# `Nebulex.Adapters.Nil`
[🔗](https://github.com/elixir-nebulex/nebulex/blob/v3.1.0/lib/nebulex/adapters/nil.ex#L1)

The Nil adapter is a special cache adapter that turns off the cache. It
drops everything written to it, behaves as if every key were missing on
reads, and reports success on all writes. It is mostly useful for tests.

## Shared options

All of the cache functions accept the following options
when using this adapter:

* `:before` (function of arity 0) - Pre-hook function. It calls the given function before the cache command
  is executed.

* `:after_return` (function of arity 1) - Post-hook function. It calls the given function after the cache command
  is executed, and the returned result is passed as an argument.

## Example

Suppose you have an application that uses Ecto for database access and
Nebulex for caching, so it defines a repo and a cache. In some tests you may
want to turn the cache off. For example, a test that expects no data from
the database can still get data from the cache if the cache was not
flushed, so you would have to delete all entries before each test. Instead
of adding code to flush the cache before each test, you can define a test
cache that uses the Nil adapter.

The application cache is defined in `lib/my_app/cache.ex`:

    defmodule MyApp.Cache do
      use Nebulex.Cache,
        otp_app: :my_app,
        adapter: Nebulex.Adapters.Local
    end

The test cache is defined in `test/support/test_cache.ex`:

    defmodule MyApp.TestCache do
      use Nebulex.Cache,
        otp_app: :my_app,
        adapter: Nebulex.Adapters.Nil
    end

Then tell the app which cache to use per environment: `MyApp.TestCache` in
tests and `MyApp.Cache` otherwise. One way is a config parameter that picks
the cache module. In `config/test.exs`:

    config :my_app,
      nebulex_cache: MyApp.TestCache,
      ...

The last piece is to read the config parameter and start that cache. In
`lib/my_app/application.ex`:

    def start(_type, _args) do
      children = [
        {Application.get_env(:my_app, :nebulex_cache, MyApp.Cache), []},
      ]

      ...

`MyApp.Cache` is used by default unless the `:nebulex_cache` option points
to a different module, which is the case in the `test` environment.

---

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