Exq

Exq is a job processing library compatible with Resque / Sidekiq for the Elixir language.

• Exq uses Redis as a store for background processing jobs.
• Exq handles concurrency, job persistence, job retries, reliable queueing and tracking so you don't have to.
• Jobs are persistent so they would survive across node restarts.
• You can use multiple Erlang nodes to process from the same pool of jobs.
• Exq uses a format that is Resque/Sidekiq compatible.
  • This means you can use it to integrate with existing Rails / Django projects that also use a background job that's Resque compatible - typically with little or no changes needed to your existing apps. However, you can also use Exq standalone.
  • You can also use the Sidekiq UI to view job statuses, as Exq is compatible with the Sidekiq stats format.
  • If you don't need Resque/Sidekiq compatibility, another option to check out would be toniq which uses erlang serialization instead of JSON.
  • You can run both Exq and Toniq in the same app for different workers.
• Exq supports uncapped amount of jobs running, or also allows a max limit per queue.
• Exq supports job retries with exponential backoff.
• Exq supports configurable middleware for customization / plugins.
• Exq tracks several stats including failed busy, and processed jobs.
• Exq stores in progress jobs in a backup queue (using the Redis RPOPLPUSH command). This means that if the system or worker is restarted while a job is in progress, the job will be re_enqueued when the node is restarted and not lost.
• Exq provides an optional web UI that you can use to view several stats as well as rate of job processing.
• When shutting down Exq will attempt to let workers terminate gracefully, with a configurable timeout.
• There is no time limit to how long a job can run for.

Do you need Exq?

While you may reach for Sidekiq / Resque / Celery by default when writing apps in other languages, in Elixir there are some good options to consider that are already provided by the language and platform. So before adding Exq or any Redis backed queueing library to your application, make sure to get familiar with OTP and see if that is enough for your needs. Redis backed queueing libraries do add additional infrastructure complexity and also overhead due to serialization / marshalling, so make sure to evaluate whether it is an actual need or not.

Some OTP related documentation to look at:

• GenServer: http://elixir-lang.org/getting-started/mix-otp/genserver.html
• Task: https://hexdocs.pm/elixir/Task.html
• GenStage: https://hexdocs.pm/gen_stage/GenStage.html
• Supervisor: http://elixir-lang.org/getting-started/mix-otp/supervisor-and-application.html
• OTP: http://erlang.org/doc/

If you need a durable jobs, retries with exponential backoffs, dynamically scheduled jobs in the future - that are all able to survive application restarts, then an externally backed queueing library such as Exq could be a good fit.

If you are starting a brand new project, I would also take a look at Faktory. It provides language indepedent queueuing system, which means this logic doesn't have to be implemented across different languages and can use a thin client such as faktory_worker_ex.

Getting Started:

Pre-requisite:

This assumes you have an instance of Redis to use. The easiest way to install it on OSX is via brew:

> brew install redis

To start it:

> redis-server

Screencast on elixircasts.io:

If you prefer video instructions, check out the screencast on elixircasts.io which details how to install and use the Exq library: https://elixircasts.io/elixir-job-processing-with-exq

Installation:

Add exq to your mix.exs deps (replace version with the latest hex.pm package version):

  defp deps do
    [
      # ... other deps
      {:exq, "~> 0.14.0"}
    ]
  end

Then run mix deps.get.

Configuration:

By default, Exq will use configuration from your config.exs file. You can use this to configure your Redis host, port, password, as well as namespace (which helps isolate the data in Redis). If you would like to specify your options as a redis url, that is also an option using the url config key (in which case you would not need to pass the other redis options).

Configuration options may optionally be given in the {:system, "VARNAME"} format, which will resolve to the runtime environment value.

