do: "https://tiles.example.com/#{z}/#{x}/#{y}.png"

  @impl true
  def info(_opts), do: %{attribution: "© Example", tile_size: 256, zoom: 0..19, scales: [1]}
end

To add headers a server needs, such as an API key, implement headers/2. To replace how tiles are fetched entirely, override fetch_cacheable/3: it is the one function every fetch goes through, cached or not. fetch/2 cannot be overridden, because the cache never calls it, so an override there would be silently skipped.

Every request:

• sends User-Agent: StaticMaps/<version> (+<contact>), with the contact from config :staticmaps, contact: "…", which no source or option can replace;
• has a connect timeout and a receive timeout;
• is retried with backoff on connection errors and on 408, 429, 500, 502, 503 and 504, honouring Retry-After up to a limit and giving up rather than waiting longer;
• counts as a tile only when the response is 200 with an image/* content type.

Configuration

config :staticmaps, :http,
  connect_timeout: 5_000,
  receive_timeout: 10_000,
  max_retries: 2,
  retry_base: 500,
  max_retry_after: 10_000

retry_base is the first backoff in milliseconds, doubling on each retry. max_retry_after is the longest Retry-After the fetcher will wait, in milliseconds.