## RequestCache
[](https://github.com/MikaAK/request_cache_plug/actions/workflows/test-actions.yml)
[](https://codecov.io/gh/MikaAK/request_cache_plug)
[](https://hex.pm/packages/request_cache_plug)
This plug allows us to cache our graphql queries and phoenix controller requests declaritevly
We call the cache inside either a resolver or a controller action and this will store it preventing further
executions of our query on repeat requests.
The goal of this plug is to short-circuit any processing phoenix would
normally do upon request including json decoding/parsing, the only step that should run is telemetry
### Installation
This package can be installed by adding `request_cache_plug` to your list of dependencies in `mix.exs`:
```elixir
def deps do
[
{:request_cache_plug, "~> 1.0"}
]
end
```
Documentation can be found at <https://hexdocs.pm/request_cache_plug>.
### Config
This is the default config, it can all be changed but without any configuration setup this will be used:
```elixir
config :request_cache_plug,
enabled?: true,
verbose?: false,
graphql_paths: ["/graphiql", "/graphql"],
conn_priv_key: :__shared_request_cache__,
request_cache_module: RequestCache.ConCacheStore,
default_ttl: :timer.hours(1),
default_concache_opts: [
ttl_check_interval: :timer.seconds(1),
acquire_lock_timeout: :timer.seconds(1),
ets_options: [write_concurrency: true, read_concurrency: true]
]
```
### Usage
This plug is intended to be inserted into the `endpoint.ex` fairly early in the pipeline,
it should go after telemetry but before our parsers
```elixir
plug Plug.Telemetry, event_prefix: [:phoenix, :endpoint]
plug RequestCache.Plug
plug Plug.Parsers,
parsers: [:urlencoded, :multipart, :json],
pass: ["*/*"]
```
We also need to setup a before_send hook to our absinthe_plug (if not using absinthe you can skip this step)
```elixir
plug Absinthe.Plug, before_send: {RequestCache, :connect_absinthe_context_to_conn}
```
What this does is allow us to see the results of items we put onto our request context from within plugs coming after absinthe
After that we can utilize our cache in a few ways
#### Utilization with Phoenix Controllers
```elixir
def index(conn, params) do
conn
|> RequestCache.store(:timer.seconds(60))
|> put_status(200)
|> json(%{...})
end
```
#### Utilization with Absinthe Resolvers
```elixir
def all(params, _resolution) do
# Instead of returning {:ok, value} we return this
RequestCache.store(value, :timer.seconds(60))
end
```
#### Utilization with Absinthe Middleware
```elixir
field :user, :user do
arg :id, non_null(:id)
middleware RequestCache.Middleware, ttl: :timer.seconds(60)
resolve &Resolvers.User.find/2
end
```
### Specifying Specific Caching Locations
We have a few ways to control the caching location of our RequestCache, by default if you have `con_cache` installed,
we have access to `RequestCache.ConCacheStore` which is the default setting
However we can override this by setting `config :request_cache_plug, :request_cache_module, MyCustomCache`
Caching module will be expected to have the following API:
```elixir
def get(key) do
...
end
def put(key, ttl, value) do
...
end
```
You are responsible for starting the cache, including ConCacheStore, so if you're planning to use it make sure
you add `RequestCache.ConCacheStore` to the application.ex list of children
We can also override the module for a particular request by passing the option to our graphql middleware or
our `&RequestCache.store/2` function as `[ttl: 123, cache: MyCacheModule]`
##### With Middleware
```elixir
field :user, :user do
arg :id, non_null(:id)
middleware RequestCache.Middleware, ttl: :timer.seconds(60), cache: MyCacheModule
resolve &Resolvers.User.find/2
end
```
##### In a Resolver
```elixir
def all(params, resolution) do
RequestCache.store(value, ttl: :timer.seconds(60), cache: MyCacheModule)
end
```
##### In a Controller
```elixir
def index(conn, params) do
RequestCache.store(conn, ttl: :timer.seconds(60), cache: MyCacheModule)
end
```
### telemetry
Cache events are emitted via :telemetry. Events are:
- `[:request_cache_plug, :graphql, :cache_hit]`
- `[:request_cache_plug, :graphql, :cache_miss]`
- `[:request_cache_plug, :rest, :cache_hit]`
- `[:request_cache_plug, :rest, :cache_miss]`
- `[:request_cache_plug, :cache_put]`
For GraphQL endpoints it is possible to provide a list of atoms that will be passed through to the event metadata; e.g.:
##### With Middleware
```elixir
field :user, :user do
arg :id, non_null(:id)
middleware RequestCache.Middleware,
ttl: :timer.seconds(60),
cache: MyCacheModule,
labels: [:service, :endpoint],
whitelisted_query_names: ["MyQueryName"] # By default all queries are cached, can also whitelist based off query name from GQL Document
resolve &Resolvers.User.find/2
end
```
##### In a Resolver
```elixir
def all(params, resolution) do
RequestCache.store(value, ttl: :timer.seconds(60), cache: MyCacheModule, labels: [:service, :endpoint])
end
```
The events will look like this:
```elixir
{
[:request_cache_plug, :graphql, :cache_hit],
%{count: 1},
%{ttl: 3600000, cache_key: "/graphql:NNNN", labels: [:service, :endpoint]}
}
```
##### Enable Error Caching
In order to enable error caching we can either setup `cached_errors` in our config
or as an option to `RequestCache.store` or `RequestCache.Middleware`.
The value of `cached_errors` can be one of `[]`, `:all` or a list of reason_atoms as
defined by `Plug.Conn.Status` such as `:not_found`, or `:internal_server_error`.
In REST this works off the response codes returned. However, in order to use reason_atoms in GraphQL
you will need to make sure your errors contain some sort of `%{code: "not_found"}` response in them
Take a look at [error_message](https://github.com/MikaAK/elixir_error_message) for a compatible error system
### Notes/Gotchas
- In order for this caching to work, we cannot be using POST requests as specced out by GraphQL, not for queries at least, fortunately this doesn't actually matter since we can use any http method we want (there will be a limit to query size), in a production app you may be doing this already due to the caching you gain from CloudFlare
- Caches are stored via a MD5 hashed key that correlates to your query in GraphQL, or in REST your url path + query parameters
- Absinthe and ConCache are optional dependencies, if you don't have them you won't have access to `RequestCache.Middleware` or `RequestCache.ConCacheStore`
- If no ConCache is found, you must set `config :request_cache_module` to something else
### Caching Header
When an item is served from the cache, we return a header `rc-cache-status` which has a value of `HIT`. Using this you can tell if the item was
served out of cache, without it the item was fetched.
We can also invalidate specific items out of the cache, by using the `rc-cache-key` header which returns the key being used for the cache
### Example Reduction
In the case of a large (16mb) payload running through absinthe, this plug cuts down response times from 400+ms -> <400μs
<img width="704" alt="image" src="https://user-images.githubusercontent.com/4650931/161464277-713e994b-1246-43ac-82a1-fb2442cd7bce.png">