Other options include:

• The queues list specifies which queues Exq will listen to for new jobs.
• The concurrency setting will let you configure the amount of concurrent workers that will be allowed, or :infinite to disable any throttling.
• The name option allows you to customize Exq's registered name, similar to using Exq.start_link([name: Name]). The default is Exq.
• If the option start_on_application is false, Exq won't be started automatically when booting up you Application. You can start it with Exq.start_link/1.
• The shutdown_timeout is the number of milliseconds to wait for workers to finish processing jobs when the application is shutting down. It defaults to 5000 ms.
• The mode option can be used to control what components of exq are started. This would be useful if you want to only enqueue jobs in one node and run the workers in different node.
  • :default - starts worker, enqueuer and api.
  • :enqueuer - starts only the enqueuer.
  • :api - starts only the api.
  • [:api, :enqueuer] - starts both enqueuer and api.
• The backoff option allows you to customize the backoff time used for retry when a job fails. By default exponential time scaled based on job's retry_count is used. To change the default behavior, create a new module which implements the Exq.Backoff.Behaviour and set backoff option value to the module name.

config :exq,
  name: Exq,
  host: "127.0.0.1",
  port: 6379,
  password: "optional_redis_auth",
  namespace: "exq",
  concurrency: :infinite,
  queues: ["default"],
  poll_timeout: 50,
  scheduler_poll_timeout: 200,
  scheduler_enable: true,
  max_retries: 25,
  mode: :default,
  shutdown_timeout: 5000

Concurrency:

Exq supports concurrency setting per queue. You can specify the same concurrency option to apply to each queue or specify it based on a per queue basis.

Concurrency for each queue will be set at 1000:

config :exq,
  host: "127.0.0.1",
  port: 6379,
  namespace: "exq",
  concurrency: 1000,
  queues: ["default"]

Concurrency for q1 is set at 10_000 while q2 is set at 10:

config :exq,
  host: "127.0.0.1",
  port: 6379,
  namespace: "exq",
  queues: [{"q1", 10_000}, {"q2", 10}]

Job Retries:

Exq will automatically retry failed job. It will use an exponential backoff timing similar to Sidekiq or delayed_job to retry failed jobs. It can be configured via these settings:

config :exq,
  host: "127.0.0.1",
  port: 6379,
  ...
  scheduler_enable: true,
  max_retries: 25

Note that scheduler_enable has to be set to true and max_retries should be greater than 0.

Dead Jobs:

Any job that has failed more than max_retries times will be moved to dead jobs queue. Dead jobs could be manually re-enqueued via Sidekiq UI. Max size and timeout of dead jobs queue can be configured via these settings:

config :exq,
  dead_max_jobs: 10_000,
  dead_timeout_in_seconds: 180 * 24 * 60 * 60, # 6 months

OTP Application:

You can add Exq into your OTP application list, and it will start an instance of Exq along with your application startup. It will use the configuration from your config.exs file.

  def application do
    [
      applications: [:logger, :exq],
      #other stuff...
    ]
  end

When using Exq through OTP, it will register a process under the name Elixir.Exq - you can use this atom where expecting a process name in the Exq module.

If you would like to control Exq startup, you can configure Exq to not start anything on application start. For example, if you are using Exq along with Phoenix, and your workers are accessing the database or other resources, it is recommended to disable Exq startup and manually add it to the supervision tree.

This can be done by setting start_on_application to false and adding it to your supervision tree:

config :exq,
   start_on_application: false

    # Define workers and child supervisors to be supervised
    children = [
      # Start the Ecto repository
      supervisor(MyApp.Repo, []),
      # Start the endpoint when the application starts
      supervisor(MyApp.Endpoint, []),
      supervisor(Exq, []),
    ]

Sentinel:

Exq uses Redix client for communication with redis server. The client can be configured to use sentinel via redis_options. Note: you need to have Redix 0.9.0+.

config :exq
  redis_options: [
    sentinel: [sentinels: [[host: "127.0.0.1", port: 6666]], group: "exq"],
    database: 0,
    password: nil,
    timeout: 5000,
    name: Exq.Redis.Client,
    socket_opts: []
  ]

Using iex:

If you'd like to try Exq out on the iex console, you can do this by typing

mix deps.get

and then

iex -S mix

Standalone Exq:

You can run Exq standalone from the command line, to run it:

> mix do app.start, exq.run

Workers

Enqueuing jobs:

To enqueue jobs:

{:ok, ack} = Exq.enqueue(Exq, "default", MyWorker, ["arg1", "arg2"])

{:ok, ack} = Exq.enqueue(Exq, "default", "MyWorker", ["arg1", "arg2"])

## Don't retry job in per worker
{:ok, ack} = Exq.enqueue(Exq, "default", MyWorker, ["arg1", "arg2"], max_retries: 0)
## max_retries = 10, it will override :max_retries in config
{:ok, ack} = Exq.enqueue(Exq, "default", MyWorker, ["arg1", "arg2"], max_retries: 10)

In this example, "arg1" will get passed as the first argument to the perform method in your worker, "arg2" will be second argument, etc.

