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plug_http_cache

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An Elixir plug that caches HTTP responses

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# PlugHTTPCache

A Plug that caches HTTP responses

This plug library relies on the [`http_cache`](https://hexdocs.pm/http_cache)
library. It supports all caching features of
[RFC9111](https://datatracker.ietf.org/doc/html/rfc9111) and more
(such as conditional requests and range requests).

See [`http_cache`](https://hexdocs.pm/http_cache/) documentation for more information.

![Screenshot of pug_http_cache_demo Grafana dashboard](https://raw.githubusercontent.com/tanguilp/plug_http_cache/master/media/grafana.png)

Screenshot from the [`plug_http_cache_demo`](https://github.com/tanguilp/plug_http_cache_demo)
application.

## Installation

```elixir
def deps do
  [
    {:http_cache, "~> 1.0.0"},
    {:plug_http_cache, "~> 1.0.0"}
  ]
end
```

## Configuration

In your plug pipeline, set the Plug for routes on which you want to enable caching:

`router.ex`

```elixir
pipeline :cache do
  plug PlugHTTPCache, @caching_options
end

...

scope "/", PlugHTTPCacheDemoWeb do
  pipe_through :browser

scope "/some_route" do
    pipe_through :cache

...
  end
end
```

You can also configure it for all requests by setting it in Phoenix's endpoint
file:

`endpoint.ex`

```elixir
defmodule MyApp.Endpoint do
  use Phoenix.Endpoint, otp_app: :plug_http_cache_demo

% some other plugs

plug Plug.RequestId
  plug Plug.Telemetry, event_prefix: [:phoenix, :endpoint]
  plug Plug.Parsers,
    parsers: [:urlencoded, :multipart, :json],
    pass: ["*/*"],
    json_decoder: Phoenix.json_library()
  plug Plug.Head
  plug Plug.Session, @session_options

plug PlugHTTPCache, @caching_options

plug PlugHTTPCacheDemoWeb.Router
end
```

Note that:
- caching chunked responses is *not* supported
- some responses (called "cacheable by default") can be cached even when no
`cache-control` header is set. For instance, a 200 response to a get request is
cached 2 minutes by default, unless `cache-control` headers prohibit it
- Phoenix automatically sets the `"cache-control"` header to
`"max-age=0, private, must-revalidate"`, so by default no response will ever
be cached unless you override this header

You can also configure `PlugHTTPCache.StaleIfError` to return expired cached responses.
This is useful to continue returning something when the backend experiences failures
(for example if the DB crashed and while it's rebooting).

## Plug options

Plug options are those documented by
[`:http_cache.opts/0`](https://hexdocs.pm/http_cache/http_cache.html#t:opts/0).

The only required option is `:store`.

This plug sets the following default options:
- `:type`: `:shared`,
- `:auto_compress`: `true`,
- `:auto_accept_encoding`: `true`
- `:stale_while_revalidate_supported`: `true`

## Stores

Responses have to be stored in a separate store backend (this library does not come with one), such
as:
- [`http_cache_store_memory`](https://github.com/tanguilp/http_cache_store_memory): responses are
stored in memory (ETS)
- [`http_cache_store_disk`](https://github.com/tanguilp/http_cache_store_disk): responses are
stored on disk. This library uses the `sendfile` system call and therefore benefits from the kernel's
memory caching automatically

Both are cluster-aware.

To use it along with this library, just add it to your mix.exs file:

`mix.exs`

```elixir
{:http_cache, "~> ..."},
{:plug_http_cache, "~> ..."},
{:http_cache_store_memory, "~> ..."},
```

## Security considerations

Unlike many HTTP caches, `http_cache` allows caching:
- responses to authorized request (with an `"authorization"` header)
- responses with cookies

In the first case, beware of authenticating before handling caching. In
other words, **don't**:

```elixir
PlugHTTPCache, @caching_options
MyPlug.AuthorizeUser
```

which would return a cached response to unauthorized users, but **do** instead:

```elixir
MyPlug.AuthorizeUser
PlugHTTPCache, @caching_options
```

Beware of not setting caching headers on private responses containing cookies.

## Useful libraries

- [`PlugCacheControl`](https://github.com/krasenyp/plug_cache_control) can be used
to set cache-control headers in your Plug pipelines, or manually in your controllers
- [`PlugHTTPValidator`](https://github.com/tanguilp/plug_http_validator) *should* be used
to set HTTP validators as soon as cacheable content is returned. See project
documentation to figure out why

## Telemetry events

The following events are emitted:
- `[:plug_http_cache, :hit]` when a cached response is returned.
- `[:plug_http_cache, :miss]` when no cached response was found
- `[:plug_http_cache, :stale_if_error]` when a response was returned because an error
occurred downstream (see `PlugHTTPCache.StaleIfError`)

`conn` is added to the events' metadata.

The `http_cache`, `http_cache_store_memory` and `http_cache_store_disk` emit other events about
the caching subsystems, including some helping with detecting normalization issues.

## Normalization

The underlying http caching library may store different responses for the same URL,
following the directives of the `"vary"` header. For instance, if a response can
be returned in English or in French, both versions can be cached as long as the
`"vary"` header is correctly used.

This can unfortunately result in an explosion of stored responses if the headers
are not normalized. For instance, in this scenario where a site handles both these
languages, a response will be stored for any of these requests that include an
`"accept-language"` header:
- fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7, *;q=0.5
- fr-CH, fr;q=0.9, en;q=0.8, de;q=0.7,*;q=0.5
- en
- de
- en, de
- en, de, fr
- en;q=1, de
- en;q=1, de;q=0.9
- en;q=1, de;q=0.8
- en;q=1, de;q=0.7
- en;q=1, de;q=0.6
- en;q=1, de;q=0.5

and so on, so potentially hundreds of stored responses for only 2 available
responses (English and French versions).

In this case, you probably want to apply normalization before caching. This
could be done by a plug set before the `PlugHTTPCache` plug.

See [Best practices for using the Vary header](https://www.fastly.com/blog/best-practices-using-vary-header)
for more guidance regarding this issue.

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