Using Cachex in Phoenix - working example?

fireproofsocks

I’m new to Elixir and Phoenix and I’m trying to implement Cachex. I’m struggling to get this working. I added the dependency to my mix.exs and I modified my application function so it references the Cachex app:

def application do
    [
      mod: {MyApp.Application, []},
      extra_applications: [:logger, :runtime_tools],
      applications: [:cachex]
    ]
  end

deps.get downloaded the package and that all seems to be working.

In my application.ex, I have tried to add the appropriate block to the Supervisor.start_link:

  def start(_type, _args) do
    import Supervisor.Spec

    # Define workers and child supervisors to be supervised
    children = [
      supervisor(MyApp.Repo, []),
      supervisor(MyApp.Endpoint, []),
      worker(Cachex, [:my_cache, []]),
    ]

    opts = [strategy: :one_for_one, name: MyApp.Supervisor]
    Supervisor.start_link(children, opts)
    
  end

As soon as I try to run the server, I get an error:

mix phx.server
Compiling 16 files (.ex)
Generated my_app app

=INFO REPORT==== 12-Feb-2018::15:32:28 ===
    application: logger
    exited: stopped
    type: temporary
** (Mix) Could not start application my_app: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: MyApp.Repo
    ** (EXIT) exited in: GenServer.call(Ecto.Registry, {:associate, #PID<0.367.0>, {MyApp.Repo, MyApp.Repo.Pool, [name: MyApp.Repo.Pool, otp_app: :my_app, repo: MyApp.Repo, timeout: 15000, pool_timeout: 5000, adapter: Ecto.Adapters.MySQL, username: "my_user", password: "xxxxx", database: "my_db", hostname: "my.host.tld", pool_size: 10, pool: DBConnection.Poolboy]}}, 5000)
        ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started

I’m not sure why it is choking. I can at least compile the app and start the server if I add :cachex to the “extra_applications” instead of to the “applications” bit. Can someone shed some light on this? Is it viable to use extra_applications instead?

17 comments

/phoenix #cache

0 7381 17

2018-02-14 17:14:01 UTC

First Post!

Show Best Posts
Show All (oldest first)
Show All (newest first)

fireproofsocks

I solved the above by using “extra_applications” instead of “applications” (but not both), but still, I haven’t actually gotten a working example. E.g. in my context, I have something like this:

def get_cached_result!(slug) do

    Cachex.fetch(:my_cache, "key", fn(slug) ->
      Repo.get_by!(MyRecord, slug: slug, parent_id: 0)
    end)

  end

But that generates an error:

function Cachex.fetch/3 is undefined or private

Post #1

Most Liked

outlog

try with Cachex.get - not sure a fetch exists - Cachex — Cachex v4.1.1

so maybe:

def get_cached_result!(slug) do

    Cachex.get(:my_cache, "key_#{slug}", fn(slug) ->
      Repo.get_by!(MyRecord, slug: slug, parent_id: 0)
    end)

end

fwiw I have this from some old code (untested recently):

{ _status, post } = Cachex.get(:my_cache, "post_#{id}", fallback: fn(_params) ->
  Post
  |> Repo.get!(id)
  |> Repo.preload([:comments])
end)

EDIT: from a quick look: fetch is being added in Cachex 3.0 - so thats probably why you find it on the github examples:/

Post #4

fireproofsocks

Thank you for your patient guidance – it helps, and in retrospect, it all seems obvious.

Post #11

outlog

it does blow up with a bang.. so a bit more defensive code is needed, unless you are doing bangs on purpose..

if you simply remove the bang and use get_by it will return nil on not found, and that will be cached - depending on your caching strategy you might not want that - and you can use the ignore feature.. that way the nil/not found is not cached and the db is always queried on slugs that are not found - but it all depends..

here is an example.. using :ignore so the nil is not cached.. else just remove the bang in get_by

  def get_cached_thing!(slug) do
    Cachex.get(
      :my_cache,
      slug,
      fallback: fn slug ->
        case result = Repo.get_by(Mything, id: slug) do
          %Mything{} ->
            # happy path - commit to cache
            {:commit, result}

          nil ->
            # not found - ignore tuple so it's not cached
            {:ignore, nil}
        end
      end
    )
    |> case do
      # It's not common to return error strings, rather you'd probably want an error tuple
      {:error, _} ->
        {:error, "error - devops needed?"}

      # the ignore returns an :loaded tuple - so match nil
      {:loaded, nil} ->
        {:error, :not_found}

      {success, result} when success in [:ok, :loaded] ->
        {:ok, result}
    end
  end

Post #15

Last Post!

OvermindDL1

It’s in the error handling section of the phoenix docs, you can either explicitly state what error in the conn and then return the conn, or you can raise an exception that specifies to phoenix what error to return.