You can also enqueue jobs without starting workers:

{:ok, sup} = Exq.Enqueuer.start_link([port: 6379])

{:ok, ack} = Exq.Enqueuer.enqueue(Exq.Enqueuer, "default", MyWorker, [])

You can also schedule jobs to start at a future time: You need to make sure scheduler_enable is set to true

Schedule a job to start in 5 mins

{:ok, ack} = Exq.enqueue_in(Exq, "default", 300, MyWorker, ["arg1", "arg2"])

Schedule a job to start at 8am 2015-12-25 UTC

time = Timex.now() |> Timex.shift(days: 8)
{:ok, ack} = Exq.enqueue_at(Exq, "default", time, MyWorker, ["arg1", "arg2"])

Creating Workers:

To create a worker, create an elixir module matching the worker name that will be enqueued. To process a job with "MyWorker", create a MyWorker module. Note that the perform also needs to match the number of arguments as well.

Here is an example of a worker:

defmodule MyWorker do
  def perform do
  end
end

We could enqueue a job to this worker:

{:ok, jid} = Exq.enqueue(Exq, "default", MyWorker, [])

The 'perform' method will be called with matching args. For example:

{:ok, jid} = Exq.enqueue(Exq, "default", "MyWorker", [arg1, arg2])

Would match:

defmodule MyWorker do
  def perform(arg1, arg2) do
  end
end

Job data from worker

If you'd like to get Job metadata information from a worker, you can call worker_job from within the worker:

defmodule MyWorker do
  def perform(arg1, arg2) do
    # get job metadata
    job = Exq.worker_job()
  end
end

Dynamic queue subscriptions:

The list of queues that are being monitored by Exq is determined by the config.exs file or the parameters passed to Exq.start_link. However, we can also dynamically add and remove queue subscriptions after exq has started.

To subscribe to a new queue:

# last arg is optional and is the max concurrency for the queue
:ok = Exq.subscribe(Exq, "new_queue_name", 10)

To unsubscribe from a queue:

:ok = Exq.unsubscribe(Exq, "queue_to_unsubscribe")

To unsubscribe from all queues:

:ok = Exq.unsubscribe_all(Exq)

Middleware Support

If you'd like to customize worker execution and/or create plugins like Sidekiq/Resque have, Exq supports custom middleware. The first step would be to define the middleware in config.exs and add your middleware into the chain:

  middleware: [Exq.Middleware.Stats, Exq.Middleware.Job, Exq.Middleware.Manager,
    Exq.Middleware.Logger]

You can then create a module that implements the middleware behavior and defines before_work, after_processed_work and after_failed_work functions. You can also halt execution of the chain as well. For a simple example of middleware implementation, see the Exq Logger Middleware.

Using with Phoenix and Ecto

If you would like to use Exq alongside Phoenix and Ecto, add :exq to your mix.exs application list:

  def application do
    [mod: {Chat, []},
     applications: [:phoenix, :phoenix_html, :cowboy, :logger, :exq]]
  end

Assuming you will be accessing the database from Exq workers, you will want to lower the concurrency level for those workers, as they are using a finite pool of connections and can potentially back up and time out. You can lower this through the concurrency setting, or perhaps use a different queue for database workers that have a lower concurrency just for that queue. Inside your worker, you would then be able to use the Repo to work with the database:

defmodule Worker do
  def perform do
    HelloPhoenix.Repo.insert!(%HelloPhoenix.User{name: "Hello", email: "world@yours.com"})
  end
end

Using alongside Sidekiq / Resque

To use alongside Sidekiq / Resque, make sure your namespaces as configured in exq match the namespaces you are using in Sidekiq. By default, exq will use the exq namespace, so you will have to change that.

Another option is to modify Sidekiq to use the Exq namespace in the sidekiq initializer in your ruby project:

Sidekiq.configure_server do |config|
  config.redis = { url: 'redis://127.0.0.1:6379', namespace: 'exq' }
end

Sidekiq.configure_client do |config|
  config.redis = { url: 'redis://127.0.0.1:6379', namespace: 'exq' }
end

For an implementation example, see sineed's demo app illustrating Sidekiq to Exq communication.

