# Shoehorn
[](https://circleci.com/gh/nerves-project/shoehorn)
[](https://hex.pm/packages/shoehorn)
Shoehorn provides full control over the application lifecycle in Elixir.
## Usage
`Shoehorn` acts as a shim to the initialization sequence for your application's
VM. Using `Shoehorn`, you can ensure that the VM will always pass initialization.
This provides the running node the ability of using Elixir / Erlang to control
the full application lifecycle through the exposure of new system phases.
This level of control is important when the Erlang VM is fully responsible
for the entire runtime, including its own updates. In these situations, if
the VM were to fail to start it would never be able to recover from a bad
update. This is especially useful when running [Nerves](https://circleci.com/gh/nerves-project).
Here's how it works.
Include `shoehorn` into your application release plugins:
```elixir
# rel/config.exs
release :simple_app do
set version: current_version(:simple_app)
plugin Shoehorn
end
```
Then, produce a release:
```sh
mix release
```
Next, go to the release directory and boot your app using `shoehorn`:
```sh
_build/dev/rel/simple_app/bin/simple_app console_boot $(pwd)/_build/dev/rel/simple_app/bin/shoehorn
```
From here we can see that shoehorn was started, but `simple_app` was not.
```elixir
iex(simple_app@127.0.0.1)1> Application.started_applications
[{:iex, 'iex', '1.4.0'}, {:shoehorn, 'shoehorn', '0.1.0'},
{:elixir, 'elixir', '1.4.0'}, {:compiler, 'ERTS CXC 138 10', '7.0.3'},
{:stdlib, 'ERTS CXC 138 10', '3.2'}, {:kernel, 'ERTS CXC 138 10', '5.1.1'}]
```
Booting the shoehorn.boot script with zero application config will bring up the
Erlang VM and only run the `shoehorn` app.
Now let's configure `shoehorn` to do something more interesting by adding some
minimal configuration:
```elixir
# config/config.exs
config :shoehorn,
app: :my_app,
init: [:nerves_runtime, :nerves_init_gadget, :nerves_firmware_ssh]
```
Shoehorn will call `Application.ensure_all_started/2` on each app in the `init`
list, followed by the main `app`. In the example above, the boot sequence would be
`[:nerves_runtime, :nerves_init_gadget, :my_app]`.
Use the `init` application list to prioritize OTP applications that are needed for
error recovery. In the example above, we initialize the runtime, bring up the network,
and ensure that we can receive new firmware updates. Now, if `my_app` fails to start,
the node would still be in a state where it can receive new firmware over the network.
You can also specify an `{m, f, a}` in the `init` list for performing
simple initialization time tasks. Shoehorn will call [Kernel.apply/3](https://hexdocs.pm/elixir/Kernel.html#apply/3) for each `{m, f, a}`-formatted entry.
```elixir
# config/config.exs
config :shoehorn,
app: :my_app,
init: [{IO, :puts, ["Init"]}, :nerves_runtime]
```
## Application Failures
The Erlang VM will respond to application failures differently, depending on the
_permanence type_ specified when the application started. There are three permanence types:
`:permanent` - if the application terminates, all other applications and the entire node
are also terminated.
`:transient` - if the application terminates with `:normal reason`, it is reported but no
other applications are terminated. However, if the application terminates
abnormally, all other applications and the entire node are also terminated.
`:temporary` - if the application terminates, it is reported but no other applications are
terminated (the default behaviour).
Shoehorn will start all applications as `:temporary` and monitor application
events by registering with the erlang kernel [error_logger](http://erlang.org/doc/man/error_logger.html).
Application start and exit events will attempt to execute a callback to the
configured `Shoehorn.Handler` module. By default, the module `Shoehorn.Handler.Ignore`
will be called. This module is configured to continue the Erlang VM if any OTP
application were to exit, for any reason. In production, you may want to customize
the action on failure so you can gather forensics or perform updates to the node.
You can do this by overriding the handler in the prod env of your application config.
```elixir
# config/prod.exs
config :shoehorn,
handler: MyApp.ShoehornHandler
```
More advanced failure cases can be handled by providing your own module that implements
the `Shoehorn.Handler` behaviour. For example, the erlang `:ssh` application is prone to
exiting when undergoing a brute force attack. Instead of the default production behaviour of
forcing the node to restart, we can restart the application.
```elixir
defmodule Example.RestartHandler do
use Shoehorn.Handler
def application_exited(app, _reason, state) do
IO.puts("Application stopped: #{inspect(app)} #{inspect(state)}")
Application.ensure_all_started(app)
{:continue, state}
end
end
```
The `application_exited/3` callback is limited in the amount of time is has to execute by
setting a shutdown timer. If the callback does not return within the defined shutdown time,
the node is instructed to halt. The default shutdown time is 30 seconds but this value can
be changed in the application config:
```elixir
# config/config.exs
config :shoehorn,
shutdown_timer: 50_000 # 50 Seconds
```
Have a look at the [example application](https://github.com/nerves-project/shoehorn/tree/main/example) for more info on implementing custom strategies.
## Distillery overrides
Shoehorn will alter the release defaults to omit `:mix` and `:distillery` from
the list of default applications to include. If you depend on these applications
at runtime, you can add `:distillery` to the `extra_applications` list and or
`:mix` to the `included_applications` list in the `application/0` callback in
your `mix.exs` file.
