# `Storehouse.Mirror`
[🔗](https://github.com/hattoriai/storehouse/blob/v0.2.0/lib/storehouse/mirror.ex#L1)

A store that keeps its files in a primary store and copies them to
others, its mirrors: to move to another provider without stopping (new
files go to both, the old ones are copied, then the app reads from the
new one), or to keep a second copy in another provider or region.

    defmodule MyApp.Media do
      use Storehouse.Mirror, primary: MyApp.S3Media, mirrors: [MyApp.R2Media]
    end

    config :storehouse, adapter: MyApp.Media

It is a `Storehouse.Adapter`, used wherever a store is:

  * **Writes** go to the primary: a file stored, streamed, uploaded
    straight from a browser (`Storehouse.verify_upload/2`) or promoted.
    Once the primary has it, each mirror is given a copy of it
    (`Storehouse.copy/2`), out of the request.
  * **Deletes** go to the primary, then to each mirror.
  * **Reads**, URLs and uploads straight from a browser are the
    primary's.

## A mirror that fails

A mirror's copy or deletion never fails the primary's: it is a job of
the app's runner (`Storehouse.Runner`), tried again until the mirror
answers. With Oban's (`Storehouse.Runner.Oban`), the jobs are kept in
the database and outlive a deploy; with `Storehouse.Runner.Memory`
they live in a process of the app's, and those not done when the app
stops are not done. `Storehouse.copy_prefix/2` (or `mix storehouse.copy`)
copies what a mirror is missing, and only that, to catch it up.

## Moving to another store

  1. Write to both: `use Storehouse.Mirror, primary: Old, mirrors: [New]`.
  2. Copy the files there were: `mix storehouse.copy --from Old --to New`.
  3. Read from the new one: `primary: New, mirrors: [Old]`, while the
     old one is still wanted, then `New` alone.

## Options

  * `:primary` - the store files are written to and read from.
  * `:mirrors` - the stores given a copy of each.
  * `:runner` - the runner of its jobs, in place of the app's
    (`Storehouse.Runner`).

---

*Consult [api-reference.md](api-reference.md) for complete listing*