Redix

Superfast, pipelined, resilient Redis client for Elixir.

Redix is a Redis client written in pure Elixir with focus on speed, correctness and resiliency (that is, being able to automatically reconnect to Redis in case of network errors).

Installation

Add the :redix dependency to your mix.exs file:

defp dependencies do
  [{:redix, ">= 0.0.0"}]
end

Then run $ mix deps.get to install it.

Why

As for the why you would want to use Redix over the more battle-tested redo or eredis Erlang clients, well, it's pure Elixir :). Also, it appears to be slightly faster than the above-metioned clients (see the section about speed). It surely lacks on battle-testing, but we can make up for that by using it a lot!

Usage

Redix is very simple in that it doesn't wrap Redis commands with Elixir functions: it only provides two functions (with their bang! variants), command/3 and pipeline/3. A Redis command is expressed as a list of strings making up the command and its arguments.

Connections are started via start_link/0, start_link/1 or start_link/2. These functions accept Redix-specific options as well as all the options accepted by GenServer.start_link/3 (e.g., :name for registering the connection process under a name).

{:ok, conn} = Redix.start_link
{:ok, conn} = Redix.start_link(host: "example.com", port: 5000)
{:ok, conn} = Redix.start_link("redis://localhost:6379/3", name: :redix)

Commands can be sent using Redix.command/2-3:

Redix.command(conn, ~w(SET mykey foo))
#=> {:ok, "OK"}
Redix.command(conn, ~w(GET mykey))
#=> {:ok, "foo"}

Pipelines are just lists of commands sent all at once to Redis for which Redis replies with a list of responses. They can be used in Redix via Redix.pipeline/2-3:

Redix.pipeline(conn, [~w(INCR foo), ~w(INCR foo), ~w(INCR foo 2)])
#=> {:ok, [1, 2, 4]}

command/2-3 and pipeline/2-3 always return {:ok, result} or {:error, reason}. If you want to access the result directly and raise in case there's an error, bang! variants are provided:

Redix.command!(conn, ["PING"])
#=> "PONG"

Redix.pipeline!(conn, [~w(SET mykey foo), ~w(GET mykey)])
#=> ["OK", "foo"]

A note about Redis errors: in the non-bang functions, they're returned as Redix.Error structs with a :message field which contains the original error message.

Redix.command(conn, ~w(FOO))
#=> {:error, %Redix.Error{message: "ERR unknown command 'FOO'"}}

# pipeline/2 returns {:ok, _} instead of {:error, _} even if there are errors in
# the list of responses so that it doesn't have to walk the entire list of
# responses.
Redix.pipeline(conn, [~w(PING), ~w(FOO)])
#=> {:ok, ["PONG", %Redix.Error{message: "ERR unknown command 'FOO'"}]}

In command!/2-3, they're raised as exceptions:

Redix.command!(conn, ~w(FOO))
#=> ** (Redix.Error) ERR unknown command 'FOO'

command!/2-3 and pipeline!/2-3 raise Redix.ConnectionError in case there's an error related to the Redis connection (e.g., the connection is closed while Redix is waiting to reconnect).

Transactions are supported naturally as they're just made up of commands:

Redix.command(conn, ~w(MULTI))
#=> {:ok, "OK"}
Redix.command(conn, ~w(INCR counter))
#=> {:ok, "QUEUED"}
Redix.command(conn, ~w(INCR counter))
#=> {:ok, "QUEUED"}
Redix.command(conn, ~w(EXEC))
#=> {:ok, [1, 2]}

PubSub

Redix supports the PubSub functionality provided by Redis.

