Opus

A framework for pluggable business logic components.

example-image

Installation

The package can be installed by adding opus to your list of dependencies in mix.exs:

def deps do
  [{:opus, "~> 0.3"}]
end

Conventions

Each Opus pipeline module has a single entry point and returns tagged tuples {:ok, value} | {:error, error}
A pipeline is a composition of stateless stages
A stage returning {:error, _} halts the pipeline
A stage may be skipped based on a condition function (:if option)
Exceptions are converted to {:error, error} tuples by default
An exception may be left to raise using the :raise option
Each stage of the pipeline is instrumented. Metrics are captured automatically (but can be disabled).
Errors are meaningful and predictable

Usage

defmodule ArithmeticPipeline do
  use Opus.Pipeline

  step  :add_one,         with: &(&1 + 1)
  check :even?,           with: &(rem(&1, 2) == 0), error_message: :expected_an_even
  tee   :publish_number,  if: &Publisher.publishable?/1, raise: [ExternalError]
  step  :double,          if: :lucky_number?
  step  :randomize,       with: &(&1 * :rand.uniform)
  link  JSONPipeline

  def double(n), do: n * 2
  def lucky_number?(n) when n in 42..1337, do: true
  def lucky_number?(_), do: false
end

ArithmeticPipeline.call(41)
# {:ok, %{number: 84.13436750126804}}

Pipeline

The core aspect of this library is defining pipeline modules. As in the example above you need to add use Opus.Pipeline to turn a module into a pipeline. A pipeline module is a composition of stages executed in sequence.

Stages

There are a few different types of stages for different use-cases. All stage functions, expect a single argument which is provided either from initial call/1 of the pipeline module or the return value of the previous stage.

An error value is either :error or {:error, any} and anything else is considered a success value.

Step

This stage processes the input value and with a success value the next stage is called with that value. With an error value the pipeline is halted and an {:error, any} is returned.

Check

This stage is intended for validations.

This stage calls the stage function and unless it returns true it halts the pipeline.

Example:

defmodule CreateUserPipeline do
  use Opus.Pipeline

  check :valid_params?, with: &match? %{email: email} when is_bitstring(email), &1
  # other stages to actually create the user
end

Tee

This stage is intended for side effects, such as a notification or a call to an external system where the return value is not meaningful. It never halts the pipeline.

Link

This stage is to link with another Opus.Pipeline module. It calls call/1 for the provided module. If the module is not an Opus.Pipeline it is ignored.

Available options

The behaviour of each stage can be configured with any of the available options:

:with: The function to call to fulfill this stage. It can be an Atom referring to a public function of the module, an anonymous function or a function reference.
:if: Makes a stage conditional, it can be either an Atom referring to a public function of the module, an anonymous function or a function reference. For the stage to be executed, the condition must return true. When the stage is skipped, the input is forwarded to the next step if there's one.
:raise: A list of exceptions to not rescue. Defaults to false which converts all exceptions to {:error, %Opus.PipelineError{}} values halting the pipeline.
:error_message: A String or Atom to replace the original error when a stage fails.
:retry_times: How many times to retry a failing stage, before halting the pipeline.
:retry_backoff: A backoff function to provide delay values for retries. It can be an Atom referring to a public function in the module, an anonymous function or a function reference. It must return an Enumerable.t yielding at least as many numbers as the retry_times.
:instrument?: A boolean which defaults to true. Set to false to skip instrumentation for a stage.

Retries

defmodule ExternalApiPipeline do
  use Opus.Pipeline

  step :http_request, retry_times: 8, retry_backoff: fn -> lin_backoff(10, 2) |> cap(100) end

  def http_request(_input) do
    # code for the actual request
  end
end

The above module, will retry be retried up to 8 times, each time applying a delay from the next value of the retry_backoff function, which returns a Stream.

All the functions from the :retry package will be available to be used in retry_backoff.

Stage Filtering

You can select the stages of a pipeline to run using call/2 with the :except and :only options.

Example:

# Runs only the stage with the :validate_params name
CreateUserPipeline.call(params, only: [:validate_params]

# Runs all the stages except the selected ones
CreateUserPipeline.call(params, except: :send_notification)

Instrumentation

Instrumentation hooks which can be defined:

:before_stage: Called before each stage
:stage_skipped: Called when a conditional stage was skipped
:stage_completed: Called after each stage

You can disable all instrumentation callbacks for a stage using instrument?: false.

defmodule ArithmeticPipeline do
  use Opus.Pipeline

  step :double, instrument: false
end

You can define module specific instrumentation callbacks using:

defmodule ArithmeticPipeline do
  use Opus.Pipeline

  step :double, with: &(&1 * 2)
  step :triple, with: &(&1 * 3)

  instrument :before_stage, fn %{input: input} ->
    IO.inspect input
  end

  # Will be called only for the matching stage
  instrument :stage_completed, %{stage: %{name: :triple}}, fn %{time: time} ->
    # send to the monitoring tool of your choice
  end
end

You can define a default instrumentation module for all your pipelines by adding in your config/*.exs:

config :opus, :instrumentation, YourModule

# but you may choose to provide a list of modules
config :opus, :instrumentation, [YourModuleA, YourModuleB]

An instrumentation module has to export instrument/3 functions like:

defmodule CustomInstrumentation do
  def instrument(:stage_completed, %{stage: %{pipeline: ArithmeticPipeline}}, %{time: time}) do
    # publish the metrics to specific backend
  end

  def instrument(:stage_completed, _metadata, %{time: time}) do
    # publish the metrics to common backend
  end
end

Graph

You may visualise your pipelines using Opus.Graph:

Opus.Graph.generate(:your_app)
# => {:ok, "Graph file has been written to your_app_opus_graph.png"}

Opus.Graph is in fact a pipeline and its visualisation is:

graph-png

You can customise the visualisation:

Opus.Graph.generate(:your_app, %{filetype: :svg})
# => {:ok, "Graph file has been written to your_app_opus_graph.svg"}

Read the available visualisation options here.