If you would like to exclusively send some jobs from Sidekiq to Exq as your migration strategy, you should create queue(s) that are exclusively listened to only in Exq (and configure those in the queue section in the Exq config). Make sure they are not configured to be listened to in Sidekiq, otherwise Sidekiq will also take jobs off that queue. You can still Enqueue jobs to that queue in Sidekiq even though they are not being monitored:

Sidekiq::Client.push('queue' => 'elixir_queue', 'class' => 'ElixirWorker', 'args' => ['foo', 'bar'])

Security

By default, your Redis server could be open to the world. As by default, Redis comes with no password authentication, and some hosting companies leave that port accessible to the world.. This means that anyone can read data on the queue as well as pass data in to be run. Obviously this is not desired, please secure your Redis installation by following guides such as the Digital Ocean Redis Security Guide.

Node Recovery

A Node can be stopped unexpectedly while processing jobs due to various reasons like deployment, system crash, OOM, etc. This could leave the jobs in the in-progress state. Exq comes with two mechanisms to handle this situation.

Same Node Recovery

Exq identifies each node using an identifier. By default machine's hostname is used as the identifier. When a node comes back online after a crash, it will first check if there are any in-progress jobs for its identifier. Note that it will only re-enqueue jobs with the same identifier. There are environments like Heroku or Kubernetes where the hostname would change on each deployment. In those cases, the default identifier can be overridden

config :exq,
   node_identifier: MyApp.CustomNodeIdentifier

defmodule MyApp.CustomNodeIdentifier do
  @behaviour Exq.NodeIdentifier.Behaviour

  def node_id do
     # return node ID, perhaps from environment variable, etc
     System.get_env("NODE_ID")
  end
end

Heartbeat

Same node recovery is straightforward and works well if the number of worker nodes is fixed. There are use cases that need the worker nodes to be autoscaled based on the workload. In those situations, a node that goes down might not come back for a very long period.

Heartbeat mechanism helps in these cases. Each node registers a heartbeat at regular interval. If any node misses 5 consecutive heartbeats, it will be considered dead and all the in-progress jobs belong to that node will be re-enqueued.

This feature is disabled by default and can be enabled using the following config.

config :exq,
    heartbeat_enable: true,
    heartbeat_interval: 60_000,
    missed_heartbeats_allowed: 5

Web UI:

Exq has a separate repo, exq_ui which provides with a Web UI to monitor your workers:

See https://github.com/akira/exq_ui for more details.

Starting Exq manually:

Typically, Exq will start as part of the application along with the configuration you have set. However, you can also start Exq manually and set your own configuration per instance.

Here is an example of how to start Exq manually:

{:ok, sup} = Exq.start_link

To connect with custom configuration options (if you need multiple instances of Exq for example), you can pass in options under start_link:

{:ok, sup} = Exq.start_link([host: "127.0.0.1", port: 6379, namespace: "x"])

By default, Exq will register itself under the Elixir.Exq atom. You can change this by passing in a name parameter:

{:ok, exq} = Exq.start_link(name: Exq.Custom)

Testing

Exq.Mock module provides few options to test your workers.

# change queue_adapter in config/test.exs
config :exq,
  queue_adapter: Exq.Adapters.Queue.Mock

# start mock server in your test_helper.exs
Exq.Mock.start_link(mode: :redis)

Exq.Mock currently supports three modes. The default mode can provided on the Exq.Mock.start_link call. The mode could be overriden for each test by calling Exq.Mock.set_mode(:fake)

redis

This could be used for integration testing. Doesn't support async: true option.

fake

The jobs get enqueued in a local queue and never get executed. Exq.Mock.jobs() returns all the jobs. Supports async: true option.

inline

The jobs get executed in the same process. Supports async: true option.

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Questions? Issues?

For issues, please submit a Github issue with steps on how to reproduce the problem.

Contributions

Contributions are welcome. Tests are encouraged.

To run tests / ensure your changes have not caused any regressions:

mix test --no-start

To run the full suite, including failure conditions (can have some false negatives):

mix test --trace --include failure_scenarios:true --no-start

Maintainers:

Anantha Kumaran / @ananthakumaran (Lead)

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