PubSub connections are different than regular Redix connections and have to be started using Redix.PubSub.start_link/1-2. Clients can then subscribe processes to channels (or patterns) using Redix.PubSub.subscribe/4 (or Redix.PubSub.psubscribe/4). After that, recipient processes will receive (Elixir) messages for:

• successful subscriptions
• messages published on subscribed channels or patterns
• successful unsubscriptions (Redix.PubSub.unsubscribe/4 and Redix.PubSub.punsubscribe/4)
• disconnections and reconnections of the PubSub client

Here's an example usage of the PubSub functionality:

{:ok, conn} = Redix.PubSub.start_link

:ok = Redix.PubSub.subscribe(conn, "ch1", self())

# We receive a message for each channel we subscribe to. We are receiving it
# because we passed self() to subscribe/3.
receive do msg -> msg end
#=> {:redix_pubsub, :subscribe, "ch1", 1}

{:ok, other_conn} = Redix.start_link
{:ok, _} = Redix.command(other_conn, ~w(PUBLISH ch1 hello_world))

receive do msg -> msg end
#=> {:redix_pubsub, :message, "hello_world", "ch1"}

Resiliency

Redix takes full advantage of the connection library by James Fish to provide a resilient behaviour when dealing with the network connection to Redis. For example, if the connection to Redis drops, Redix will automatically try to reconnect to it periodically at a given "backoff" interval (which is configurable). Look at the documentation for the Redix module for more information on the available options and on the exact behaviour.

Speed

I'm by no means a benchmarking expert, but I ran some benchmarks (out of curiosity) against Redis clients for Erlang like redo and eredis. You can find these benchmarks in the ./bench directory. I used benchfella to make them.

It appears from the benchmarks that:

• Redix is roughly as fast as redo and eredis (with a < 10% margin) for single commands sent to Redis.
• Redix is slightly faster than redo and eredis when sending a few pipelined commands (a few means 5 in the benchmarks), with a margin of 15%-20%.
• Redix is substantially faster than eredis (~35% faster) and redo (~300% faster) when sending a lot of pipelined commands (where "a lot" means 10k commands).

For now, I'm quite happy with these benchmarks and hope we can make them even better in the future.

Using Redix in the Real World™

Redix is "low level" in some ways as it tries to be as close to the Redis API as possible and at the same time remain as flexible as it can. A Redix connection is basically just a GenServer, so it fits nicely in OTP supervision trees and the such. Most of the times you will want to supervise Redix connections and often to use a pool of them: for example, if you have a web application that uses Redis, you wouldn't want to connect to Redis each time a new request arrives to the server.

This section provides some sample code to serve as an example for how to supervise a pool of Redix connections. Pooling will be done using the very famous poolboy Erlang library.

A simple way to do this is create a supervisor for the poolboy pool that we can later start when we start our application. The code for this supervisor would look something like this.

defmodule MyApp.RedixPool do
  use Supervisor

  @redis_connection_params host: "example.com", password: "secret"

  def start_link do
    Supervisor.start_link(__MODULE__, [])
  end

  def init([]) do
    pool_opts = [
      name: {:local, :redix_poolboy},
      worker_module: Redix,
      size: 10,
      max_overflow: 5,
    ]

    children = [
      :poolboy.child_spec(:redix_poolboy, pool_opts, @redis_connection_params)
    ]

    supervise(children, strategy: :one_for_one, name: __MODULE__)
  end

  def command(command) do
    :poolboy.transaction(:redix_poolboy, &Redix.command(&1, command))
  end

  def pipeline(commands) do
    :poolboy.transaction(:redix_poolboy, &Redix.pipeline(&1, commands))
  end
end

Using this is straightforward:

MyApp.RedixPool.command(~w(PING)) #=> {:ok, "PONG"}

Now, all you need to do to is add the MyApp.RedixPool supervisor to the supervision tree of your application (using something like supervisor(MyApp.RedixPool, []) in the list of children to supervise).

Just as a short aside, note that you could also use :poolboy.child_spec/3 directly in the list of supervised children of your application, without creating a dedicated supervisor for the pool of Redix connection. Here, we did this for clarity but both strategies make sense in different scenarios.

Contributing

Clone the repository and run $ mix test to make sure everything is working. For tests to pass, you must have a Redis server running on localhost, port 6379.

License

MIT © 2015 Andrea Leopardi, see the license file.