# `Arc.Application`
[🔗](https://github.com/Cratis/Arc.Elixir/blob/main/lib/arc/application.ex#L4)

The runtime of one Arc application: discovery, configuration and the processes that serve
observable queries.

    defmodule MyApp.Arc do
      use Arc.Application, otp_app: :my_app
    end

Start it in your supervision tree, after the Chronicle client:

    children = [
      {Chronicle.Client, ...},
      MyApp.Arc
    ]

There is no global configuration. Everything is scoped to the application module, so tests
and several Arc applications in one VM stay isolated.

## Options

Options are given to `use` and merged over `config :my_app, MyApp.Arc, [...]`.

  * `:otp_app` - (required) the OTP application whose modules are scanned for commands and
    read models (anything defining `__arc__/0`)
  * `:artifacts` - an explicit list of modules instead of scanning `:otp_app`
  * `:route_prefix` - default `"api"`
  * `:segments_to_skip` - leading module-name segments dropped from routes, slice locations
    and proxy folders; default `1` (the application name)
  * `:chronicle` - options given to every Chronicle call, for example `[client: MyApp.Chronicle]`
  * `:authentication` - a list of `Arc.Authentication` handler modules, run in order
  * `:policies` - `[name: PolicyModule]` for `Arc.Policy`
  * `:filters` - `Arc.Filter` modules: cross-cutting rules that run for every command and query
  * `:tenant_header` - default `"x-cratis-tenant-id"`; the header the `:header` tenancy resolver
    reads, and the fallback of the `:subdomain` one
  * `:tenancy` - how the tenant is resolved, see `Arc.Tenancy`: `resolver` (`:header`, `:query`,
    `:claim`, `:subdomain`, `:fixed`, `:development` or a module), `header`, `query_parameter`,
    `claim_type`, `base_domain` and `tenant_id`. A named tenant also selects the Chronicle
    namespace of every call made through the context
  * `:correlation_header` - default `"x-correlation-id"`
  * `:max_body_bytes` - request body limit; default `1_000_000`
  * `:introspection` - who may read the discovery routes (`/.cratis/commands`, `/.cratis/queries`,
    `/.cratis/identity-details/schema`, `/.cratis/users`, `/.cratis/tenants`): `:authenticated`
    (default), `:anonymous` or `false` to remove them. `/.cratis/me` is not affected
  * `:expose_exceptions` - include exception messages and stack traces in results; default
    `false`, which replaces them with a generic message (the detail is always logged)
  * `:identity_details` - the `Arc.Identity.Provider` module to use; by default the single
    provider found in `:otp_app`. Without a provider `/.cratis/me` is not served
  * `:trust_forwarded_identity_headers` - let `Arc.Authentication.MicrosoftIdentityPlatform` accept the
    `x-ms-client-principal*` headers; default `false`. Enable only behind an ingress that
    authenticates callers and strips client-supplied identity headers
  * `:forwarded_identity` - signed forwarded identity, see `Arc.Identity.SignedForwarding`:
    `enabled`, `audience`, `keys: [[key_id:, secret_base64:]]`
  * `:on_behalf_of` - acting on behalf of a user, see `Arc.Identity.OnBehalfOf`: `enabled`, `issuer`,
    `audience`, `service`, `clock_skew` (seconds), `signing_keys: [[key_id:, public_key_pem:]]`

# `artifacts`

```elixir
@spec artifacts(module()) :: %{
  commands: [Arc.Descriptor.t()],
  read_models: [Arc.Descriptor.t()]
}
```

The artifact descriptors of an application: `%{commands: [...], read_models: [...]}`.

---

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